What roof underlayment goes under a metal roof?
Why heat rating matters
A metal panel in the Georgia sun transfers heat directly to whatever sits under it. Standard self-adhered membranes and felt contain asphalt that can soften, ooze or bond to the back of the panels. That can stain, trap the panels so they cannot expand freely, and make future repairs or replacement much harder. High-temperature products are formulated to stay stable at those temperatures.
Common setups
- Synthetic over the field: a heat-rated synthetic sheet across the open deck, fastened with cap fasteners.
- High-temperature peel and stick at critical areas: valleys, around chimneys and skylights, at low-slope sections, and at eaves in the mountains.
- Full high-temperature membrane: some owners and some standing seam systems call for full coverage for a sealed deck.
- Slip sheet: some manufacturers want a separator over self-adhered membranes so panels can slide as they expand.
Moisture and ventilation
Fully adhered membranes block vapor, so moisture rising from the house cannot escape through the deck. That is fine with a well-ventilated attic but risky over poorly vented attics or vaulted ceilings in humid Georgia weather. Metal also cools quickly at night and can form condensation underneath, which the underlayment has to handle.
Follow the panel maker
The metal manufacturer's installation guide lists approved underlayments, and using something else can affect its warranty. Code also requires underlayment under most metal roof coverings.
See metal roofing in Georgia, synthetic underlayment vs felt and ice and water shield in Georgia.