Do I need plywood under a metal roof?
Short answer. On most homes, yes. The residential code requires metal roof panels to go over a solid or closely fitted deck unless the panel is specifically designed for spaced supports, so standing seam needs plywood or OSB, while some exposed fastener panels can span open purlins, which is mainly done on barns and sheds.
Solid deck vs spaced supports
- Standing seam: clips fasten to a solid deck, and the panels rely on it for support and a flat plane. Plywood or OSB is standard.
- Exposed fastener panels: some are rated to span between purlins, often wood strips, at set spacing. Common on outbuildings, less common on homes because of condensation, noise and the lack of a continuous underlayment.
- Stone coated steel and metal shingles: usually installed over solid decking, sometimes with battens on top per the maker's system.
Why solid decking is better on a house
- It supports the required underlayment, which is your backup against leaks and condensation.
- It quiets rain noise.
- It gives clips and fasteners consistent holding power.
- It adds stiffness against wind uplift.
Plywood or OSB
Both are used. Some panel makers set a minimum deck thickness or specify plywood for clip pull out values, so check the installation guide before buying materials. Replace any soft or delaminated sheets first; a metal roof will not hide a bad deck for long.
Going over old shingles
Some installers put metal over existing shingles, sometimes on furring strips. Code limits on recovering still apply, and you lose the chance to inspect and repair the deck. Read OSB vs plywood roof decking, metal roofing in Georgia and roofing over existing shingles.