Gaps in the Roof Deck (Asphalt Shingles)

By Tom Feiza, Mr. Fix-It, Inc.,

 Inspecting a roof covering gets complicated because of access issues, snow, rain, and an unlimited variety of roof and shingle designs. One thing a smart  inspector should do is check the underside of the roof in the attic if it is accessible. In the attic, inspect the roof deck – the boards, plywood or oriented strand board (OSB) supporting the asphalt shingle assembly. We can’t see the roof deck from the outside, because the deck is covered (illustration R067).

Most installation instructions for new asphalt shingles use some version of this statement: “Roof deck – The deck surface must be clean, bare, gap-free and flat.” Most manufacturers assume the use of a plywood or OSB deck. But what happens with older homes?

 When you’ll encounter spaced boards

 In older homes, asphalt shingle roofing is usually installed over OSB (photo – Roof Deck Boards Plus OSB). From the attic side, you can often see the underside of the OSB placed over the spaced roof boards.


You will also find plywood placed over the spaced boards (Photo – Roof Deck Boards Plus Plywood). In the case of OSB and plywood decking, decking should be nailed to roof rafters at the edges, and there should be no gaps larger than 1/8 inch between the sheets.


What if you see black roof felt between boards?

 When black roof felt or a synthetic underlayment is visible through the spaced roof deck board, this may indicate a serious problem (photo – Roof Deck Large Gap, Roof Felt). Try to reach the space between the roof boards; push on the felt to check for a roof deck of plywood or OSB.  If there is no roof deck, you found a problem. You may also see the shingle, felt and nails pushing into the attic through the gap because there is no proper roof deck.


In the old days, some gaps were tolerated

 Twenty years ago, gaps of up to ¾ inch between boards were often ignored, and the asphalt shingle assembly was installed over the spaced boards. At that time, these smaller gaps seemed to cause no issues, but now the rules have changed. New asphalt shingles must have a roof deck that isclean, bare, gap-free and flat.”

 What should you note?

 If you see excessive gaps and a relatively new asphalt shingle, note this in your inspection report as a defect requiring further evaluation. Gaps over ¾ inch are always an issue.

Weather Tight Corp

Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through (, he provides high-quality marketing materials that help professional home inspectors educate their customers. Copyright © by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.

To learn more, attend Tom’s technical presentations at educational sessions for ASHI chapters and local groups. Tom can also provide his knowledge for your educational event; contact him at


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