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Q. Does OSB sag more than plywood when installed horizontally over 24-inch-center rafters?

A. It depends on the materials used to make the OSB, which can be manufactured from a variety of species. These include aspen, southern pine, sweet-gum, yellow poplar, and birch. The Modulus of Elasticity (MOE) of the wood used will determine the relative flexibility of an OSB panel. The list at right shows the MOE values for some of the woods used to make OSB and plywood (from NDS Supplement)

On the same roof with rafters spaced at 24-inch centers, a plywood panel made from high-grade Douglas Fir-Larch veneers is going to deflect less than an OSB panel made from Aspen. Run the same test using an OSB panel made from similar materials and you will most likely find no difference in their deflections.

Probably the reason that OSB has the reputation for flexing more than plywood is that much of the OSB sold is manufactured from lower grade fibers. This is why OSB typically costs a lot less than good plywood.

—S.M.

Wood Species MOE
Aspen 800,000 to 1,100,000
Yellow Poplar 1,100,000 to 1,500,000
Beech-Birch-Hickory 1,200,000 to 1,700,000
Douglas Fir-Larch 1,300,000 to 1,900,000
Southern Pine 1,200,000 to 1,900,000