Causes of Telegraphing through Roof Shingles: Part 2

A recent inspection on a family residence revealed that sheathing panel edges were telegraphing through the roofing shingles. Since the roof was properly ventilated and there was no moisture present, we conducted an inspection to see what was causing the problem.

Incorrect panel spacing

Incorrect panel spacing

Buckling and ridging of the roofing panels was caused by incorrect panel spacing. Both the manufacturer of the sheathing panels and the APA recommend a 1/8” space between panels to allow for swelling as panels adjust to the moisture conditions onsite. The 1/8” spacing acts as a mini expansion joint.

From the APA Technical Note D481: “Buckling of wood structural panel sheathing such as plywood or oriented strand board (OSB) occasionally results when increased moisture conditions cause the wood to expand. Such buckling may occur between supports or between nails along supports. Although structural properties are unaffected, the waviness affects the building’s appearance and may lead to complaints.

Wood will expand when it gets wet. Even increases in the moisture conditions around the panel can cause expansion. If at all possible, allow the panels to acclimatize to the conditions where they are being installed. Always use the proper fastener size and type and never forget to provide the 1/8” spacing on the edges. Panels are cut to a size that allows for that 1/8” spacing.

The tendency of expansion to cause buckling in structural panels is related to mechanical properties, physical properties and natural characteristics of wood. It is also a function of not spacing panel ends and edges to allow for expansion. A thin panel has a greater tendency to buckle upon expansion than a thicker panel. Include the natural growth variability of wood, moisture absorption rate of the wood, and always refer to the manufacturer’s instructions and leave a 1/8” gap between panels.

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