Do you need sheathing under a metal roof?
Short answer. It depends on the panel system. Standing seam panels on homes are almost always installed over solid sheathing with underlayment, while some exposed-fastener panels are designed to span open purlins, as on barns and sheds. On a house, solid sheathing is usually the better choice and is often required by the panel maker, a fire or wind rating, or the local building official.
What each system needs
- Standing seam. The concealed clips fasten into a solid deck, and the panels rely on it for support and a smooth surface. Plywood or OSB with high-temperature underlayment is standard.
- Exposed-fastener panels. Some are rated to span between wood purlins, strips of lumber run across the rafters. Many homeowners still put a solid deck under them for strength, quiet and a continuous underlayment.
- Stone-coated steel and metal shingles. Typically installed over a solid deck, sometimes with battens.
Why solid sheathing is usually better on a house
- It carries a continuous underlayment, a second line of defense against wind-driven rain and condensation.
- It stiffens the roof against wind uplift and keeps panels from flexing when walked on.
- It reduces rain noise and makes attic insulation simpler.
- It is often required for the panel maker's warranty and for certain fire and wind ratings.
Purlins over old shingles
Some installers fasten purlins over an existing shingle roof and lay metal on top. That traps the old roof, hides decking problems and adds weight. Ask whether the local building department and the panel maker allow it, and whether the existing deck is sound before anything covers it.
For deck materials, see OSB vs plywood roof decking; for metal basics, metal roofing in Georgia.