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Roof Plywood Installation and Rough Lumber Expansion

Plywood Bumps

You might have seen a freestanding unused plywood which has humps, scratches, bumps and swelling. These kinds of damages are the results of exposure to moisture or water vapor.

Now, imagine these damages on pieces of your roof plywood. You would surely get disgusted and extremely troubled. You would question your roofer or builder why those unsightly bulges appeared. In the recesses of your mind, you hope that the bulges will flatten and return to their original condition and shape. But they would not.

Plywood and Rough Lumber Expansion

Plywood and other kinds of wood or rough lumber easily absorb water because of their fibrous and cellular structure. They absorb water, either in the form of water vapor or moisture. When the cells within plywood absorb water, they expand. This expansion is largest along the grain of the plywood. A plywood sized 4 x 8 would expand across the four-foot side and the 8-foot side. Plywood expands both across its length and across its width. When these grains are exposed to considerable moisture, the plywood would expand rapidly, resulting to bulges and bumps.

1/8-Inch Spacing Between Plywood Panels

Skilled builders and carpenters know all about wood expansion, contraction and wood swelling. They know that when plywood pieces are laid out side by side without any space, the edges of the plywood would pucker up when they expand because they have nowhere to go when they expand. In very bad cases, the plywood pieces even damage the rough lumber and roof framing components.

The standard practice in relation to roof plywood installation, as advised by plywood manufacturers, is to allow a one-eighth-inch gap around the edges of the plywood panel. This gap will reduce or prevent puckering caused by bulging.

Plywood Nailing Tips

You should use the recommended size of nails to attach your plywood pieces. The more commonly used size and type is the 16d sinker nail. You would observe that the nail is about 1/8 of an inch in thickness. Check also the location and spacing of the nails.