What underlayment goes under a metal roof installed over shingles?
Short answer. When metal goes directly over existing shingles, use a synthetic underlayment or slip sheet rated for metal roofing and high temperatures as a separator, so the panels do not rub against granules or stick to softened asphalt. With a purlin system, an underlayment over the shingles is often still recommended to manage condensation, and the panel manufacturer's instructions decide what is required.
Why a separator is needed
- Abrasion: panels move as they heat and cool. Without a separator, the back of the metal grinds against shingle granules and wears through its coating.
- Heat: metal gets hot enough to soften asphalt, which can stick to the panels.
- Moisture: a separator helps manage condensation that forms under metal on humid Georgia nights.
- Backup protection: it gives the new roof its own water barrier rather than relying on old shingles.
Choosing the underlayment
Pick a synthetic underlayment the metal manufacturer approves for its panels and for high temperatures. Self-adhered membranes generally need a smooth, clean surface to bond, so they do not stick well to granular shingles; check the product before planning to use them over old roofing.
Prep before it goes down
- Confirm the roof is a single layer and the deck is sound. Codes do not allow a recover over two layers or over wet or deteriorated roofing.
- Flatten or remove curled shingles, nail pops and old patches, since thin panels show every bump.
- Remove ridge caps and hip caps if they create humps under the new panels.
- Plan new edge trim, vents and pipe flashings for the added thickness.
- Fasten the underlayment with cap fasteners in the pattern the maker specifies.
With purlins
If wood purlins go over the shingles, the panels sit on the purlins with an air gap. Many installers still lay underlayment first, and the eaves and rakes need closures so birds and insects cannot move into the gap.
See roofing over existing shingles and synthetic underlayment vs felt.