Why Flooring Professionals Must Always Test For Excessive Moisture And Alkalinity

Why Flooring Professionals Must Always Test For Excessive Moisture And Alkalinity

Why Flooring Professionals Must Always Test For Excessive Moisture And Alkalinity

Moisture can cause a wide range of hardwood flooring problems

It’s no mystery. The leading cause of flooring failures is moisture.

Industry experts say approximately 85% of all installation failures result from moisture problems. In fact, moisture accounts for more than $1 billion in damages annually.

Moisture can cause a wide range of hardwood flooring problems that may occur soon after installation or, in some cases, months or even years down the road. These problems include cupping, buckling, blistering, crazing, adhesive failure or delamination, warping, scaling, pop-outs, efflorescence, discoloration, gaps or cracks, swollen joints, and even mold or mildew growth.

The reason moisture is the root cause of many flooring failures is simple. Concrete slabs, wood subfloors, and the wood flooring placed above a slab or subfloor are by their very nature porous. So they easily absorb and release moisture.

While it’s critical to ensure hardwood flooring reaches its equilibrium moisture content (EMC) with the air (that is, attain an optimum moisture level before installation), it’s equally – and perhaps even more – important to accurately measure the moisture content of the concrete. Why? Because moisture passing from or through concrete has become the number one cause of floor covering failures in this country.

For that reason, most U.S. producers of floor coverings, adhesives and resinous coatings require accurate moisture testing of both concrete and wood in order to avoid flooring failures.

It’s important then that wood flooring professionals have an understanding for the need of both concrete and wood floor moisture testing. They should also know the difference between the two recognized concrete test methods and the two types of hardwood moisture meters.

Let’s look at how moisture affects concrete before moving on to moisture’s effect on wood flooring.
Concrete Trowel

Concrete and Moisture

Concrete is porous material. The more porous it is, the greater the potential for moisture vapor to move at a volume harmful to the floor covering. Concrete slabs with a high moisture emission rate and/or too high a pH level will always be detrimental to a successful flooring installation.

Excess moisture in concrete can occur for various reasons:

  • There may be too much water in the concrete mix
  • Too little curing or drying time
  • Rainfall from incomplete roofing systems
  • Lack of HVAC climate control
  • Poor landscaping that fails to drain water away from building foundations.
  • Buffing the concrete smooth to iron out imperfections that may “telescope” into the floor covering. This practice hinders drying time because it seals the concrete’s pores.

High Alkalinity and Time

A related problem to excess moisture – too much alkali. High alkalinity destroys the bond between the adhesive and floor covering. High alkalinity occurs when too much moisture moves through the slab.

Alkalinity is a natural constituent of all concrete. The good: the internal alkaline state of concrete prevents reinforcing steel from rusting. The bad: when the surface of a concrete slab has an alkalinity above 9 on a pH scale (usually 10 and above), adhesive and bonding systems can be compromised.

To ensure concrete is at the proper state of alkalinity, testing with pH paper or a pH meter must be performed. Moisture causes damage, but moisture at a high pH is disastrous.

Still another problem is time. Many building contractors work on a fast-track construction schedule. As a result, concrete is not given enough time to naturally dry before installation of floor covering materials and coatings.

Making matters worse, is the use of curing compounds which inhibit or prevent concrete from drying . . . and the use of floor covering adhesives and coatings which are more sensitive to moisture and alkali assault than previous materials.

Keep in mind, flooring installers are attempting to adhere floor covering materials using water based adhesive systems to a water-based agglomerate called concrete. Excessive moisture emission from concrete that has not sufficiently dried will almost always interfere with the ability of an adhesive to bond or cure properly.
Concrete Slab

Concrete versus Cement

To avoid confusion, it’s important to distinguish “cement” from “concrete.” Though they’re often considered the same thing, they are not. Cement powder is the material, typically grey, combined with sand, water, gravel, or crushed stone to make concrete. Concrete, therefore, is a combination of cement and aggregate materials.

There are two types of cement: hydraulic cement and non-hydraulic cement.

Hydraulic cement (e.g., Portland cement) hardens by reacting with water to form a water-resistant product. It sets extremely fast and hardens even under water. It’s used to stop water and leaks in concrete and masonry structures, and is used widely where structures are submerged in water.

Non-hydraulic cement (e.g., lime and plaster of Paris) cannot harden while in contact with water. It must be kept dry to gain strength and hold the structure. When used in mortars, those mortars can set only by drying out, and therefore gain strength very slowly.

Both types of cement can be combined with fly ash, lime, silica fume, blast furnace slag, and other additives to give a variety of strengths and colors to the various cement blends.
Leveling Concrete

Curing versus Drying

When cement is mixed with sand, water, and aggregate, a chemical reaction occurs binding the materials together to form concrete. It takes roughly four weeks for this chemical process to be complete in a 4-inch slab. This process is called “curing.”

It’s a common misconception to say that concrete dries when actually it’s curing. Curing is not the same thing as drying.

Curing requires adequate moisture, temperature, and time to allow the concrete to achieve its desired properties. When a slab is considered “cured,” it still holds an appreciable amount (about two-thirds) of the moisture from the original concrete mixture. Obviously, this is way too much to install a flooring product over, so the concrete needs additional time to dry thoroughly.

A concrete slab typically cures in 28 days. Drying time continues after curing – about one month for each inch of slab thickness.

So even after curing, drying continues. Moisture moves through the slab to the surface where it then evaporates. It will, however, be replaced by more moisture drawn up through the entire slab.

Thus, it’s important to keep in mind that even when a slab may be considered cured or even “dry,” it may not be fully dry enough. Environmental conditions like temperature and air humidity can affect the drying process.

It’s imperative then that accurate moisture testing be done to ensure the slab is dry enough to apply a floor covering.

