Wood Floor Cupping

cupped floor


In daily life, some people are surprised how quickly a floor that is not flat. The sides edge of flooring are higher than the center of the board, the surface of the board has a concave shape,Yes, the floor was cupped. It is important that wood floor manufacturer consistently produce good quality floors, but, as you can see all the time, cupping is a persistent problem in flooring industry.

To assist in diagnosing the source of the cupping problem or prevent wood floor failures, we should get better understanding what makes a floor (solid flooring or engineered flooring) cup and what can we do once the cupping has happened.


Why Does Cupping Happen on wood flooring?


The driving force for cupping comes from wood moisture content (MC) in both solid and engineered flooring. Wood swells if the MC is increased and shrinks if the MC is decreased below the Fiber Saturation Point (typically 28% to 30%). The mechanism that causes swelling can be understood by looking at the cellular structure and organization in wood.


Cupping occurs in wood flooring when an elevated MC in the bottom of the flooring is higher than the MC on the surface. The 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. 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. 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.


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 react 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.




What can we do to Prevent Wood Floor Cupping in summer?


In summer months, air has water vapor mixed into it as relative humidity (RH) above zero . If the MC is increased, the wood flooring 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.


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.


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.


Crowning After Sanding

crowning after sanding

photo of wood flooring showing cupping 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.




What can we do to Prevent Wood Floor Gaps in Winter?


If the MC of this flooring is decreased, the face material attempts to shrink approximately 10 times faster than the plywood backer. As he 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.



Creating Cupping

creating cupping


The left pic 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.





creating cupping 2


when this board was dried out, however, it flattened again.




creating cupping 3

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.



Cupping: Step by Step

Here's a quick glimpse at how cupping progresses:

step by step 1


1) At the beginning of the process, water is introduced to the bottom of a dry board.






step by step 2




2) As the water penetrates the wood, the wetter material on the back expands, but the dry face does not, so the board deforms.





step by step 3



3) If the process is allowed to continue, the entire board swells. At this point, it is likely that a floor would buckle.  






Tight Squeeze

This solid cherry flooring was ...

1 - Brought to a low MC

2 - Secured to a substrate

3 - Constrained from swelling across the face

4 - Allowed to acclimate to ambient conditions, which resulted in a concave surface:

tight squeeze