Can you put a metal roof over an existing roof, including old metal?
What the code allows
The International Residential Code's reroofing rules prohibit a recover when the existing roof is water-soaked or too deteriorated to serve as a base, when the existing covering is slate, clay, cement or asbestos-cement tile, or when there are already two or more layers. The code also recognizes complete, separate roofing systems, such as some standing seam systems, that carry their loads straight to the structure without relying on the old roof. Your local building department applies the edition Georgia has adopted, so confirm there.
Metal over shingles
Panels can go directly over one flat layer with an approved underlayment or slip sheet, or over wood purlins fastened through to the rafters. Purlins create an air space and a flat plane over lumpy shingles. Read the panel maker's requirements, because many specify one method or the other.
Metal over old metal
Retrofit systems use purlins or engineered clips fastened through the old panels into the framing, sometimes with insulation between. The old metal stays as a backup layer. The key is that fasteners reach the structure, not just the old sheet metal.
The tradeoffs
- Saves tear-off labor and disposal.
- Hides the deck, so soft or rotted wood stays in place.
- Traps any moisture already in the old roof.
- Makes the next replacement a two-layer removal.
- Requires new trim at every edge and penetration, since the roof plane rises.
Compare this with the shingle version in roofing over existing shingles, and weigh metal itself in metal roofing in Georgia.