Under certain conditions, engineered wood flooring may be able to be installed directly over concrete or in below grade applications. This is something that cannot typically be done with solid wood floors. However, vapor transmission testing must be done first to determine if the level of concrete slab vapor emissions or hydrostatic pressure is acceptable. If not, excessive moisture in a basement slab or concrete slab can ruin an engineered wood flooring installation.
Wood Flooring Over Concrete

Floor Covering Problems

When a floor covering shows signs of moisture-related failures, the problem is likely due to one of the following;

  • The concrete slab has a higher moisture emission rate than the floor covering can tolerate.
  • The concrete was not cured or dried sufficiently before installation of the floor covering.
  • No moisture testing was performed or it wasn’t done correctly.
  • The moisture testing didn’t show future concrete slab behavior.
  • Alkalinity is too high in the concrete due to a high moisture emission rate.

Alkalinity is measured by pH from 1-14. Ideally, concrete should have a pH of 7-9 before installing a floor covering; otherwise, the high alkalinity (high pH) combined with moisture will destroy flooring adhesive bonds.

Freshly mixed concrete is highly caustic and extremely alkaline – well above 10. This condition may compromise adhesives and flooring materials. As concrete ages and reacts with carbon dioxide in the air (part of the curing process), the alkalinity (or pH level) of its surface gradually decreases.

Before flooring is installed, the pH level of the concrete surface should be between 7-9. However, flooring installers should check the pH level recommended by the adhesive manufacturer as well as other manufacturers’ products that might be used on the subfloor, such as moisture barriers, epoxy coatings, and leveling compounds.

ASTM states that a pH test should be performed along with every moisture test. Many flooring manufacturers require pH testing be conducted before flooring is installed.

Measuring pH

There are a couple of ways to measure pH. One way is to use paper test strips which change color according to the alkalinity of the concrete. But a more accurate way is to use a pH meter, which gives a numerical reading on a digital display.

To test pH, one must:

  1. Sand a small section of the concrete surface with 200 grit sandpaper. This removes any impurities from the surface that might affect the test results.
  2. Remove the dust with a vacuum cleaner.
  3. If the surface doesn’t pass the water drop test, that is, the surface is still not porous after sanding, the non-porous material will have to be removed using a hand grinder. If the water beads on the surface, it will not give an accurate pH reading because only the water drop itself will be tested
  4. Put several drops of distilled or deionized water on the prepared surface. Use the test strip or meter to see if the water has a pH reading of 7 before using it.
  5. Leave the drops on for 60 seconds and then place the test strip or meter in the water. Wait for the period of time specified by the manufacturer.
  6. If test strips are used, compare the color of the strip to the pH color chart. Or, if using a pH meter, read the meter’s digital display.

Proper and Accurate Concrete Moisture Testing

Testing concrete for moisture must not be done haphazardly. In order to attain accurate results, proper testing procedures must be followed to avoid costly problems later.

For instance, testing the slab’s surface only once is inadequate when trying to make an intelligent decision about installing flooring. Since a slab may dry unevenly, proper testing should include a number of different spots on the slab as well as below the slab’s surface.

Also, a combination of tests is better than a single test. The American Society for Testing and Materials (ASTM) has written standards for testing moisture content using two different test methods: Calcium Chloride testing (ASTM F1869) and In-Situ Relative Humidity testing (ASTM F2170).

There is a growing number of floor covering manufacturers suggesting or requiring both of these tests to determine dryness and suitability for installing their products.

Calcium Chloride Test ASTM F-1869

The calcium chloride test, long considered the standard for testing concrete moisture vapor emissions, has been in use since the 1940s. This test produces a moisture vapor emissions rate (MVER), which is a reading of how much moisture content is released from 1000 square feet of concrete slab over a 24-hour period. The results are expressed as pounds of moisture.

As a desiccant, calcium chloride absorbs moisture from the air. Typically, a small plastic dish of anhydrous calcium crystals is weighed on a gram scale before placement to determine moisture content. The weight, date and time the test was started is recorded. The dish of crystals is placed on the concrete for 60 to 72 hours, covered, and sealed with plastic tape to the concrete.

During this time, the only source of moisture absorbed by the anhydrous crystals is what can evaporate out of the covered concrete surface area. At the end of the test, the cover is removed and the lid is placed back on the dish and sealed. Again the dish is weighed on the gram scale and the date and time are marked. The change in weight is multiplied by a constant and divided by hours to provide an estimated rate of evaporation in pounds.

For example, water weighs 8.3 pounds per gallon. “Pounds” is the equivalent weight of water that evaporates out of a 1,000 sq. ft. surface area during 24 hours. If the test reports 8.3 pounds emission, then one-gallon of water is leaving a 1,000 sq. ft. surface area in 24 hours. When flooring is to be installed, an allowable amount of moisture emission, as expressed by the calcium chloride test, is 3.0 pounds per 1000 sq. ft. per 24 hours.

For new construction, ASTM requires 3 kits for the first 1,000 sq. ft. and at least 1 kit for each additional 1,000 sq. ft. The temperature of the surface should be between 50-70º F. The internal conditions in the room should be 40-60% humidity and 65-85º F.

Shortcomings of the Calcium Chloride Test

The Calcium Chloride test, however, is not foolproof. It has a number of shortcomings, and as a result, some floor manufacturers no longer recognize Calcium Chloride test results for determining concrete dryness.

