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What You Need to Know about Roof Sheathing

Obviously, the surface material of a roof is very important. Whether you choose to install metal, asphalt, tile, fiber cement, wood shakes, etc., the top of your roof is what gives it its aesthetic value and durability characteristics. No matter the material, however, a roof wouldn't be possible without the sheathing below.

 

Roof sheathing provides an area where the surface material can be attached, but it also helps keep the trusses and/or rafters properly spaced. Sheathing is installed over multiple trusses/rafters, so it essentially turns them into one cohesive unit instead of individual structural members. Sheathing works with other components of the roof as well. Ventilation, for example, is needed so that the sheathing doesn't warp or swell under humid conditions. Felt wrap is installed over the sheathing to create a barrier between it and the bottom of the roofing material above.

 

The question that arises is, “does sheathing material matter?” At first glance it wouldn't seem so. As long as you are using a durable, substantial 4 x 8-foot sheet of building material, the trusses should have something to keep them together and the roofing material something to fasten to. In actuality however, the perfect sheathing materials are lightweight, dimensionally stable, strong, and affordable. Here's how some popular sheathing options stack up in those categories.

 

 

Oriented-Strand Board (OSB)

The “big two” when it comes to roof sheathing is oriented-strand board (OSB) and plywood. They are the most readily available sheathing and can accommodate virtually any type of roofing material.

 

Made from strands of wood that are cut and stripped from logs and glued together in cross layers using a strong adhesive under high pressure to form a strong structural board, OSB has cornered the lion's share of the market when it comes to walls and flooring. But many contractors still have mixed feelings when it comes to using the material on a roof. The reason to use OSB would be cost – about $3 less per sheet than plywood – but the reason to avoid it would be that OSB is more susceptible to damage from moisture than plywood.

 

 

Plywood

Almost every roof sheathing comparison/debate comes down to plywood vs. OSB. Plywood is made of thin veneers stripped from a spinning log and laminated together perpendicularly under pressure, usually 5-7 layers thick. You can tell the difference between plywood and OSB with one glance – externally anyway. Plywood presents a smooth, continuous surface whereas OSB’s surface looks like a pieced-together puzzle of wood fragments.

 

A panel of OSB is typically heavier than plywood, yet not as stiff, so it could be prone to some sagging between trusses/rafters, especially if they are set 2 feet on center. Coupled with OSB's weakness to moisture resistance (especially at the edges), some people just feel safer with plywood and are willing to pay extra.

 

Both OSB and plywood should be installed with the long edge perpendicular to the trusses/rafters. The panels may vary in thickness somewhat, but typically 1/2-inch-thick structural wood panels are used on roofs with trusses/rafters at 2 feet on center, though panels down to 7/16 inch may be used, particularly stronger plywood. 

 

 

Tongue & Groove Boards

Tongue-and-groove boards were used for decades prior to the widespread adoption of plywood as a sheathing material in the 1950s and 1960s. The main reason to use tongue-and-groove boards these days is appearance – from beneath the roof. Say what you want about them but OSB and plywood do not offer a very attractive “finished” look. T&G roof decking, however, looks classy and rich.

 

In fact, T&G boards are most frequently used when the sheathing is visible from the inside such as in cathedral ceilings or in some cabin-style homes, especially if the span of the rafters is greater than 2 feet. T&G boards must be 5/8 inch thick if used on 2-foot-on-center rafters. For spans greater than 2 feet – up to 6 feet – the boards must be 1-1/2 inches thick.

 

 

Fiberboard

Wood fiberboard is most commonly used in wall sheathing because it provides insulating and soundproofing properties. The material is soft and dense which means it doesn't stand on its own as a pitched roof sheathing. It can be installed over plywood or OSB sheathing, but the most common use is on flat slope roofs where there is already a hard surface underneath.

 

 

Rigid Foam

Rigid foam is very similar to wood fiberboard in application. It is dense and insulating but not sturdy enough to be the sole sheathing on a pitched roof. Foam, like fiberboard, can be installed over the top of OSB, plywood, or T&G boards in a cathedral ceiling application, but for roofing purposes it is most commonly installed on flat surfaces with built-in support underneath.

 

 

Gypsum Board

One of the main problems in using gypsum board as a dedicated roof sheathing is the fact that it's not designed to hold fasteners. When it's used as a sheathing for siding for example, fasteners need to go through the gypsum but also only into wall studs. Using gypsum as sheathing is rather rare for residential jobs, but it is still found in commercial / low-slope roofs in the same manner as fiberboard and rigid foam.

 

Most of your decisions about roof sheathing will come down to OSB vs. plywood. But it's nice to know that if you do have special needs – such as an exposed interior ceiling – there are options that can accommodate you. 

 

Footnote: The lead image of this article is courtesy International Association of Certified Home Inspectors. For more information, click here.

 

 

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