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Hi, Just discovered this forum, it's great! My 1898 N. Minnesota farmhouse needs a roof, and I'm thinking metal. Its rafters are 2x4 (the old ones that actually measure 2" x 4"), and it looks like there are three layer of asphalt shingles. The roof is pitched quite steeply--so the snow should slide right off--but I'm worried about the weight if I leave on the old shingles before installing the metal. Any insights and/or warnings? Thanks!
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First of all, if you are in an area subject to a building code, that code probably requires no more than two layers of roofing. You might be able to get an exception.
Most metal roofs weigh from 45 to 125 pounds per square. The lower weight ones are aluminum. This contrasts with 250 - 425 pounds for composition shingles.
Chances are that your last layer of old shingles has lost 50 or more pounds since it was installed due to worn off granules and evaporated oils.
If you have no existing signs of a weightload problem, going over the existing will likely be okay. However, you should confer with the manufacturer of your roofing before proceeding and make sure they are okay with this and that they can fasten through the layers of old roofing.
Also, as an option, you could hire a structural engieer to do an evaluation
8/9/2004
Or copy this URL: https://www.metalroofing.com/spirit/comment/3667/find/8/9/2004
Or copy this URL: https://www.metalroofing.com/spirit/comment/3668/find/