Here are some of the cons for the Calcium Chloride test:

  • Too many tests are being set without floor preparation as required by ASTM F1869. Surface contaminants and residue from paint, adhesive curing or parting compounds can reduce vapor emission at the test site and produce inaccurate test results. In short, the concrete pores must be open.
  • It requires a specified waiting period and the correct number of tests performed based on square footage. This test measures vapor emissions at the time of testing, which can change as the building environment changes.
  • It measures moisture vapor emissions only from the top ¼” to ½” of the slab, so it doesn’t indicate moisture deep in the slab. If ambient environmental conditions immediately preceding testing have been extremely wet or dry, the concrete surface may be affected and test results may be skewed.
  • The subject building must be acclimated at or near the temperature and relative humidity levels anticipated during occupancy or use. This is often a difficult requirement to meet on a new construction project. Significant variance between the test environment and intended use environment should cause test data to be questioned.
  • The most current revision of ASTM F1869 specifically excludes Calcium Chloride tests from use on lightweight aggregate concrete.
  • “Homemade” Calcium Chloride test kits are being used and in some cases by very reputable labs. Some of these kits do not meet the apparatus requirements of ASTM F-1869-09 and are delivering questionable results.

In-Situ Relative Humidity Testing ASTM F-2170

ASTM Committee F.06 on Resilient Flooring has developed and published a standard for In-Situ Testing of Concrete Relative Humidity. This test method has been used extensively in Europe. It enables flooring professionals to check the moisture level within a concrete slab through relative humidity (RH) testing using in situ probes.

After side-by-side testing with Calcium Chloride kits, there is evidence that the In-Situ Relative Humidity (RH) data is more useful and meaningful than Calcium Chloride test results. In other words, RH probes are less sensitive to changes in ambient air humidity and temperature above the slab than calcium chloride testing.

In that regard, In-Situ RH testing provides more meaningful data under conditions that may not be acceptable for Calcium Chloride testing.  As a result, some industry experts believe this test is a more accurate way of predicting what will happen to the slab in the future.

Probes Are Inserted Into The Slab

rapid rh at 40%

This test involves drilling holes at a diameter of 5/8” to a depth equal to 40% of the slab thickness. The hole is then lined with a plastic sleeve and the sleeve is capped. The test site must be permitted to acclimate for 24 hours before reading relative humidity levels.

After or during acclimation or equilibrium, a probe is placed in the sleeve that permits readings to be obtained from the bottom of the hole, thus offering a method to measure moisture content inside of the concrete slab reported as a relative humidity level.

It is critically important that probe sensor temperature is at equilibrium with concrete slab temperature. Testing should take place in an acclimated building and at the same test placement density as noted above.

This concrete moisture test method is less affected by conditions occurring at the concrete surface, which may negatively influence Calcium Chloride test results. Consequently, this test provides the best picture of the moisture conditions the adhesive and finished flooring product would encounter if they were installed at that time.

In-situ RH testing performed at multiple depths permit a testing agency to develop a profile of moisture conditions through the thickness of a concrete slab. This information permits the user to make a more informed decision regarding the installation of floor coverings or the need to consider other alternatives.

The water content in concrete migrates from the bottom of a slab to the surface, where it evaporates according to changes in RH. It’s imperative, therefore, that the concrete’s internal moisture be allowed to dry to the proper level in order to avoid problems with adhesives and flooring products after installation.

Unfortunately, many builders mistakenly believe that surface moisture content levels reflect those of the whole slab. They neglect internal slab moisture levels. A concrete moisture meter will tell them relative moisture on the surface, but only relative humidity (RH) testing gives them an internal, and more accurate, assessment.

Although relative humidity test probes are not sold as reusable (some are, but require re-calibration for each test site), the sensors remain intact. They can be re-used over and over.
Finishing Concrete

RH Test versus Calcium Chloride Test

In terms of cost-efficiency, relative humidity testing is better than anhydrous calcium chloride testing. Calcium Chloride test materials must be discarded after each test procedure is complete. This means new kit test costs. Relative humidity testing, however, produces multiple sites throughout a concrete slab which can be assessed repeatedly.

One advantage of the RH test is that it’s less impacted by ambient temperature and relative humidity conditions than calcium chloride type tests. Thus, conditions that are not acceptable for calcium chloride testing may not generate meaningful data.

Wagner Meters, with its state-of-the-art Rapid RH® system for RH testing, saves on total material costs and preparatory work for flooring installers. The Rapid RH in-situ test method saves more than 150 hours of installation time (based on industry estimates for 13 slab tests).

Here is a breakdown of time (cost) savings:

  • Preparation: 130 minutes
  • Site protection: 120 minutes
  • Test processing: 125 minutes
  • Clean-up: 13 minutes

Smart Sensors equilibrate to produce meaningful RH data within one hour of installation, though the ASTM standard requires a 24-hour reading before making an installation decision. The Wagner Meters’ Smart Sensors continue to digitally read moisture content levels at the point of installation. This enables builders to repeat tests at each test location on site to capture an ongoing RH assessment. Project managers for large commercial construction projects find this to be especially valuable.

The Rapid RH offers multiple benefits. Users realize built-in cost savings since the system offers the lowest cost per test than other In-Situ RH test systems, plus lower time and labor costs. It’s also easy to use and 10 times faster than other RH tests.

Once installed, the Smart Sensor and Easy Reader work together to provide accurate, instant readings at a touch. And, the new, award-winning Rapid RH Datamaster helps streamline data records and ASTM-compliant reports.

Wagner Meters’ Rapid RH system offers flooring and building professionals the concrete moisture data they need to meet ASTM F2170 standards. And each disposable smart sensor comes with an NIST traceable factory calibration certificate, providing reliable accuracy and completely eliminating the need for periodic calibrations typically required by other sensing technologies.

In addition to meeting industry standards, the Rapid RH system also has received impressive industry accolades and reviews.

Other Common Tests

There are other tests that can be used to determine moisture and alkalinity levels, but they are not quantitative like the calcium chloride or in-situ relative humidity tests. Although they may indicate the presence of moisture or alkalinity, many flooring manufacturers insist that one or both of the ASTM endorsed tests be conducted to satisfy their requirements.

