Tips for installing metal roofing over asphalt shingles include checking with the local building authority for code requirements, using shingles instead of standing seam roofing, and adding a framework of 2x2 lumber to support the metal while providing ventilation. While the metal roof is more expensive than asphalt shingles, a properly installed roof has a lifespan of 50 or more years.
Prior to installation, check with local building authorities. In some locations, codes allow installation of a second layer of roofing over a first one. However, if the roof already has two layers of shingles or code prevents installing over an existing layer of roofing, it is necessary to remove the asphalt shingles before proceeding with the project.
The metal shingles install more like asphalt shingles. They eliminate the need of handling long sheets of roofing that run from the eves of the roof to the ridge. Most homeowners find them less expensive and installation to be easier.
Some cheaper, do-it-yourself metal roofing kits install directly over existing shingles. However, this approach does not allow for ventilation under the metal. Installation directly over shingles causes problems with heat-curled asphalt shingles. It also increases the chances of the formation of ice dams when temperatures are below freezing. Better kits require the frame that lifts the roofing to provide installation and allow room for the curled shingles.