What Type of Plywood Is Used for Roofing?
Plywood is a favorite material for many builders because of its water resistance, structural stability, and relatively low cost. These qualities also make it a great choice for the sheathing that covers the rafters on the roof of a building. But another material, oriented strand board, is a popular choice as well. With the same resistance and stability, OSB can come at a lower cost than plywood. Both materials are often chosen by professional roofers, though OSB has a higher market share at this time, likely because it can come in at $3 less per sheet than plywood for sale. But despite the similarities, there are some notable differences between plywood and OSB that can be important to know.
Plywood Material Differences
One of the main ways that OSB and plywood diverge is the materials that make them each. Plywood, which is used for concrete wall forms, is composed of a series of thin sheets of veneer that have been cross-laminated and bonded together using a hot press. A plywood sheet will have an odd number of layers, allowing the panel to balance around the center and reducing the change of swelling, shrinkage, or warping. When you see plywood for sale, this is what you are purchasing.
On the other hand, OSB is made of thin wood strands that have been ground from logs. These strands are dried, mixed with glue and wax, and then formed into large, thick mats that are hard-pressed together to form a panel. The strands have been aligned and positioned perpendicular to each other in alternating layers, allowing the OSB to support higher amounts of weight than plywood usually can.
Plywood’s Varying Thickness
Using density as a measure, OSB usually comes out the winner. Plywood is on average 5 to 7 plies thick, while OSB can have as many as 50 strand layers compressed together. The thickness may depend on how spaced apart the rafters are, but plywood usually comes in at ½ to 5/8 inch thick when sheathed on a roof. In contrast, OSB is usually 7/16 inches thick, making it 2 times stronger in shear value than plywood. Additionally, because the two-knot holes overlap, OSB doesn’t have the potential for soft spots like plywood. However, OSB is also significantly heavier than plywood, making it difficult to work with.
Performance Against Water
All wood expands when it is wet, but OSB and plywood behave differently in this regard. OSB expands at a faster rate around the edges of the panel, and these edges will remain swollen after they’ve dried out. In the case of plywood, the swelling is consistent across the entire panel and will shrink back down when it dries. Plywood also dries more quickly than OSB without retaining moisture, meaning it is less likely to degrade than OSB.
One of the things to consider when looking to purchase used plywood is its stiffness. Plywood is manufactured in such a way that it is consistently less stiff than OSB, by up to about 7% on average.
Because OSB absorbs moisture slowly and dries out slowly, it is not ideal in areas with a lot of rain or snow. Rain, snow, and ice can cause moisture to lock into the OSB panel and cause it to rot. Without consistent roof maintenance, a humid or wet climate will make OSB a difficult choice. As shingles shift, OSB can become exposed to the moisture in the environment and deteriorate quickly. On the other hand, plywood is better able to resist moisture and dries faster, which is also helpful when considering construction time.
If your concern is sustainability, OSB is generally considered to be the better option as it can be made out of much smaller-diameter trees that grow more quickly and can be easily farmed. The larger-diameter trees used for plywood have to be grown for longer periods and harvested from old-growth forests. However, OSB is still created using formaldehyde, while plywood has phased out this chemical almost completely and is available with soy-based glues and other materials. Depending on your goals for sustainability, both OSB and plywood have pluses.
As mentioned, OSB often goes for a significantly lower price than plywood. However, if you want the benefits of plywood without the additional cost, a good option can be to look for used plywood for sale. Plywood that has been removed from concrete forms is ready to be used for roofing and a number of other projects at a reasonable rate.