These other tests include:

Polyfilm Test
In this test, several pieces of plastic film, 18” – 24” square, are placed at key points on the cement, then sealed with silver duct tape on all four sides. The plastic film is removed after 24 hours (72 hours is better) and inspected for signs of condensation. Use of a heat source, such as a 40 to 60 watt light bulb placed 18” above the plastic “accelerates” this test.

If beads of water are found on the subfloor or the concrete appears darker, this serves as notice that further testing is necessary. Installation can proceed if there’s no indication of moisture.

Phenolphthalein Test
This test requires drilling dime-sized 1/4-inch deep holes in various areas of the slab, especially around the walls, and then applying in each hole two drops of a 3 percent phenolphthalein solution in water-free ethyl alcohol. If there’s no color change in the solution, this means moisture and alkalinity are not at levels to affect the installation.

Should the phenolphthalein turn pink or dark red within five minutes and the pH is 9.0 or higher, further testing must be done with a more precise method.

Wood Subfloor

Improper moisture conditions in a wooden subfloor can also lead to moisture-related problems associated with a concrete slab. Flooring specialists can test the moisture content of a wood subfloor using the pinless wood moisture meter. Similar to the hardwood floor, the wood subfloor must have an acceptable moisture content to avoid flooring failures over time.

Flooring specialists should test several areas of a wood subfloor to be certain conditions are acceptable for the final flooring installation. A good rule of thumb is that the subfloor moisture content should be within 3-4 percentage points of the flooring to be installed before proceeding.

Wood Flooring

Regardless of whether hardwood or engineered flooring is to be installed, all material should be checked with an accurate wood moisture meter. This involves opening and testing several bundles to ensure the wood flooring is the same MC throughout, and is compatible with the subfloor which the flooring will cover when installed.

As with the wood subfloor, pin-style and pinless meters can give an indication of the MC to ensure a professional and long-lasting flooring installation.

The pinless meters haves some unique advantages over the pin-style meters. Since they don’t use pins, they won’t damage the wood flooring with unsightly pin holes. That means no filing or sanding on the final floor. They also enable installers to measure entire bundles in a matter of seconds.

The Wagner Meters Orion line are pinless floor moisture meters which offers flooring professionals a versatile range of options for measuring and monitoring wood moisture content. Considered one of the world’s most accurate hardwood moisture meters based on third-party and university tests, it comes with dual-depth reading options and Wagner’s signature IntelliSense™ technology for accuracy that is virtually unaffected by surface moisture.

The Orion floor moisture meter also features a specific gravity (SG) range that covers most wood species including dense, tropical species. It provides a moisture measuring range of 5% to 30%. And, it displays moisture readings to one-tenth of a percent (0.1%) resolution on an easy-to-read digital display.

Moisture levels in the average household are considered to be between 6-9% and wood flooring is generally dried to this level. But regional and seasonal variations mean that even wood flooring manufactured from properly-dried lumber will need an acclimation in the installation environment to prevent extreme changes in the finished floor. For instance, even a change as small as 1/32” per 2” board multiplied across an eight-foot room equals 1 ½” of gapping or swelling.

In Closing . . .

All flooring professionals need to be proactive when it comes to proper installation of wood flooring. Accurate moisture testing is their best defense against moisture-related flooring failures that can lead to wasted time and financial loss . . . and even a tarnished reputation.

Hopefully, this article has provided a basic understanding of why moisture testing is extremely important, and why it applies not only to the wood flooring, but also the concrete slab and wood subfloor.

Interested in moisture testing?

Common Hardwood Flooring Problems: Prevention and Cures for Installation and Finishing Issues

Common Hardwood Flooring Problems: Prevention and Cures for Installation and Finishing Issues

Common Hardwood Flooring Problems

Prevention and Cures for Installation and Finishing Issues


The finish pulls away from itself, causing ridges in the finish similar to an alligator’s skin. This condition can occur in both water-based and oil-modified finishes.

Prevention: Alligatoring can be prevented by avoiding the following finish application mistakes:

  • Poor wetting of the finish.
  • Contamination of the finish.
  • Finish application under cold temperatures.
  • Application of a new finish coat before the previous coat has dried.
  • Application of a heavier finish coat than is recommended.
  • Use of thinners that cause the finish to dry too quickly.
  • Application of oil-modified finish over waterborne finish or vice versa when the finish is not completely cured.

Cure: Screen and recoat after the finish has dried sufficiently.

Applicator Streaks

After the floor dries, marks still are visible from the path of the applicator. It usually is associated more with water-based finishes than other types of finishes, although it may affect other types, as well.

Prevention: Avoid the following mistakes when applying finish:

  • Using an applicator that has hardened spots.
  • Improper spread rate—too much or too little finish is applied.
  • The finish is not applied evenly.
  • Excessive air movement and abnormally high temperatures causing the finish to dry too quickly, resulting in a wet edge of finish being pulled over one already dried.
  • Applying a satin or semi-gloss finish that has not been stirred properly.
  • Applying finish in directly sunlit areas or other areas that are hot.

Cure: Screen and recoat after the finish has dried sufficiently.

Bleed Back

Occurs when excess stain seeps up from the grain or from the spaces between boards.

Prevention: Avoid the following:

  • Excessive stain application.
  • High-viscosity or highly pigmented stain.
  • Excessive heat during application.
  • Knots or areas that contain higher amounts of sap.

Cure: Wipe off the excess stain or burnish/buff the floor with a white pad to remove the excess and even out the stain color. Then, let the stain dry thoroughly before applying another coat. If finish already has been applied over bleed-back, a complete re-sand is required. Trowel filling a floor can help prevent bleed back. Cover windows during application to prevent hot spots on the floor.


Dried bubbles are visible on the surface of the finish.

