It can be surprising how quickly some people in our industry pick out a floor that is not flat. Not long ago I was standing with an experienced wood flooring professional when a room scene flashed on the screen. We immediately observed together: "That floor was cupped!" It is important that wood floor installers consistently produce floors of good quality, but, as you can see all the time, cupping continues to be a persistent problem in our industry, and for all types of products.
Understanding what makes a floor cup—whether solid or engineered—can help prevent wood floor failures, or, once the cupping has happened, can assist in diagnosing the source of the problem. Here's a rundown on why cupping happens.
Cupping has occurred when the sides of flooring are higher than the center of the boards-the surface of the board has a concave shape. Solid and engineered wood flooring can both cup. We'll discuss each construction separately, as the dynamics are different. The driving force for cupping comes from a response of the wood to a change in moisture content (MC) in both cases. Below the Fiber Saturation Point (typically 28% to 30%), wood swells if the MC is increased and shrinks if the MC is decreased. The mechanism that causes swelling can be understood by looking at the cellular structure and organization in wood. (To find out more about the cellular structure of wood, see my article "Understand the Science of Water and Wood Floors.")
Cupping in Solid Wood Floors
Cupping occurs in solid wood flooring as a result of an elevated MC in the bottom of the flooring compared with the MC of the face. The general effect is easy to demonstrate by putting a small strip of paper onto a small drop of water. The paper will curl up away from the water. It is not hard to see the effect in wood flooring, either.
For many species and aspect ratios, a piece of flooring placed on a towel that is kept moist will show some cupping in a day or two. Very noticeable cupping will occur within a few days. With those demonstrations, the source of the water is easy to identify because the water is liquid, but wood can also take on water vapor from the surrounding air to cause swelling in the same way.
Any air that has a relative humidity (RH) above zero has water vapor mixed into it. Given enough time, the MC of wood will come to an equilibrium value based on the RH of the air. If the MC is increased, the wood will swell, just as it does when liquid water is introduced. The vapor-driven process is slower than when liquid water is present. Problems caused by water vapor can take several weeks or even months to become evident.
One instance that involved a delayed manifestation of a cupping problem was a wood floor installed on the second of three levels in an existing structure in a warm, humid climate. After the floor had performed well for a year and a half, the floor suffered a serious failure within the period of a few weeks and had to be replaced.
The failure was due to a hidden defect in the original wall construction that allowed air to enter the space between the ceiling of the first floor and the subfloor of the second level. In this case, the moisture reached a level in the flooring that was sufficient to generate areas of buckling and splits in some boards.
When the water that causes cupping comes from plumbing, appliance or building leaks, or from site grading problems, it is often straightforward to diagnose the problem. The source of problems caused by moisture vapor can be more difficult to find, especially in today's building envelopes with complex water vapor dynamics. Changes to air, moisture or thermal barriers in an existing building can cause problems with wood floors that have performed well for years in the past. It is not unusual to have seasonal cupping in these cases. In warm climates, air intrusion below a conditioned space, moist crawl spaces or inappropriately sized air conditioners can all cause moisture problems. (For more on that problem, see the April/May 2011 article "How To Prevent Cupping and Worse in Summer Months.")
Water vapor emission from concrete or a wet subfloor can cause a cupping problem in any climate. Swelling is the largest in the tangential direction, which is across the face of plainsawn flooring or across rotary-peeled faces. The amount of cupping will depend on the species (swelling coefficient), cut, width, thickness and the restraining forces that may be present.
A floor that has water introduced to it from the top may cup as the water goes between the boards and enters the wood from the bottom. This is likely to be the case when a wood floor has finish on it. Floor finish slows water movement but does not stop moisture passage into or out of the flooring completely, so wet floors that have finish on them dry out slowly.
As wood responds to environmental changes, the boards in a floor can experience forces from adjacent boards and from the subfloor. When increasing MC causes a board to expand, the rest of the floor pushes back and restricts the swelling to some extent. Wood has an elastic property in response to a force applied to it. If a board is bent a small amount, it will go back to the original shape when the bending force is removed. Similarly, wood constrained from swelling (in the range where the size would have changed less than one percent) returns to the original size when the MC returns to the initial value.
On the other hand, if the MC increases enough while the wood is not allowed to swell by adjacent boards, it acquires a set and will not go back to the initial size when brought back to the original MC. This is called "compression set." The result is gapping between boards when the floor is dried because the boards are narrower. If you hit a board with a hammer lightly, it bounces with no damage. A hard hit will cause a dent. In severe cases, floors that have been at an elevated MC may experience compression set and have permanent gaps between the boards. Set can often be reversed by treating with steam, but this is not a viable solution for an installed floor.
