Can you put a metal roof over shingles, and how is it done?

When the code says no

Under the International Residential Code, a new roof covering cannot go over an existing roof that is water soaked or deteriorated, or that already has two or more layers. Standing seam systems designed to carry loads directly to the structure are treated differently. Georgia jurisdictions enforce the IRC with state amendments, so confirm with your building department before you plan around a recover.

Method one: directly over the shingles

A synthetic underlayment rated for metal goes over the shingles as a separator so the panels do not rub on granules or bond to softened asphalt. The surface has to be flat, because thin panels show every lump. Nail pops and curled shingles must be flattened first.

Method two: purlins

Rows of 1x4 or 2x4 purlins are fastened through the shingles into the rafters or trusses, and the metal attaches to the purlins. This creates an air gap, evens out bumps, and gives screws solid wood to bite. Eaves and rakes need trim deep enough to close off the gap from birds and insects.

The tradeoffs

• Savings: no tear-off labor or disposal.
• Hidden problems: soft or rotted decking stays buried, and nobody can inspect it.
• Weight: an extra layer on the framing.
• Ventilation and edges: existing vents and trim details must be reworked to fit the new layer.

Check the metal manufacturer's warranty terms, which may set conditions for recovers, and your HOA rules. See roofing over existing shingles and metal roofing in Georgia.