At one time, a roof deck was 1x6 or wider dimensional lumber or plywood with dimensional lumber roof joists. Today, oriented strand board (OSB) is king: Virtually all new roof decks are made of OSB on a structural roof truss. OSB is a proven product if installed correctly. Nailing, spacing, clips and spans are all-important.
How wide is wide enough?
During a visual inspection, we really can’t see fasteners, edge joints and thickness, but we can see the width of the OSB spanning between rafters or roof joists (Photo 1).
Photo 1. Attic thin strips of OSB.
Can you tell whether this narrow strip is properly installed near the ridge of this roof? The OSB has not collapsed or shown obvious signs of failure.
Clips between sheets are installed on only one side of the ridge. What is the required minimum width when we span 24 inches between roof trusses? Installing thin strips of OSB between roof trusses 24 inches on center just doesn’t seem right.
Failure of thin OSB strips
Here’s a more obvious problem (Photo 2). In this case, the thin strips are about the same width—4 inches. The strips, which span 24 inches, are bulging and twisting. I don’t see any clips between the sheets of OSB.
Photo 2. Attic failed OSB at ridge.
OK at the ridge vent?
In another case, the OSB strips are a little wider—maybe 6 inches (Photo 3). A 2½-inch-wide gap was cut for the ridge vent. The clips are also visible. Is this OK?
Photo 3. Attic thin strips of OSB at ridge vent.
Let’s check OSB construction standards
I spent some time checking design standards for OSB roof decks and I quizzed several wood engineering scientists. I learned that the general standard for OSB roof decking requires a minimum width of 24 inches. This assumes that a roof truss system has the typical 24-inch spacing between trusses and a typical OSB thickness. If the panel width is less than 24 inches, panel clips or lumber blocking is suggested.
If the width is less than 12 inches, special blocking and support for both edges is required. When installed at a ridge vent, one edge must be supported by blocking.
It appears that design requirements are often ignored at the roof ridge. During an inspection, we should always be concerned with OSB roof panels that are not at least 12 inches wide and are not supported by clips. If the edge is not supported at a ridge vent, that is additional cause for concern.
Any visible failure of the OSB decking should be reported as a major concern to be evaluated by a specialist. All narrow panels should also be reported for further evaluation.
To learn more, attend Tom’s technical presentations at educational sessions for ASHI chapters. Tom can also provide his knowledge for you educational event; contact him at Tom@HTOYH.com.
Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through HowToOperateYourHome.com, he provides high-quality marketing materials that help professional home inspectors educate their customers. Copyright © 2019 by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.