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Why is my roof sheathing sagging between the trusses?
Wednesday, June 13, 2018
Structurally Inadequate or Deteriorated Over Time
Roof sheathing sags for the same reason any component of a home that spans between two bearing points sags: because it was structurally inadequate when installed or has deteriorated over time. Here’s some examples of how it can happen:
- The span between trusses exceeds rating of the sheathing - In other words, the sheathing is too thin. Each panel of plywood or OSB has a grade stamp on one face, like the example shown at below, with two big numbers near the top. The first one is the rated span in inches for roof sheathing, the second number is for flooring, and the stamp will be visible from inside the attic.
- Panel installed incorrectly - The long dimension of a panel is stronger than the short dimension, so a typical 4’x8’ panel must be installed with the 8’ edge perpendicular to the trusses. Also, the panel must cross a minimum of two spans to make it less flexible under a load.
- Missing H-clips -Small metal connectors, called H-clips, provide additional stiffness at the unsupported edges of panels. They are not always specified by the manufacturer as required, but are usually installed by the builder anyway, because they also the create the necessary expansion gap between panels and reduce panel warp at edges. Missing H-clips can cause or contribute to a sagging problem.
- Poor attic ventilation - The moist, hot air in a Florida attic must be ventilated, or the high humidity will allow wood rot to begin. Trapped attic air can be the result of not enough soffit and roof vents being installed during the home’s construction, or because bathroom vent fans dump moist air from showering into the attic, or maybe insulation was added to the attic without installing baffles to keep the soffit vents from becoming blocked.
- Roof leak - Leakage is most likely to occur around roof penetrations and at flashings at roof edges and at valleys. Even a small leak can cause wood rot and structural deterioration at the surrounding sheathing. This is the most common cause of sagging sheathing and will usually be limited to the area around the leak, while poor attic ventilation—the second most likely reason—can cause the problem over a larger swath of the roof.
Here’s another view of the same roof shown at the top of the page, and you can see that the sagging extends over a large area. In this case, the problem was due to a combination of poor attic ventilation and two bathrooms with vent fans dumping moist air into the attic. The two slant-back roof vent boxes near the ridge are a recent addition in an attempt to solve the problem but, at the time of our inspection, the bathroom vent fans still terminated in the attic.
It’s important to determine the reason for sheathing sag and fix the cause before proceeding with repairs. Minor sag may only be a cosmetic issue but the underlying defect should be addressed before it progresses to a structural problem.
And here’s an example of an occurence of this problem in a home just a few months after completion of construction.
To Learn More
You can learn about other warping or sagging roof problems at our blog posts What causes shingles to buckle along a line on the roof? and What causes a lump or dip in the roof?
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To learn more about roofs and attics, see these other blog posts:
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