Prevention: Avoid the following:

  • Soap or some other contaminant was not removed before coating.
  • Applying hot oil-modified finish onto a cold floor.
  • Applying finish to a hot floor.
  • Overworking finish during application.
  • Air movement across the floor that dries bubbles into place before they can flow out.
  • Floor not screened or sufficiently cleaned between coats of finish.

Cure: Problems in the topmost finish coat can be screened and recoated, while cases of delamination require complete sanding and refinishing. Cover windows during application to prevent hot spots on the floor.


The wood flooring becomes separated from the subfloor, usually is accompanied by cupping and swelling. Buckling is caused by: excessive moisture at the job site; a house left vacant with no ventilation (see Greenhouse Effect); grade conditions; pipe leaks; a wet slab.
Extra Resource: Article: Learn The Warning Signs

Prevention: Excessive moisture is usually the main cause of buckling. Be aware of these improper installation techniques, which can aggravate the problem:

  • For nail-down products: inadequate nailing, incorrect fasteners, and incorrect subfloor construction.
  • For glue-down products, incorrect adhesive, insufficient adhesive, the wrong trowel, inadequate adhesive transfer (due to over-flashing or not rolling the floor), subfloor separation or subfloor contamination.
  • Inadequate expansion space left by the installer.

Cure: Fix the excessive-moisture condition and allow the floor to dry to normal levels. Spot repair/replacement, reinstallation, re-nailing and refinishing are also options. In most cases where the flooring has loosened from the subfloor throughout the installation, reinstallation or replacement may be necessary.

Chatter or Wave Marks

Chatter marks are consistent sanding imperfections across the grain of the wood varying from 1⁄4 inch to 1 inch apart. Wave marks are two or more “upsets” occurring along the direction of travel of a sander. They generally are 1 to 3 inches from peak to peak.
Extra Resource: Article on recommended flooring tools

Prevention: Most chatter marks are caused by the sanding drum. Before you begin, check the drum for balance and round, hard spots, incorrect paper installation or compressed rubber. Other problems include:

  • Poor splice/seams on the abrasive belt, drive belts and fan belts
  • Running the machine in the wrong direction (for belt sanders, right to left; for drum machines,left to right)
  • Worn pulleys
  • Bad bearings in the fan housing
  • Loose flooring

Most wave marks are caused by imperfections already present on the floor that are transferred through the wheels of the machine to the sanding job. Check for:

  • Wheels on the big machine that are out of round.
  • Improper electrical hookup (voltage too high or low).
  • Undulation of the floor from inadequate joist design.

Cure: First, determine and correct any problems with the sanding machine. Then, use a hard plate, paper disc or multi-disc sander (A sanding screen only highlights the chatter and causes the floor to dish out). Use a disc sander and hard plate while working right to left, traveling down and back in the same path and working the disc sander at slight angles for the best cut of the unit. Repeat the same procedure, overlapping the last cut one-half the size of the first disc.

Another technique is to do a light sanding with the machine at a 7-degree angle, then go over the floor again, this time straight. If using a multi-disc sander, walk slowly with the grain from side to side, always overlapping the unit as you move from right to left. This blends in the floor and prevents deep scratches. If joist design or loose flooring is the problem, the structural flaws must be corrected before the floor is re-sanded and refinished.


Dried finish separates from the surface in the form of flakes or chips.

Prevention: Avoid the following:

  • Applying a less elastic finish on top of a more elastic one.
  • Improper adhesion between coats.
  • Spot contamination.

Cure: Screen and recoat. More than one coat may be needed, or problem areas may be spot-coated before screening and recoating the entire floor.

Cloudy Finish

The finish appears cloudy or milky.

Prevention: Do not applying finish over a coat that isn’t dry.

Cure: Screen and recoat, being sure to increase the dry time between coats. Check with a damp rag before reapplication of finish to make sure the cloudiness has disappeared—if the finish appears clear when it is dampened, the problem probably has been eliminated, and the floor is ready to be coated over.


Often mistaken for bubbles, this problem resembles craters on the moon.

Prevention: Avoid the following:

  • Contamination of the floor or finish.
  • Application of finish over previous coats that are not dry or have not gassed off.

Cure: Sand the crater out by hand-sanding, and then screen, pad and recoat. Oftentimes, hand-sanding out the craters will leave an unevenness in the floor. To eliminate low spots, spot finishing the areas may be necessary before the entire floor is recoated. If the problem is severe enough, the floor may need to be re-sanded.

As a general rule, trowel-filling the floor may help prevent problems due to contaminants in the cracks between boards.


The center of the flooring pieces appear to be higher than the edges.
Extra Resource: Cupping and Crowning: Spotting Trouble

Prevention: While it’s possible that excessive moisture could cause crowning, it is more likely that the floor cupped and then was sanded flat before it could dry and flatten on its own. When the floorboards did dry to a normal condition, their edges had been removed, making them lower than the center of the board. Gaps are generally formed as the flooring dries.

Cure: First, determine if the moisture content is normal and if all of the crowning from the original cupped condition has occurred. After the floor has stabilized, re-sand and finish.


Occurs across the width of the individual pieces of flooring. The edges are high, and the center is lower.
Extra Resource: Cupping and Crowning: Spotting Trouble

Prevention: Cupping is usually caused by an excessive moisture differential within individual pieces of flooring and excessive moisture on the underside of the flooring. More subtle cupping can be caused by lack of proper acclimation (this is generally permanent cupping). Check for potential sources of excessive moisture, including:

  • Building leaks
  • Poor drainage
  • Plumbing leaks or overflows
  • Leaks from dishwashers or refrigerator ice making units
  • Wet or damp basements/crawlspaces
  • Concrete subfloors that have not cured
  • Plywood subfloors with excessive moisture
  • Poor or no ventilation
  • HVAC system not operating.

Flooring may also cup when a wood floor experiences conditions that cause rapid drying on the surface. This condition occurs with gaps as the flooring shrinks.

