The answer is: “Yes it can.” And by doing so, you save in the labor and hassle of tearing off the old roof and you won’t be contributing to filling our landfills. Another added benefit is that the layers of your old roof will further insulate your home. Metal is the best choice due to its very low weight per square foot and its unique design. Metal roofing weighs less than one pound per square foot. Asphalt shingles can weigh up to four pounds per square foot depending on the brand.
If you don’t want to take our word for it check with your local building department. In some areas building officials only allow a re-roof over one layer of shingles. Other areas will allow you to re-roof over two layers. In most cases it’s not even an issue if you are only going over one layer of shingles.
Referencing Section R907.3 of the 2006 International Residential Code Book® best answers this question, it reads as follows:
“Complete and separate roofing systems, such as standing seam metal roof systems, that are designed to transmit the roof loads directly the buildings structural systems and that do not rely on existing roofs and roof coverings for support, shall not require the removal of existing roof coverings.”
When you decide to go ahead with this project there are a couple of precautions regarding the underlying material:
If there are any existing roof leaks, you need to address them and replace any rotten sheeting or rafters. If there are any signs of problems caused from the previous roofing weighing too much, they must also be addressed. This could include things like sagging decking or buckling trusses.
A couple of options:
A layer of underlayment installed on top of the existing shingles will help keep the granules of the old shingles from rubbing against the back of the metal and causing possible corrosion. The industry standard for underlayment is often thought of as felt paper (tar paper) but there are also many newer synthetic underlayments that are designed to work well with metal roofing. Standard tar paper has a tendency to stick to the back of metal roofing. This can cause the paper to tear when the metal roofing expands or contracts as it is designed to do. The new synthetic underlayments do not have this problem as their design far exceeds the felt paper standards.
“E-Foil” or foil faced, bubble wrap type, reflective barrier insulation can also be used as a cushion between the existing roof and the new metal roofing. The reflective barrier insulation can also reduce your cooling cost during the summer months by reflecting heat away from your homes roof. Radiant barrier insulation will perform the best when used with a corrugated panel that has a built in airspace due to the corrugation. The airspace will also allow natural venting to take place.
I hope this article has been helpful. But here’s one more thing for you to consider: Do you realize that if you select metal roofing that this will likely be the last decision you make about the roof of your home?