Should a metal roof be installed over shingles?
Short answer. It can be done when the shingles are a single, flat, dry layer over a sound deck, using a slip sheet or purlins between the shingles and the metal. In many cases it is still better to tear off, because curled shingles telegraph through thin panels, the deck goes uninspected, and moisture or rot underneath stays hidden.
When it can work
- Only one layer of shingles, lying flat.
- No leaks, soft spots or attic stains.
- The metal panel maker approves installation over shingles and specifies the method.
- Your local building department allows it for your roof.
Two installation methods
- Direct over shingles with a slip sheet. A layer of approved underlayment separates the panels from the shingle granules, which can abrade metal finishes as panels expand and contract. Lumpy, curled shingles show through as waves in the panels.
- Over wood purlins. Strips fastened through the shingles into the rafters create a flat plane and an air gap. This handles uneven shingles better but raises the roofline, so every trim piece and flashing has to be planned for the new height.
The risks that make many roofers recommend tear-off
- Hidden decking. Soft or rotted wood stays in place under a roof meant to last decades.
- Trapped moisture. Any water already in the old shingles has nowhere to go.
- Fastener holding. Screws must reach solid wood through the shingle layer; long screws into tired decking hold poorly.
- Buried flashing. Old step flashing at walls and chimneys gets covered rather than replaced.
- Ventilation. The attic still needs intake and exhaust, and an overlay can block existing vents.
The general overlay rules are in roofing over existing shingles, and the long-term case for metal here is in metal roofing in Georgia.