Can I install a new metal roof over an asphalt shingle roof?
Thursday, September 12, 2019
Yes, you can. It is specifically allowed in the 2017 Residential Edition of the Florida Building Code (FBC) at R908.3, Exception 1, with a similar citation in the International Residential Code (IRC):
A separator sheet between the shingles and the back of metal roofing is often used, but not required. Asphalt or synthetic felt, or even coated kraft paper, can be used. Some roofers prefer to lay down wood purlin strips to screw the metal panels to, thereby creating an air gap between the two roofs, but this technique is more likely to lead to “oil canning,” slight waves of the flat panels that create irregular surface reflections and shading, especially on hot days. It is only a cosmetic defect, but is sometimes quite pronounced. See our blog post What are roofing purlins and battens?
Installing the metal directly over the shingles saves you the tear-off and disposal expense for the existing roof. Also, in Florida, it means you do not have to re-nail the roof sheathing to meet current hurricane-resistance code standards.
See our blog posts What is the difference between galvanized and galvalume metal roofing? and Can metal roofing be used on a low slope/pitch roof? for more on metal roofs.
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