Long Aluminum Shingle Run Buckles After a Hot Day
A crew is installing 24-gauge aluminum shingles finished in dark bronze on a 12/12 gable roof 60 ft long at the eave and 32 ft from ridge to eave. The deck is 5/8-in. plywood covered with 30-lb felt. Panels have factory slots at the top of each pan; the manufacturer’s published instructions require one fastener per pan, centered in the slot, with the head snug but not compressing the shingle. The crew installs the lower 20 courses on a cool morning, then finishes the ridge at 3 p.m. when the panel surface is 150°F. The next morning the owner reports a wave in the field and a 3/8-in. gap opening at the rake trim; several fasteners are visibly overdriven, and the ridge cap has lifted at one end. No sealant has been applied. The contractor wants to add larger washers and seal the ridge cap rather than remove panels.
1. Calculate the approximate thermal movement of a 60-ft aluminum shingle run for a 130°F surface-temperature swing, and explain how that movement should be accommodated in fastening and edge details. State any assumptions.
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Assume a coefficient of linear expansion for aluminum of about 1.3 × 10^-5 per °F and an unrestrained length of 60 ft = 720 in. For a surface swing from about 20°F to 150°F, ΔT ≈ 130°F:
- ΔL = 720 × 1.3×10^-5 × 130 ≈ 1.2 in.
If the panel field is restrained near its center, each end must be free to move roughly 0.6 in. The slots, rake trim, ridge cap, and any end laps must therefore allow that travel. Fasteners belong centered in the slots, driven perpendicular to the deck, with the head touching but not flattening the slot; overdriving clamps the panel and converts thermal movement into buckling or slot tear-out. Do not fasten both ends of a long run or lock panels with sealant. Verify the manufacturer’s maximum panel-run length; if the run exceeds it, use the specified expansion joint or break. A 3/8-in. opening at the rake after a hot day is consistent with restrained expansion, not with a need for larger washers.
2. The contractor proposes larger washers and sealant at the ridge. Evaluate this repair and describe a proper correction.
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The proposed repair worsens the problem. Larger washers increase clamping force and bearing area over the slot, which further restricts movement; sealant at the ridge bonds the cap to the field and creates a hard restraint. A proper correction is to remove the ridge cap and the deformed panels, back out or replace overdriven fasteners, and reinstall panels with the specified fastener centered in each slot and snug-not compressed. Any slot that is elongated, torn, or deformed should be treated as damaged; replace the panel or use only a manufacturer-approved repair. Check that the rake trim and ridge detail are designed to slip and that eave closure does not pin the lower edge. Confirm fastener length is correct for the deck thickness; too long a fastener can bottom out or deform the panel, while too short a fastener may not engage. After correction, inspect at the hottest part of the day and again when cool to see whether the wave and gap change. Do not rely on sealant to hide a movement problem.
3. What inspection steps and documentation would you require before accepting the corrected roof?
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Before acceptance, inspect every course from the eave up: fastener centered in slot, head snug, no dished or torn slot, no missing fasteners, and no fasteners placed outside the slot. Check the ridge and rake for uniform gaps and free movement; confirm closures and trim are not caulked to the panel field. Verify underlayment is not torn and that all penetrations remain flashed. Photograph each elevation, record deck and surface temperatures, and note the panel run length and fastener type. Ask the manufacturer’s representative to confirm the fastening schedule and any run-length limit in writing. A short water test at the ridge and rake is useful, but it does not replace the movement check. If the field still waves after fasteners are corrected, the cause may be panel distortion from the original installation, and affected panels should be replaced rather than accepted.