I frequently am asked whether old shingles should be removed from a home’s roof before a new metal roof is installed. My answer often surprises people. However, it is based upon experience with tens of thousands of installations over many years.
The vast majority of installations of the metal roofing systems produced by my company are over existing shingles. The low weight of metal roofing is one thing that encourages this. Steel shingles weigh about 1/4 what asphalt shingles weigh and aluminum shingles weigh about 1/8 what standard shingles weigh. I would say that in most cases, less weight is being added to the home by the new metal roof than what the current shingles have lost due to granule wear and oil evaporation. In other words, the total roof weight was probably greater when the last layer of shingles was installed than it will be when the metal roof is completed.
One important thing for folks to keep in mind if they are trying to decide what type of roof to buy is that new asphalt shingles should not be placed on top of old asphalt shingles. With most asphalt shingle manufacturers, doing so will void the warranty on the new roof. This is partly because the old shingles will not allow the sun’s heat to pass so easily into the attic so the new shingles end up staying at a warmer temperature, shortening their life.
That is not the case with metal — temperature does not damage the metal or the paint finish. The warranty on your new metal roof, at least from my company, will not be impacted at all by going over the old shingles.
In over 30 years and tens of thousands of installations over old shingles, I have yet to go back on a job later and find myself saying “Hmmmm … maybe the old shingles should have been removed.” It just never becomes an issue. And many of those installations actually have been over wood shingles and thinner wood shakes! I have also re-roofed personal properties with our products five times now over the years — every time was over the old shingles, and no regrets.
I like going over the old shingles for three reasons:
1) It stops the need to fill up landfills with old shingles.
2) It ultimately increases the thermal resistance (R Value) of the roof assembly, actually increasing energy efficiency compared to tearing off the old shingles. The benefit of this is during the summer.
3) It allows the property owner to spend discretionary dollars on a better roof rather than on removing the old roof and disposing of it.
Going over the old shingles also avoids the potentially damaging issues that can occur, like unexpected rainstorms, when old roofs are torn off.
Still, ultimately, it is the property owner’s decision but I would have no qualms about going over a layer of old shingles in most casaes. It’s not uncommon to go over multiple layers either actually. On older homes, of course, I do put the caveat out there that if there are signs of an existing weight issue or old leaks, those need to be addressed and that may require removing the old shingles. If there are concerns about the integrity of the roof decking, then pull-out resistance tests using the roof fasteners can be performed.
One other thing to keep in mind is that building codes, where active, usually require no more than two layers of roofing of any type on a structure. There have been instances of building inspectors waiving that requirement but that is not an easy thing to get accomplished.
In a typical year, approximately 90% of the roofs our company manufactures are installed over old shingles. In my opinion, going over the old shingles is worthy of consideration because of the benefits of doing so. Every roof needs to be evaluated individually. Please feel free to email me your questions or circumstances for my input. My email is todd@asktoddmiller.com
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