What ice and water shield should go under a metal roof?
Why metal needs a different membrane
Metal panels heat up fast in the sun, and the heat goes straight into whatever is beneath. A standard rubberized asphalt membrane can soften, flow and stick to the panel undersides. High temperature membranes are formulated to stay stable at those temperatures.
Thermal movement
Long metal panels lengthen and shorten as temperatures swing between a Georgia summer afternoon and a cool night. Standing seam clips are designed to allow that movement. If the panels bond to the membrane, the movement is restricted, which stresses clips and seams. That is why some panel makers call for a slip sheet over the membrane.
How much of the roof to cover
- Valleys, eaves and penetrations at minimum.
- Full coverage is common under metal, especially on low slopes and in specifications that call for it.
- Follow the metal manufacturer's requirements, which may name approved underlayments.
Condensation
Metal cools quickly at night, and moisture can condense on its underside. A membrane protects the deck from that moisture, and good attic ventilation keeps the deck dry from below.
Questions for your metal roof bid
- Is the membrane specified as high temperature, with its rating listed?
- Is a slip sheet included if the panel maker requires one?
- Does the underlayment meet the panel maker's warranty requirements?
More on metal in this climate in metal roofing in Georgia, and on membranes in ice and water shield in Georgia.