Cure: Never attempt to repair a cupped floor until all sources of excessive moisture have been located and eliminated. This can be verified only with a moisture meter that takes readings of the underlying subfloor. As long as the wood is not permanently deformed or damaged, the flooring will return to its original shape and size when the excessive moisture is removed. This may take weeks, months, or even an entire heating season.

Attempting to sand a cupped floor while it is still too wet may cause subsequent crowning when the floor dries. Flooring that does not return to its original shape, even after completing an entire heating season, probably is permanently deformed (Taking moisture readings at different levels in the wood flooring also can help determine this—if there is a gradient of 1 percent or more between the top and bottom of the boards, they probably are not done drying). If the boards are permanently deformed, the cupped edges may be sanded off. For floors that have cupped due to drying, relative humidity should be increased. Relative humidity below 20 percent is considered very dry for wood flooring, and it is suggested that humidification be provided under such conditions.


Crushed spots in the wood.

Prevention: Institute good floor maintenance procedures, such as removing high heels and using floor protectors.

Cure: If wood fibers are not broken, attempt to draw fibers back up with an electric iron over a dampened cloth. If fibers are broken, remove and repair the damaged boards. The entire floor may need to be re-sanded and refinished.


The floor changes color over time. Some areas may darken more than others.

Prevention: Understand that changes in a floor’s coloring over time is natural and to be expected. Despite the pervasive myth that an oil-modified finish recoated with waterborne finish will stop ambering, ambering of oil-modified finishes cannot be prevented. Wood lying in direct sunlight will also change color over time, and wood also changes color through oxidation. Shading the floor can minimize lightening. Customers should be informed that certain species, particularly cherry and many exotics, will change color greatly as they age.

Cure: If marks are left on the floor by area rugs or furniture, moving them around can equalize the change in color.

Dish Out

Areas on the wood floor where softer parts of the wood appear to have been sanded more than other areas. Occurs between areas of annual rings or between mixed species of varying hardness together on a floor, such as in feature strips, borders and medallions.

Prevention: Ensure you use the proper angles while sanding.

Cure: Re-sand the floor using a slight angle with the big machine. A hard plate or multi-disc sander may be needed on softer woods.

Excessive/Early Finish Wear

The appearance of too much wear on a relatively new finish.

Prevention: Most excessive or early finish wear is due to improper maintenance procedures. Avoid the following:

  • Failure to fully remove grit from the floor’s surface.
  • Using water to clean the floor, or using strong cleaners on the floor.
  • Pet nails and chair legs

Mistakes during finish installation can also cause excessive or early finish wear. Avoid:

  • Not enough finish applied to the floor initially.
  • Applying finish over coats that have not had enough time to gas-off and dry.
  • Improper sanding procedures—when the floor is left too rough, finish accumulates in the bottoms of the grooves in the floor, leaving little coverage on the “peaks,” where the finish then wears through. This may give the appearance of ridges in the flooring.

Cure: Follow proper maintenance procedures, including regular dust-mopping with an approved wood-floor cleaner, use of throw rugs and use of floor protectors. If caused by improper sanding, re-sand and recoat.


A circular, cloudy haze with a clear center that can measure up to about 1 inch in diameter.

Prevention: Avoid contaminating the surface—the new coat “crawls” away from the wet or contaminated areas, giving the appearance of fisheyes when the finish sets. Also, if the finish container has sat undisturbed for some time and has not been properly agitated, a disproportionate amount of flow and leveling agents may be put on the floor, causing a fish-eyed appearance.

Cure: Screen and recoat.

Flooded Floors

Standing water on the wood floor.
Extra Resource: Disaster Relief and Flooding

Prevention: Common sense measures prevail, although flooding, like other natural disasters, can often be an event beyond your control.

Cure: Remove the water and dry the floor as quickly as possible. Elevate the temperature, dehumidify and increase airflow using fans. In basement houses, dry from below. In crawl space homes, use exhaust fans. If the flooding was of a long duration on a surface-finished floor, rough sanding to remove the finish will accelerate drying. Lightly sand at a slight angle of about 7 degrees. Or, use a buffer or orbital sander with a 120-grit screen backed by a soft pad. (Sanding this way should not remove the edges, which could cause crowning later). Do not sand down to bare wood, but rather remove the majority of the finish.

Do not repair the floor until moisture meter readings on the top and bottom of the boards and subfloor are at normal levels. When flooring is stabilized, determine the damage. If the flooring has loosened from the subfloor, repair the necessary areas or the entire floor. If it is cupped, sand it flat. If the floor is flat, fill if necessary and screen and recoat. If the subfloor is plywood over concrete, it is unlikely that the plywood and concrete subfloor will dry out in a reasonable time. Full removal to concrete usually is best to allow the slab to dry.

In cases where you have determined that the flooring system has not returned to normal levels, do not succumb to pressure from involved parties for a quick fix. If you must proceed, have a full release signed due to the risk of more moisture problems.

Gaps, Normal

Gaps between strips/planks that appear between individual boards and open and close with changes in humidity.
Extra Resource: Article on Wood Flooring Movement

Prevention: Most normal gaps are caused by seasonal fluctuations in relative humidity—the floor expands with high humidity and contracts during periods of low humidity. This type of expansion and contraction is considered to be normal and expected for solid wood floors. In solid 21⁄4-inch floors, gaps may be the thickness of a dime (1⁄32 inch) or wider. Wider boards have even wider gaps.

If normal gapping is a concern to you, chose light –colored floors, which show gaps less than dark floors, and beveled floors, in which gapping as not as obvious as in square-edged floors.

Cure: Normal gaps can be minimized by using the HVAC system to control fluctuations in humidity in the building. The use of humidifiers or dehumidifiers can narrow the overall fluctuation range.

