Which Thickness of Plywood is for a Roof?

By Mark Morris

While framing is what holds the walls up, the structural integrity of your house lies primarily in the foundation and the roof. The frame of your roof serves as the skeleton, while the decking protects your home from the elements. Selecting the right decking is essential to the longevity of your roof. The thickness required is dependent on the type of decking and the spacing of your rafters, or trusses.

Plywood Decking

Exterior grade plywood decking can be used to deck a roof, directly over rafters. It is typically nailed down. The thickness is selected by the spacing of your roof rafters. For 16 inch on center rafters, thinner 15/32- or 1/2-inch plywood can be used. While this is sufficient, the thicker the decking, the stronger your roof will be. For 24 inch on center rafters, the decking must be thicker at a minimum of 5/8 inch thick. All decking plywood should be exposure 1-rated plywood and rated for structural use.

Oriented Strand Board

Oriented strand board, or OSB, is a type of chipboard, typically categorized as plywood. It is becoming more common as roof decking and house sheathing, due to the lesser cost. OSB thickness is regulated in the same way. For 16 inch on center rafters, the minimum thickness is 15/32, or 1/2 inch. It is not recommended for use on wider spacing. Panels used as roof decking should be rated similar to plywood panels.

Wood Plank Decking

Many older homes have plank decking. This method of roofing is more expensive due to both material costs and installation time. Plank decking is typically 3/4 inch, which is the minimum allowed thickness. Planks are recommended to be less than 8 inches in width to minimize warping that is common in wider planks. Homes that have wood shingles, or shakes, may be decked with spaced plank decking. This is made up of 1-by-4-inch lumber planks, spaced at about two-thirds of the width of the planks for ventilation.

Replacing Decking

If you have experienced roof damage, decking may need to be replaced. Although a much smaller area may be directly affected, it should only be replaced in full 4-by-8-foot sheets, or whatever portion of a full sheet makes up the affected section of roof. It is best to use the same material and thickness as the original, so that your roof will react evenly to changes in humidity and temperature. If you find that less than minimal decking was installed, you may be required to replace the deck to bring the roof up to code.

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