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How and Why Do Floors "Cup?"

Posted: Wednesday, November 10, 2010

Cupping and Crowning

You've probably seen it happen…a once beautiful hardwood floor is suddenly reminiscent of a wagon-worn trail. This creation of dips and divots in wood is called dry cupping. But even if the effect of cupping is not so visually disastrous, even slight cupping can cause installation problems for new hardwood flooring, especially when working with borders and feature strips. Cupping is just one of many ways wood can deform, and most, including cupping, are related to — you guessed it — moisture.

As we all know, every living thing (or once living, in this case) is made up of cells. They come in many different shapes and sizes and are made up of various elements, compounds, polymers, and proteins, but the cell walls of wood are comprised mainly of cellulose, a super strong substance that water molecules like to bond to. Swelling happens when the humidity around the wood increases, and the moisture in the air causes lots of this H2O/cellulose bonding. Wood then expands, likely causing some pre- or post-installation problems.

The flip side of swelling is cupping, which is caused by the moisture levels of wood actually decreasing. The moisture content of a given plank of engineered flooring is at a certain level at the time of its production and storage. Then let's say the planks suddenly find themselves in a different, drier environment—trip to Phoenix, anyone?

But the problem isn't the solid wood itself shrinking; cupping only happens because of the layered construction of engineered hardwood flooring. Typically, engineered flooring is made up of a thin strip of solid, sawn wood glued on top of a piece of plywood. Plywood is much more stable when it comes to moisture, so in cupping, the top layer, for all intents and purposes, shrinks, while the bottom layer stays quite possibly unchanged or at least significantly less affected, resulting in a pulling away or curling of the top layer. Major culprits are moves to arid climactic zones or colder regions where humidity levels drop significantly due to periodic artificial heating.

So the takeaway here is that to avoid cupping, the humidity levels must be similar at the time the hardwood flooring was manufactured to the driest point in the floor's daily life.