Ventilation, Thermal Movement, and Moisture Management
Metal shingles are ventilated roofs and moving roofs at the same time. This domain tests balanced attic ventilation, condensation control, thermal breaks, and movement allowances at details.
Ventilation and moisture
- Provide intake at the soffit or eave and exhaust at the ridge or high point; balance them. Code generally allows 1:150 of vent area to attic floor area, or 1:300 with a vapor retarder and balanced vents; check the local code.
- Keep insulation out of the eave path with baffles. Blocked intake is the most common ventilation failure.
- Metal is a vapor barrier. In cold climates, air leaking from the house condenses on the cold underside. Seal ceiling penetrations and use an underlayment and detail that let the assembly dry.
- Use high-temperature underlayment and a thermal break where the manufacturer requires it.
Thermal movement
- Steel moves about 0.0000065 in./in./°F, aluminum about 0.000013, copper about 0.0000094. A 40-ft aluminum run with a 100°F swing moves about 0.5 in.; steel moves about 0.26 in.
- Fix one point and let the rest move with slotted holes, clips, or laps. Do not pin both ends of a run.
- At headwalls and hips, use a cover or slip joint that allows movement without opening a gap.
Worked example
A 32-ft aluminum fascia run sees a 120°F surface swing. Movement is 0.000013 × 32 × 120 ≈ 0.50 in. The headwall detail must allow ±0.25 in. from the fixed point; a slotted clip and a cover plate do that. A hard-fastened apron would buckle or tear the sealant.
Traps
Assuming a metal roof needs no ventilation; blocking soffit intake with insulation; using a vapor barrier on both sides of the assembly; hard-fastening both ends; ignoring the difference between steel and aluminum movement.