What You Should Know About Cupping

Cupping Defined

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 of the construction separately, as the dynamics are different.  The driving force for cupping comes from a response of wood to a change in the moisture content (MC) in both cases.  Below the fiber saturation point (typical results 28%-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 of the wood.

Cupping in Solid Wood Flooring

Cupping occurs in solid wood flooring as a result of an elevated MC in the bottom of the flooring (Hardwood) compared with the MC of the face.  The general effect is easy to demonstrate by putting small drops of water on a small strip of paper and you will notice the paper start to curl 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 too moist will show cupping within days.  With those demonstrations, the source of the water is easy to identify because water is liquid however wood can also take on water vapor from surrounding air to cause swelling in the same way.  Any air that has a relative humidity (RH) above zero has the water vapor mixed into it.  Given enough time the MC of the 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.

Water emissions from a concrete or a wet sub-floor can cause a cupping problem in any climate.  Swelling is the largest in the tangential direction, which is across the face of a plain sawn flooring or across a rotary peeled face.  The amount of cupping will depend on the species of wood, cut, width and thickness of the restraining forces that may be present.

Cupping in Engineered Wood Flooring

Unlike solid wood flooring, some engineered floors can cup when the MC is lowered.  Although all engineered flooring is not designed in the same way, a common construction involves the face to be the desired species applied to a backer.  The backer  (often plywood yet sometimes HDF) 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 (CROSS PLY) when the MC changes after the flooring will perform best in the MC range near the value it had at the time of manufacture.

If the MC of the 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 way that a bimetallic thermometer changes shape by bending when the temperature changes.  Dry cupping can be a yearly occurrence in cold or extreme temperatures 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 its 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 weakness 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.