Do I need underlayment for a metal roof?
Short answer. Yes, in almost every case. Building codes and metal panel manufacturers call for underlayment under metal roofing installed on a solid deck, because panels can let wind-driven rain past laps and fasteners and because condensation forms on the underside of metal. A synthetic underlayment rated for high temperatures is the usual choice, with self-adhered membrane in valleys and low-slope areas.
Why metal needs it
- Condensation: metal cools quickly on clear nights, and moisture can form on its underside. Underlayment keeps that water off the wood deck.
- Backup protection: wind-driven rain can get past side laps, end laps, fasteners and trim. Underlayment carries it down and out.
- Code and warranty: the residential code requires underlayment beneath metal roofing on a deck, and panel makers specify what kind.
Which types work
- High-temperature synthetic: the common choice. Metal surfaces get very hot in a Georgia summer, so the product should carry a temperature rating suited to metal.
- High-temperature self-adhered membrane: for valleys, around penetrations, along eaves in the mountains, and across low-slope roofs.
- Felt: some manufacturers discourage it under metal because heat can make it stick to panels or break down; follow the panel maker's list.
Barns and open framing
Metal fastened straight to purlins with no deck, as on many barns and carports, has no underlayment. Condensation there is handled with a factory anti-condensation backing on the panels or a vapor retarder, which matters if anything stored underneath must stay dry.
Over old shingles
When metal goes over an existing shingle roof, a new underlayment layer acts as a separator and backup barrier. For the bigger picture, see metal roofing in Georgia and synthetic underlayment vs felt.