Plywood is a building material used as a cheap and efficient alternative to regular lumber. It often is used as part of the structural framing system of a home, and it may act as a reinforcement and bracing component for wall and roof framing. A number of different types of plywood and similar products are used to build roofs, and each offers its own distinct features and benefits to builders and homeowners.
Traditional plywood is the most expensive sheathing material for roof construction. Oriented strand board (OSB) has replaced plywood use in the United States for the majority of roof applications. It is more affordable and much more uniform, though it must be protected from water and moisture. Fiberboard is even more affordable, but is not as strong or durable and plywood and OSB. Fire-rated plywood is still used in areas where fire is a major concern.
Plywood is made from thin strips of wood veneer bonded together with resin. The resin is blended with a fire-retardant material in situations where fire-resistant plywood is required. OSB is made from finely shredded strips of wood that are compressed to form a solid, dense board. The strips are layed in one direction to create a uniform and consistent grain pattern. Fiberboard is made from wood fibres that are pressed together in random patterns, forming a cross between OSB and plywood.
When choosing between types of plywood and wood sheathing products, there are a number of factors to consider. OSB is used on the majority of homes because of its low cost and uniform surface. It must be protected from moisture however, and homeowners should consider paying extra for plywood to avoid future roofing problems. While most building codes consider OSB, plywood and fiberboard to be identical for sheathing purposes, some codes may require use of one of these materials over others. It is also important to consider the weight of roofing materials and the wind levels. Plywood will work better with heavy roofs, while fiberboard will not hold up against heavy materials or high winds.
Roof sheathing products come in a variety of different thicknesses, which are chosen based on the weight of the roof and the distance between joists. For most lightweight roofing materials, use 5/16-inch plywood for spans up to 20 inches, and use 1/2-inch sheets for spans up to 32 inches. Spans up to 48 inches require 3/4-inch plywood. All heavier roofing materials like clay, concrete or slate require plywood at least 1/2-inch think, with joists be set no further than 16 inches. Fiberboard must be nearly twice as thick as OSB or plywood to cover the same span.
No matter what type of roof sheathing is chosen, fasten it to the roof joists using galvanised or stainless steel nails. The plywood should be layed perpendicular to the joists, and butt the seams tightly together. Cover the entire surface with building paper or roof felt to act as a moisture barrier. It is particularly critical that OSB is properly waterproofed because of its tendency to fail when wet.