Plywood – Proper Installation & Handling
Many people have seen plywood swell. It is not a pretty sight. Individual pieces of plywood buckle, twist and sometimes bubble or delaminate. These problems are, more often than not, caused by water or water vapor.
When this swelling takes place after the plywood is installed, flooring, roofing, and or siding products can begin to look less than perfect. Corrective measures can be expensive. Hoping that the plywood will shrink and return to its original shape can be futile. Rarely, if ever, will the plywood return to its original shape and size.
The key to avoiding these problems is to plan for the swelling. This is accomplished quite easily if you simply follow established guidelines offered by plywood manufacturers and associations.
Plywood and Expansion
Wood, because of its cellular and fibrous structure, can readily absorb water. It can absorb water in the liquid state as well as the vapor state. This is extremely important to know, as many residential problems are related to water in the vapor state.
When water vapor levels rise, the cells within a piece of plywood pick up the water. The cells begin to expand ever so slightly. This expansion is greatest along the face of the grain of a piece of wood. This means that on a piece of 4 x 8 plywood, you can experience expansion due to swelling across the 4 foot dimension as well as the 8 foot dimension. Don't forget, plywood has the face grain aligned in two directions! That's why they call it plywood. The face grain of each ply, or layer, is oriented 90 degrees to the ply immediately above or below. You can see this orientation and layering when you look closely at the edge of a piece of plywood.
This means that plywood expands both lengthwise and widthwise when it is subjected to excessive moisture. Knowing this, it is important that you plan for this expansion.
I have seen hundreds of houses where the roofing plywood has been installed improperly. The evidence is unmistakable.
Depending upon the time of day, the angle of the sun in the sky and the pitch of a roof surface, sunlight will expose minor imperfections in either the shingles or the plywood beneath the shingles. The humps and bumps in the roof cause the light to be reflected differently from the ceramic granules on the shingles.
If you install plywood sheets tightly against one another, the edges tend to pucker upwards when they expand. This happens because there is no other direction for them to go to relieve the swelling pressure. In severe cases, the sheets of plywood can actually create humps and dips between the roof framing members.
The Solution - 1/8" Spacing
Plywood manufacturers are aware of these problems. Virtually every one recommends that you, your carpenter or builder create a 1/8 inch gap around all edges when installing plywood. This gap will minimize or totally eliminate any puckering caused by swelling. Use a 16d sinker nail for this purpose. The shaft of the nail is slightly larger than 1/8 inch.
Nailing is Important Too!
In order to achieve the best results when installing plywood products, it is very important to use the proper size nail. The nailing pattern that is used is also very critical. Not all plywood applications require the same size nail or spacing between nails. You must check before you nail. Not only that, the spacing of nails on the edges of a panel are frequently different from the spacing within the panel.
Wind Forces and Plywood
A 4 x 8 piece of plywood when exposed to wind forces is nothing more than a large sail. I have almost been blown off roofs on windy days when handling plywood. Because of the large profile that plywood presents, it is important to nail the edges securely. Strong winds can begin to easily pull plywood from walls or roofs in the event they have not been secured properly. Many manufacturers recommend that nails be spaced no farther apart than 6 inches on the edge of a sheet.
Also, nails should be no closer than 3/8 inch from the edge of a panel and driven flush. Do not countersink these nails!