What are the underlayment options for a metal roof?
High-temperature synthetic
Woven or spunbond plastic sheets rated for the heat under metal panels, which can run far hotter than under shingles in summer sun. Light, strong, tear resistant and tolerant of some exposure. It is mechanically fastened, so it does not seal around fasteners.
High-temperature self-adhered membrane
Rubberized asphalt or butyl sheets with a sticky back that bonds to the deck and seals around fasteners. It is rated for high temperatures so it does not soften, run or stick to the panel. The best protection and the highest cost, common on low slopes and as a full-coverage upgrade.
Asphalt felt
Permitted by the code in many cases, but under metal it can dry out, and in high heat the asphalt can stick to panels. Many metal manufacturers prefer synthetic or membrane products.
Slip sheets
A layer such as rosin paper is sometimes placed between an asphalt-based underlayment and the panels, especially under copper and zinc, so the panels can move freely and do not bond to the layer below.
Choosing for your roof
- Steep, simple roof: high-temperature synthetic with membrane at valleys and penetrations.
- Low slope: full high-temperature self-adhered membrane.
- Attic prone to condensation: fix ventilation first, because a vapor-tight membrane can trap moisture in the deck if the attic is wet.
- Warranty: use what the panel maker lists.
See synthetic underlayment vs felt and ice and water shield in Georgia.