What is the best underlayment for a metal roof?
Short answer. The best underlayment for a metal roof is a high temperature rated synthetic underlayment over the field, with a high temperature self-adhered membrane at valleys, eaves and penetrations or over the whole roof on low slopes. Metal runs much hotter than shingles, and standard felt or ordinary peel and stick products can soften, stick to the panels or break down.
Why metal needs special underlayment
- Heat: metal panels transfer heat directly to what is underneath. Ordinary asphalt based membranes can soften and bleed.
- Movement: standing seam panels expand and contract. A membrane that bonds to the panel can restrict movement or tear.
- Condensation: moisture can form on the underside of metal on humid Georgia nights, so the underlayment is a real second line of defense.
The options
- High temperature synthetic underlayment: lightweight woven or spunbond sheets rated for metal. The usual choice for the main field of the roof.
- High temperature self-adhered membrane: sticks to the deck and seals around fasteners. Use in valleys, along eaves, around chimneys and skylights, and over the entire roof on low slopes.
- Slip sheet: some membranes and some standing seam makers call for a separate slip layer so panels can move freely.
- Felt: allowed by code in many cases but a weak choice under metal. It wrinkles, absorbs moisture and can stick to hot panels.
Code and manufacturer rules
The residential code requires underlayment under metal roof panels, and the panel manufacturer's instructions often name approved products. Following them protects the panel warranty.
Do not skip it on a budget job
Metal laid straight onto decking or old shingles with no proper underlayment is a common shortcut, and it shows up as rot and leaks later. See synthetic underlayment vs felt and metal roofing in Georgia.