What Is Roof Sheathing and How Does It Work?

A sturdy roof needs a good foundation, which is where roof sheathing comes in. Roof sheathing, also known as roof decking, consists of sheets of board material that support shingles and other roofing material. Learn more about roof sheathing, why it’s so important, and how to make sure it’s in good condition.

What Is Roof Sheathing?

A roof’s structure consists of multiple layers, from the rafters that frame and support the entire roof to the shingles that sit on top. In between these layers, you’ll find roof sheathing, also called roof decking. Roof sheathing is a solid board material that’s attached to the rafters and provides a solid surface to attach additional parts of the roof.

Roof sheathing not only helps support the weight of the roof but also acts as an additional layer of protection against leaks and water intrusion. Roof sheathing can also provide some fire protection—some sheathing materials have a fire-retardant treatment applied.

Types of Roof Sheathing

Roof sheathing comes in two materials—plywood and oriented strand board (OSB). Both are wood-based and contain adhesives or resins, but there are key differences between them. 

Plywood

Plywood is made by gluing and compressing thin sheets of wood to form a strong, rigid sheet of material used for roof sheathing. Plywood will absorb water, but unlike OSB, it dries very quickly and retains its original shape once dry. Plywood isn’t quite as strong as OSB, so the size of sheathing made from this material is often thicker, ranging from 1/2 to 3/4 of an inch thick. 

Oriented Strand Board (OSB)

OSB is a durable sheathing material made of wood fibers compressed with resin. It’s stronger than plywood and comes in thicknesses ranging from 7/16 to 5/8 of an inch. While its strength is an advantage, it doesn’t have the same water resistance as plywood. OSB dries much slower and can warp out of shape once wet.  

How to Identify Damaged Roof Sheathing

Damaged roof sheathing can lead to serious roof problems. As soon as you identify a problem, hire a roofer to fix the issue before it causes serious safety risks and costly repairs. Signs of damaged roof sheathing include:

• Roof leaks: Any water coming through the roof is a sure sign something isn’t right. Damaged sheathing can cause roof leaks, so always call a local roofer to investigate leaks as soon as you notice them.

• A sagging or rippling roofline: Sections of saggy roof or roofing that appear wavy can signal a problem with the roof sheathing. Your roof should have an even, uniform appearance, so you should check on anything unusual. If you see these kinds of problems right after a roof replacement, it can indicate that your roofers may not have built your roof correctly. 

• Mold or mildew formation: Compromised sheathing can allow moisture to build up under the roof, leading to mold or mildew growth. If you notice visible signs of mold, mildew, mustiness, or a sour smell in your attic, your sheathing may be damaged.

How Much Does Roof Sheathing Cost?

If you’re only replacing your roof sheathing, the cost will depend on the square footage that needs replacing. Plywood is more expensive than OSB, at an average of $1.40 to $5 per square foot compared to $0.65 to $2.40 per square foot for OSB. 

Roof sheathing is often replaced during a full roof replacement. The cost to replace a roof averages between $5,800 and $13,000, with larger roofs or those made with more expensive materials increasing the price.