Gaps, Abnormal

Gaps in the floor that remain with seasonal change. If some boards appear glued together by the surface finish, see “Sidebonding/Panelization.”
Extra Resource: Article on Wood Flooring Movement

Prevention: Check for and be aware of the the following potential causes before and during hardwood flooring installation:

  • Edge crush from prior exposure to extreme moisture (especially for solid, flat-grained flooring).
  • Hot spots in the subfloor, such as poorly insulated heating ducts, hot water plumbing lines, radiant heating systems, register openings and refrigerator motors.
  • Debris between boards during installation.
  • Improper nailing/nail position.
  • Flooring installed with an excessively high moisture content or over a subfloor with excessive moisture.
  • Flooring not installed tightly together to begin with.
  • Foundation settlement.
  • Improper subfloor materials that will not hold nails.
  • For glue-down floors, early foot traffic, incorrect adhesive, the wrong amount of adhesive transferred or used, the wrong amount of flash time for the adhesive, or not using a roller when recommended.

Cure: Eliminate the cause(s), then restore normal humidity levels. After the floor has stabilized, use filler in gaps that are small enough to be filled (typically up to 3⁄32 inch) and recoat the floor. For larger gaps, use a sliver or “Dutchman” to fill in the gap. Pulling up the entire floor and reinstalling may be necessary.

Grade Problems

Dissatisfaction with the floor due to the appearance of knots, grain pattern, color variation, etc.

Prevention: Manage customer expectations during the entire process so there is a clear understanding of what the finished product will look like. This may include educating customers about natural wood variations and distinction this lends to hardwood floors.

Grade problems can also occur through ordering mistakes by the supplier, distributor or installer; poor grading at the mill; or an improperly labeled product.

Cure: Replacing the offensive boards may be necessary.

Greenhouse Effect

Floors that shrink or swell due to an abnormal level of humidity in a vacant house.
Extra Resource: Moisture Related Problems

Prevention: The greenhouse effect happens when houses are closed up with no airflow; when sunlight through windows generates heat; and when condensation and humidity build. Avoid these potential hazards by carefully regulating moisture in the home, even when it is closed up for an extended period of time.

Cure: After the environment returns to normal, follow cures discussed for cupped floors or shrinkage gaps.


If the surface of the flooring is sagging, it is likely that termites have created eating corridors beneath the surface. The bugs are white or cream colored. If fresh holes about 1⁄16-inch wide are found, powderpost beetles, or lyctid beetles, probably are the cause. Positive identification of the infestation is necessary and should be performed by an entomologist.

Prevention: Most wood damage from insects and fungal rot can be prevented with relatively low toxicity insecticides and/or pest-targeted baits. Furthermore, lmost all wood flooring in North America is kiln-dried, and proper kiln-drying should kill any insect infestations. Check all surroundings for infested wood molding and furniture (especially bamboo, mesquite and ash).

Cure: For termites, a professional exterminator should eliminate the bugs. Then, structural damage should be repaired. Damaged floorboards should be pulled and replaced. Termite infestation is not related to wood floors, and full cost should be covered by the owner. For powderpost beetles, determine the extent of damage. If infestation is heavy, handle it the same as the termite extermination above. If it is occasional, especially in new floors, treat the individual openings immediately with insecticide injected by a syringe into the holes. Or, use aerosol insect spray through a straw in the holes. Have the owner watch for evidence of new holes (with little dust piles) and treat again. After two to three months, the holes may be filled.

Iridescent Finish

The finish dries with a metallic, colored cast to it.

Prevention: Adequately ventilate during the drying of a coat of finish, which prevents solvent saturation in the air that then settles on the floor and is coated over.

Cure: Screen and recoat using proper ventilation.

Orange Peel

The surface of the finish has a texture that resembles an orange peel.

Prevention: Avoid the following:

  • Rolling a finish that is not designed to be rolled on, causing it to dry too quickly. When that happens, the texture is “frozen” into place before the finish has a chance to flow out and level.
  • Using a finish or substrate that is too cold.
  • Use of an improper applicator that causes small bubbles to form in the finish. The bubbles then pop, leaving small dimples in the finish.

Cure: Screen and recoat.


The finish delaminates from the floor in sheets.

Prevention: Avoid the following:

  • Stain or previous finish coat that was not dry.
  • Skipping abrasion between finish coats.
  • Stain not sufficiently wiped up.
  • Improper tacking between coats.
  • Surface contamination such as wax or oil-soap cleaners.
  • Finishes that are not compatible.

Cure: Re-sand and recoat.

Picture Framing (“Halo”)

The edges of the room appear to be a slightly different color than the rest of the room.

Prevention: Avoid sanding the edges of a room differently than the field, which causes edges to be either smoother or rougher than the center of the floor, affecting the finish and/or stain appearance.

Cure: Re-sand the floor, being sure to use the same sanding procedure on all parts of the floor.

Pin Holes

Similar to fisheyes, but very small.

Prevention: do not apply a coat of finish over one that was not yet dry.

Cure: Give the floor sufficient time to totally dry, then screen and recoat using correct dry times between coats.

Poly Beads

Droplets (“BB’s”) of finish that form along strip edges. They can be soft and sticky when first formed, but become hard if left undisturbed.

Prevention: Avoid slow drying conditions and excessive amounts of sealer and/or finish, which seeps into cracks.

Cure: Time will allow the floor to expand and contract, eventually allowing all of the undried finish to surface. When soft, the beads can be smeared, leaving an unsightly appearance that may require screening and recoating. For hardened beads, the solution is to remove them with a sharp edge (i.e. scraper or plastic putty knife) and, if necessary, screen and recoat. Do not attempt to screen the hardened beads, which will cause circular scratches within the finish.

Roughness/Grain Raise

The surface of the wood floor is rough to the touch.

