As the seasons change from hot and humid in the summer to dryer, cooler weather in the fall, keep in mind that your hardwood floors will need to adjust to the weather just as we do.
You may have seen this happen; a once beautiful hardwood floor is suddenly reminiscent of the ripples on water, especially visible when light hits it. When these dips appear in the wood, it is called dry cupping. Even if the extent of the cupping is not so visually disastrous, slight cupping can cause installation problems for new hardwood flooring, especially if you have borders or feature strips. Cupping is just one of many ways wood can become deformed but most, including cupping or crowning, are related to moisture.
Wood 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. Surface swelling happens when the humidity around the wood increases, and the moisture in the air causes lots of this water/cellulose bonding. Wood then expands, called crowning, likely causing some post-installation problems. This also happens when there is standing water or a water leak in the home, onto the surface of the hardwood floor. It could be a small drip from an ice maker or a toilet leak. It is also the reason why it isn't always advisable to have hardwood flooring installed in wet locations such as bathrooms. It is very important that any standing water be wiped up immediately to prevent it from soaking into any exposed cells.
The opposite of crowning is cupping, which is caused by the moisture levels on the surface of the wood actually decreasing and differing from the moisture levels below it, as in the room below or in the subfloor. The moisture content of a hardwood plank 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. The moisture levels quickly dry up causing the wood to cup or bow.
Typically, hardwood flooring is installed over a plywood subfloor. Plywood is made up of thin layers of solid and sawn wood that are glued (laminated) together to produce a solid, durable sheet. Plywood is much more stable when it comes to moisture due to the way it's manufactured and it is fastened in numerous locations to the floor joists to keep it from warping, so in cupping, the top layer shrinks while the bottom layer stays quite possibly unchanged or is less significantly affected, resulting in a pulling away or curling of the top layer. One of the major causes of this condition is installing hardwood flooring in an arid climactic zone or in a region where humidity levels change significantly at different times of the year due to seasonal usage of artificial heating or air conditioning. It is extremely important that the plywood subfloor and the hardwood floor have a chance to acclimate to the same climatic moisture/humidity levels before the hardwood is actually installed. This usually eliminates problems down the road but it is also important that the moisture levels are maintained to eliminate crowning or cupping in the future.
HOW AND WHY DO HARDWOOD FLOORS "CROWN" AND "CUP"?