- 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
|