Prevention: Avoid the following:

  • Inadequate sanding, including skipping too many grits.
  • Contamination of the finish during dry time.
  • Not allowing sufficient dry time for waterborne sealers to flatten.
  • Moisture causing the wood grain to rise.
  • Not using enough coats of waterborne finish.

Cure: If a moisture problem is evident, this must be corrected before rescreening and recoating.

Shellout/Dishing of Springwood

Uneven wear between segments of annual rings.

Prevention: Following sound maintenance practices, such as changing casters to wide, non-marking rubber.

Cure: Sand and refinish.


The problems appear similar, but are different. With sidebonding, the bottoms of the edges of the individual strips are “glued” together by the finish. This can occur with all types of finish, although it happens more frequently with water-based products. Panelization occurs when the edges of boards are crushed and stick together as a result.

Prevention: Staining floors can help prevent sidebonding, which results from the finish seeping down into the spaces between boards and gluing the bottoms together. Panelization can happen when staples crack the tongue in some areas and not others, or when a wet plywood subfloor shrinks as it dries.

Cure: Restoring normal humidity levels can return thefloor to an acceptable appearance. If there still are gaps, see “Gaps, Abnormal.” If these methods do not fix the floor, floor replacement may be necessary.


Slivers and/or splinters protrude from the surface of the floor, especially at the edges of the boards. May tend to occur more frequently in beveled prefinished products and wire brushed products.

Prevention: Take steps to prevent unevenness caused by: expansion, cupping, subfloor irregularities, edge crushing from expansion, or grain raise from moisture. Take care not to damage the floor when nailing during the installation process, and follow proper grading guidelines.

Cure: If a new floor is producing fibers, not splinters, buff vigorously with a commercial buffer and nylon polishing pad. For slivered bevels, shave off with a razor knife and re-stain. For expansion, cupping and grain raise, correct the moisture source.

For wind shake, it may be possible to repair the boards using low viscosity, CA (cyanoacrylate) adhesive (such as Super Glue). Apply the adhesive under the seam of the shake. The adhesive will wick down and hold the shake. Because it is clear and nonambering,it can even be used between coats of finish. Or, the boards can be removed and replaced.

Squeaky/Loose Floors (“Popping”)

The floor causes objectionable squeaks or other noises.
Extra Resource: Fixing Squeaky Floors

Prevention: Squeaky floors are caused by problems in the wood flooring system, subfloor system or underfloor supports. Take precautions to insure these areas are sound during installation. Avoid inadequate or improper nailing, using a weak subfloor or improper subfloor material, or insufficient/incorrect adhesive.

Cure: Noises in only certain areas may be fixed by injecting adhesive into the problem area, screwing the floor down from below, strengthening the subfloor from below or using facenails or screws and plugs. Squeaks also may be lubricated with graphite, wax or baby powder, although such solutions will contaminate the floor for future finishing.

Floors that are noisy and loose throughout the entire area usually have to be pulled and reinstalled, correcting the problem—whether it is caused by the subfloor, fastening schedule or adhesive.


Discoloration on one area of the floor.

Prevention: Avoid the following:

  • Liquids spilled onto hardwood floors.
  • Pet stains
  • Residue from improper cleaners.
  • Continual moisture leading to mildew (black), decay (brown/white) or alkaline conditions (white).

Cure: Cloudy surface finish can be fixed by lightly rubbing with a proper cleaner and buffing, although some stains require screening and recoating. Pet stains sometimes can be fixed by re-sanding, but frequently require total board replacement. One technique to eliminate pet stains is to apply naval gel (a phosphoric acid gel commonly available at hardware stores) to wick the tannins out of the area. This will not contaminate the floor for future finishing or leave a halo mark, as attempts at bleaching the floor often do.

Sticker Stain

Light brown marks that appear on the wood surface, especially on maple, ash or other light woods. They occur across the width of the strip, measure 3⁄4 to 1 inch wide and occur about every 20 to 24 inches down the length of individual strips.

Prevention: Sticker stain is allowed in second-and-better-grade maple and No. 1 common oak. If the marks will be objectionable to the owner, do not install the flooring.

Sticker stain generally does not sand out.

Sticky Board Syndrome

The finish will not adhere or cure properly on one or more boards.

Prevention: Do not apply too much stain, and then finish, over very open grain. Another cause of sticky board is excessive tannic acid or pH imbalance in the wood. This is most common with oil-modified finishes and white oak.

Cure: When one board or several boards scattered throughout the floor will not take stain or finish, the most common solution is to repair the floor by replacing the boards. Or, boards may be taped off (using recommended tape) and scraped or hand sanded, then coated with a water-based sealer. After proper dry time, they may then be coated with an oil-modified finish. Trowel filling may help prevent sticky board syndrome.

Unevenness of Entire Floor

The entire floor as a unit appears to be uneven.
Extra Resource: How Subfloors Affect Flooring

Prevention: Unevenness can be caused by uneven, warped and loose subfloors, joists that are warped or fractured, settled support pillars or perimeter foundation. Unevenness can also be caused by a cracked and/or settled concrete slab. Although structural integrity of the subfloor system is not the responsibility of wood flooring contractors, they should check the floor for flatness before beginning an installation.

Cure: A general contractor needs to repair the subfloor before the wood floor can be repaired.

Uneven Sheen Levels

The sheen of the finish is inconsistent.

Prevention: Avoid the following:

  • Insufficient mixing of finish prior to application.
  • Uneven sanding.
  • Uneven finish thickness.
  • A contaminated finish applicator, such as a lanolin-rich lambswool applicator that hasn’t been thoroughly cleaned.
  • Inspecting the floor under inadequate lighting.

Cure: Screen and recoat. If lighting is the cause, discuss with the customer the reasonable inspection position for looking at a hardwood floor—from a standing position under normal lighting conditions.

Learn More

Hardwood Flooring Reference Tool & Definitions

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