What is the best ice and water shield for a metal roof?
Short answer. The best ice and water shield under a metal roof is a high-temperature self-adhered membrane, typically rated to roughly 250 degrees F or more, with a smooth or film surface rather than granules, and approved by the metal panel maker. Standard membranes can soften and ooze in the heat that builds up under metal, and granular surfaces are meant for shingles, not for panels that need to move.
What to look for
- High temperature rating. Metal roofs run much hotter than shingles in a Georgia summer. Look for a product sold as high temperature, with a service rating around 250 degrees F or higher.
- ASTM D1970. The standard for self-adhering steep roof underlayment; reputable products state compliance.
- Surface. A smooth film or slip-resistant surface that lets panels expand and contract. Granular membranes are designed for shingles.
- Adhesive type. Both high-temperature asphalt and butyl adhesives exist; follow the panel maker's compatibility notes.
- Exposure limit. How long it can sit uncovered if panel delivery is delayed.
- Panel maker approval. Many metal manufacturers list approved underlayments for their warranties.
Where to use it
Valleys, eaves where wind-driven rain or ice is a concern, around chimneys, skylights and pipes, at sidewalls and headwalls, and on low-slope sections. Some installers cover the whole deck under standing seam, especially on lower pitches; others use a high-temperature synthetic underlayment over the rest of the field.
Avoid
- Standard-temperature membranes under metal.
- Mixing products when the panel maker's warranty names a specific underlayment system.
- Leaving the membrane exposed past its rated period.
See ice and water shield in Georgia and metal roofing in Georgia.