When it comes to your roof, roof decking (sometimes called roof sheathing) is one of the most important, and least talked about, components. So…what do you need to know about roof decking?

What is it?

                   -roof structure before decking-

Roof decking is a vital part of the roof structure. Your roof’s basic structure is made up of trusses (rafters/basic framework) and joists (which support the roof and distribute its weight). The trusses and joists, however, are just a frame with empty spaces in between. Roof decking is a flat surface, similar to a floor, that is attached to the trusses and joists. The decking adds an extra layer of protection between your roof and your home, and also provides a surface for the weatherproofing materials to attach to (underlayment, shingles, etc).

Is there just one type?

No. There are actually several types.

  1. Tongue & Groove
    • These are typically 2×6 boards with a “tongue” on one edge which fits into a “groove” on the edge of another board—they essentially slide together.
    • These can often be seen in homes where there is not ceiling—the tongue and groove adds an architectural element to the rafters.
  2. Plank Sheathing
    • Planks are spaced about 1 1/2” apart and shingles (typically wood) are placed on top in layers.
    • This is an old decking solution, and is most often seen on homes built prior to 1970.
  3. Step Sheathing
    • Boards run perpendicular, and there is space between the planks. The space allows air to circulate under the shingles and the spacing varies depending on the specific shingle installed.
    • This decking is used when installing cedar shingles.
  4. Plywood Sheathing
    • Sheets that are typically 4’x8’ long, made of plywood.
    • Plywood must be made from mature trees, and is rigid and moisture resistant.
    • Knots in the wood can cause for lack of uniformity.
  5. OSB Sheathing
    • The most common type of roof decking used today.
    • Sheets can be 16’ long and, in some cases, even longer.
    • OSB, or oriented strand board, is created by interweaving wood strands that are then bonded together. The boards are uniform throughout.
    • OSB can be made from small trees, and is rigid and moisture resistant.

If you are replacing your roof, do you also replace your decking?

It depends. Decking is usually not completely replaced with your roof. Your roofing experts will examine the decking to look for rot, dry rot, or “flexing” of the boards—basically does your decking bend when walked on. If any of these conditions exist, then those sections of decking will be replaced. Any decking in good condition will remain.

If you have spaced or plank sheathing, OSB (or plywood) will need to be added to completely cover the roof. Without these, many of the nails used to install the shingles will fall between the sheathing, and the first wind storm will blow your shingles off.

Can a roofer determine the state of my decking before starting the roof replacement?


If your roofer can access your attic or crawlspace, then they can view the underside of the deck and they generally have a good sense of how it is holding up. They will also be able to tell if you have step or plank sheathing. Of course, once the shingles are removed, they may find some additional panels that need replacement, but overall there should be few surprises.

If a roofer cannot access the underside of your deck, then they have to wait until the shingles are off to assess the decking—although roof leaks and the “bend” in your roof when walking on it will give them some idea. Roofers may also find that even though your currently have asphalt shingles, they are covering old wood and/or cedar shingles. This means that you have spaced or plank sheathing which will need to be completely covered with OSB for a new roof to be installed.

All reputable roofers will add information into their contract about the possible need to replace sections of decking, and what that cost is per sheet. Most often, a few sheets will need to be replaced due to weakness or rot/water damage. It is rare for a complete deck to be needed—especially without any prior indication.

Are you thinking of a new roof? Our roof evaluations and estimates are always free to residential customers. Contact us anytime, 585-400-7663 or office@400roof.com.