Minor to moderately cupped flooring, .01” with the occasional piece at .02”. Crawl space subfloor readings more than 4% greater than flooring readings. Cause: elevated crawl space moisture. Repair: reduce crawl space moisture and reevaluate after heating season.

Moderate cupping when viewed from a standing position. Cupping measures .01” to .02”. This cupping is noticeable even without window glare.

The most frequent questions concerning issues with wood flooring involve cupped flooring. This article reviews excerpts of conversations I have had with homeowners, builders and even flooring contractors about cupping. The primary questions are: “What has caused the flooring to cup?” “Who is responsible for the cupping?” “What is the fix for the cupping problem?”

The answers to these questions involve establishing the history of the flooring and the conditions surrounding the flooring. Assessing the moisture conditions of the flooring system will be a critical component in resolving the cupping.

Moderate cupping of an engineered flooring caused by general acclimation to higher environmental humidity levels. Moisture readings of flooring were at  9-10%,  which is the area’s average environmental condition. Repair options are to sand and refinish the flooring.

The phone conversation often goes like this: “My flooring “warped,” “buckled,” “cupped,” after I moved into the home. After some months of complaining the flooring contractor looked at the flooring. The contractor said it was from excessive jobsite moisture and was not his fault. I can’t get anyone to do anything about the flooring.” At the root of the inquiry is the perception that there is a serious problem with the flooring. It is defective and cannot be fixed. Also, that the cupping will affect the value of the home if it is put on the market.

The first item is to find out what the owner is actually experiencing. I state that the definition of cupping is where the long edges of the flooring are raised up higher than the center.  Further explaining that “buckling” is where the flooring has actually lifted off the substrate or subflooring and is raised above the plane of the flooring. The response is most always that the flooring is cupped, that, “I can see it in the light and can feel it particularly with my bare feet.”

A noticeably cupped wide plank replacement floor. A hot water leak was the cause of the original tear out. Cupping moderate, .02” with occasional pieces to .03”. Moisture readings normal for area, 7-10% flooring, 11-12% subflooring. The cause was flooring acclimated and installed to dry winter conditions and acclimated to wetter normal environment.  Repair options are to sand and finish the flooring.

Next, I review the three causes of cupping: 1. Cupping is caused by extra or excessive moisture being absorbed by the back of flooring resulting in the back expanding more than the face to raise the edges. 2. Cupping is caused by the flooring gaining general environmental moisture and expanding. Since there is no space in the flooring for expansion as the flooring tightens it cups. 3. The face of the flooring dries or looses moisture and shrinks more than the back and the edges are pulled up or cupped. This cupping is associated with gapping. 

Minor cupping .01” and less. There is a basement with elevated humidity, subflooring moisture 13-15%. Cause of cupping was moisture from basement. Repair option: reduce basement moisture and reevaluate after a heating season.

The owner is asked, “Is there is gapping in the flooring?”  Most often the response is; “No the flooring is tight with no gaps.” So, I focus on the #1 and #2 causes. The history of the flooring is then established. I ask the following questions: When was the flooring installed, sanded and finished (applicable if not factory finished)? When did you move in or when construction was completed? When was the cupping first noticed? Was there a direct event with water, i.e. a leak or over-flow? Since noticing the cupping has it gotten worse or better? Is the cupping noticeable everywhere or only in the glare of windows? In the last 3 to 5 months has the cupping changed? Does the cupping get better or worse seasonally in the winter or summer?

Moderately cupped engineered bamboo flooring, cupped .01” with an occasional .02” and .03”. Cause: dry interior environment conditions.  

The answers to these questions allow the home owner to understand the following explanations of their particular situation where minor to moderate cupping with no buckling is involved. Flooring installed in the late fall, winter, or early spring may be dry and expand in the summer as humidity increases. Flooring installed in the summer that cups later, i.e. in the fall and winter, likely has a moisture source affecting the flooring. Cupping beginning in the summer and subsiding or lessening in the winter is directly affected by the seasonal environmental humidity. Cupping seen only in glare is generally considered minor cupping (.01” and less, cup from board edge to edge).  Cupping that has been present throughout a year (complete seasonal change) with only minor differences seasonally, is considered a permanent cup. The owner is also advised that cupping of .01” to .03” edge to edge is generally considered significant likely requiring remediation such as refinishing.

Confirming additional high moisture at the slab level using insulated needles. Repair: eliminate the elevated moisture and allow time to re-acclimate. Note that bamboo takes extra time to acclimate; a complete seasonal change may be necessary.

With a water event such as an appliance leak the cause of the cupping is obvious. The questions asked then focus on what to do. They ask whether the flooring can be saved and or how much flooring to remove. The answers directly relate to how quickly the event was stopped and the water removed.  The quicker and more thoroughly the water is removed, the more likely some or all of the flooring can be saved and repaired. The amount of resulting loose flooring and extent of prominent gapping resulting from the event after drying to a normal condition determine if partial or total replacement is required. Another situation encountered with the water event is where replacement flooring also cupped. Most of the time cupping is similar to the minor to moderately cupped floor so I go over those questions and explanations.

The one extra condition discussed is that the residual excessive moisture may not have been removed and was available to cause the cupping. Also, remedial over-drying the flooring system and installing too-dry flooring can produce cupping as the flooring adjusts to the normal environment. I report that these situations are directly related to the original repair.

For cause #3, cupping as a result of drying, the explanation is that the flooring as installed was at higher moisture than the present environment. This could be caused by handling and storage under too wet/humid conditions or being manufactured and delivered at too high moisture content. The only other condition to be blamed on the flooring is if a significant portion was manufactured too dry (below the 6% to 9% moisture standard). In these cases the owner is advised that in order to determine the specific cause a site inspection would be required. Also samples of the flooring would absolutely be needed for precise measurement and oven drying to determine the condition of manufacture.

These explanations generally diffuse the thought that the flooring is completely defective and requires replacement in all cases. The explanation also qualifies the severity of the condition so the discussion for remediation can proceed.  Specific options of remediation are also reviewed with the owner. The options begin with the fix that can be as simple as living with a minor condition, or to refinish significantly cupped flooring, or to changing the environmental conditions of the site. (Remedies for the different levels of cupping were explained in my March/April FCI article.) We also review that seasonal cupping directly associated with site environment may not be blamed on anyone. With the proper information the owner can then discuss the conditions with the contractor and determine a solution.

Part of the contractor’s responsibility is to make a proper moisture assessment of the flooring and flooring system. Moisture readings should be taken with a quality moisture meter. With cupping, insulated needle probes give the best indication of the moisture conditions. One or two readings do not make a proper assessment. For readings that are near average for the geographical area, stating that excessive or elevated moisture is the cause of cupping is incorrect. For instance, flooring readings of 8-9% and subflooring readings of 11-12% are pretty normal for my area in the warm season. This is not excessive moisture though the flooring may be cupped. But the readings can point out that normal or average conditions exist. This allows the further explanation that the flooring does not need replacement so that dialogue can continue with the options for repair.