Following the theme of effects due to forces within a floor, there seems to be a phenomenon similar in appearance to the cupping due to a moisture gradient, but with a different cause. A floor installed at too-low MC will swell as it reaches an equilibrium moisture content. Due to the taper of the side match from wider at the top to narrower at the bottom of the flooring, the boards experience more force across the face than across the bottom, and they become concave on the surface. A preliminary small-scale trial to reproduce this effect has been successful in generating a concave surface without introducing a moisture gradient that increases from the face to the back in the wood.
Remedies for Cupping
In many cases of slight to moderate cupping, eliminating the source of the water and drying out the flooring can save the floor. As noted previously, moisture movement within wood can be slow. Just running a fan or dehumidifier for a week will not solve most problems. As in a dry kiln, a combination of heat, air movement and low humidity is the most effective way to dry wood. Commercial drying services that utilize large external dehumidifiers or mats to draw air through the floor may be available. With active measures, it can take a significant amount of time to dry a floor. Without active measures, it can take months. As mentioned earlier, floor finish slows the drying process (see the NWFA "Technical Manual C200: Problems, Causes and Cures").
Sanding a cupped floor flat before it is brought to a normal MC can eventually result in a crowned floor (convex surface) once the drying process is complete. At this point, the floor may now be ruined and need to be replaced. Wet floors should not be sanded flat until the drying process is completed.
Cupping in Engineered Wood Floors
Unlike solid wood, some engineered flooring cups when the MC is lowered. Although all engineered flooring is not designed in the same way, a common construction involves the face of the desired species applied to a backer. The backer (often plywood) provides structural integrity and stability in changing moisture conditions for the relatively thin face. The MC of the face and backer should be the same at the time of manufacture. The moisture level at the time of manufacture should also be the same as the MC the product will experience during use. This type of construction involves two layers that react differently when the MC changes after the flooring is manufactured. Engineered flooring will perform best in the MC range near the value it had at the time of manufacture.
If the MC of this flooring is decreased, the face material attempts to shrink approximately 10 times faster than the plywood backer. As the face pulls across the width of the piece against the backer, the flooring starts to curl up or cup. This is similar to the way that a bimetallic thermometer changes shape by bending when the temperature changes. Dry cupping can be a yearly occurrence in cold areas with a long heating season. There are some instances reported in wide engineered flooring where the curling is more pronounced along the edge of the plank above the groove. Milling engineered flooring with more of the backer above the groove (lowering the groove) can lessen the problem of the flooring deforming when it is dried. Unlike the situation with solid flooring, extended acclimation times will not lessen the possibility that an engineered floor will cup at a low MC.
No design is a cure-all that will perform well in all environmental conditions. Different products have strengths and weaknesses in different situations, and customers are best served when their vendor is knowledgeable about the properties of each product so that the chosen wood floor can perform exactly as hoped.
Crowning After Sanding
This flooring was sanded flat when it cupped after being installed over a moist subfloor. The wood flooring subsequently crowned when it dried out and had to be replaced, even though it was a new floor.
Below is a solid board that started at a normal moisture content (8%). It was then placed on a moist towel for several days. The resulting cupping is pronounced.
When this board was dried out, however, it flattened again.
And, below, here's a cupped engineered board. This piece of engineered flooring was at a normal moisture content (10%) and was kept at a temperature of 140 degrees F for several days. The resulting cupping is easily seen.
Steps to Minimize Moisture-Related Problems
- Design or evaluate the site prior to delivery for proper moisture conditions. Arrange to eliminate any deficiencies that could cause an adverse moisture condition to arise.
- Install flooring only after the wood and site have reached proper moisture conditions they will see in service.
- Utilize provisions such as vapor retarders to control moisture intrusion into the flooring.
- Use sufficient fastening to constrain flooring movement during moderate moisture changes to take advantage of the elastic property of wood.
- Educate the user about the importance of controlling relative humidity (when possible) to achieve good performance from the floor.
- Be lucky!
Cupping: Step by Step
Here's a quick glimpse at how cupping progresses:
1) At the beginning of the process, water is introduced to the bottom of a dry board.
2) As the water penetrates the wood, the wetter material on the back expands, but the dry face does not, so the board deforms.