Why does wood warp in damp weather?

Shawn is very excited about the construction of his new house. He purchased all the necessary construction materials ahead of time and ordered wood and other wooden furniture from a factory overseas. Waiting for the other materials to be completed, he stocked all the wood just outside his house. Unfortunately, it rained for a few consecutive days, and when the sun is out, the carpenters discovered that 75 percent of the wood are misshapen due to warping. That would cost another expense for Shawn.

Where and how could the problem arise? How can the warping of wood be prevented? Shawn could have some experts or wood manufacturers.

The amount of moisture and humidity affects the shape and integrity of the wood. Most wood species cannot withstand exposure to high moisture and humidity, like in Shawn’s case, exposure to rain. However, wood from cedar, redwood, teak, and ipe, contains natural oils that protect them against moisture. Other species needed protection from rain and humidity, or else they quickly warp, crack and rot. (click here for more details)

What is the wood warping?

Wood warping is the deformation in wood that changes in its appearance, size, or shape. The wood warps because of uneven drying rates within its fibers. When a portion of a wooden board dries faster than another, the drier part will shrink faster and cause stress resulting in a change of shape of the wood. The board that was meant to lie flat now has a bend in it somewhere. (click here for more details)

A type of wood with relatively high moisture content shrinks as it dries until it reaches equilibrium with relative humidity, or also called equilibrium moisture content (EMC). On the contrary, wood with a relatively low moisture content will swell as it absorbs water from the surrounding air until reaching EMC. This direct relationship helps predict how the wood will behave as it adjusts to particular conditions.

Manufacturers of lumbers, woodworkers, and flooring installers are pretty much familiar and experts on these issues. This problem can ruin both the aesthetic and practical value of a wooden product. If you are using wood for the flooring, when warping takes place after the installation, it can damage the wood and make it uneven, leading to callbacks that waste time, labor, and materials for you and flooring installers.

In carpentry, warped wood can compromise the integrity of a structure, causing wood joists not to be correctly aligned.

There are several ways on how to prevent wood warping.

The grain pattern in the wood influences its behavior and response to moisture content. Wood is cut from the heart of the tree, is generally more stable, and can withstand higher moisture. It is firm against shrinking and warping. Regardless of wood species, very straight grain with only 1/32 in (about 0.79 mm) between grain lines is less likely to warp.

It is also noted that techniques in sawing the wood or processing the lumber can also influence warping. For example, in a quarter-sawn board, growth rings are symmetrical; thus, shrinkage will occur evenly, and cup-type warping is far less likely to happen. A flat-sawn board has both edges equidistant from the tree’s core is less likely to experience warping.

Proper storage is crucial. There is adequate storage of wood to prevent warping. Here are some best practices for stacking and storage of wood:

⦁    When placed in the same layer, all boards and the stickers placed between them in a stack should be of uniform thickness.

⦁    Stickers should lie flat or be vertically aligned.

⦁    A flat foundation is best for lumber piles.

⦁    To prevent cupping, place heavyweights on a stacked lumber pile.

⦁    Proper ventilation is also needed, best accomplished by spacing the material to make surfaces exposed to the air.

⦁    Store the wood in a clean, cool, shaded, and dry location. Storing them in an area with high relative humidity will cause about 80 percent to warp. It is better to wrap them in a material that is impermeable to vapor to prevent the absorption of significant amounts of moisture from the surrounding air. High humidity is a consideration for wood and wood furniture manufacturers in South China and Southeast Asia.