Substrate and Deck Preparation, Underlayment, and Thermal and Vapor Control
This domain tests deck readiness, underlayment selection for hot metal surfaces, and the placement of vapor and thermal control layers. Questions often ask which underlayment or retarder fits a given climate and slope.
Deck
Minimum 15/32 in. APA-rated sheathing for architectural panels; OSB is acceptable with supported edges. Steel deck typically 22 ga minimum, fastened per schedule. Deflection limit L/180 unless the manufacturer is stricter. The deck must be dry, swept, and free of protruding fasteners; set nail heads flush.
Underlayment
- High-temperature synthetic or 30 lb felt; metal surface temperatures reach 150–180°F, so standard #15 felt is inadequate.
- One layer above 3:12; two layers or a self-adhering membrane below 3:12.
- Ice-dam membrane at eaves in cold climates, extending at least 24 in. inside the wall line.
- Slip sheet where the manufacturer requires it; keep asphalt products away from zinc and copper.
Thermal and vapor
Vapor retarder classes: I ≤0.1 perm; II 0.1–1.0; III 1–10. In cold climates place it on the warm side; in hot-humid climates avoid interior retarders and rely on air sealing. Continuous insulation above the deck needs thermal blocks at clip lines.
Worked example
Climate zone 5, vented attic, steel panel over plywood: specify a Class II retarder on the warm side, high-temperature underlayment over the deck, and 24 in. of ice-dam membrane at the eave. Do not add a second interior retarder beneath the panel.
Traps
Using #15 felt under a dark panel; laying underlayment on a wet deck; and treating a vapor retarder as if it were an air barrier.