Question for roofers:
When installing exposed fastener metal roofing over an existing (asphalt) shingled roof, my thought is that 1x strapping over the shingles is the best prep/substrate for the new metal.. That is, assuming a tear-off isn’t necessary, most roofers will merely felt over the existing 3-tab (or do nothing at all) and simply screw panels right onto the shingles (thru the sheathing, one would hope).
My prob: most older roofs are wiggly and wavy and the metal transfers grossly through these imperfections..which makes the new, very expensive, roofing job look hap-hazard at best. I would/will not apply metal over such a roof w/out strapping (I will string and shim strapping when necessary). This costs customers more money than the other guy. Is this expense warranted, or am I creating an unnecessary upcharge?
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I wouldn't do it over the old shingles. Why would you want to leave something under the metal that will continue to degrade with time. They will cause nails to loosen with time as well as creating the negative appearance you mention that will only become worse as time passes. I would present your proposal both ways if that is really what they want, but I would not warranty the roof if they opt not to remove the old shingles. Just because the other guys do it that way doesn't mean you should cut corners. A smart client will realize you are looking out for their best interests (as well as your own) in seeking to do the most professional job you can.
You should also check out the manufacturers website to see what they recommend. Improper installation will void any warranty they might offer.
Of course, with any installation, I would follow manufacturers guidelines..Interesting point though, on the eventual degredation of the shingles (even under the strapping). Have you any experience w/ older metal roofs over prior asphalt installations having any such problems w/ dissolving shingles..I mean other than mineral chunks clogging the gutters?
You are doing right, the others are taking shortcuts.
thermal expansion of the metal roof will scratch the backside when in direct contact with the abrasive surface of shingles, and the cold roof system will vent and perform better in extremes of heat and cold.
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I appreciate the vote of confidence. But, I'm in Florida..and it turns out that many metal manufacturers no longer allow strapping to be used under the panels as this fails the newest wind testing standards. This sort of puts me in a pickle. Even if I strip shingles first, I still would many times be left w/ the problem of wavy roofs (due to poorly framed roofs and/or old diagonal 1x sheathing) --which make a new metal installation look like doodoo. What to do then?If strapping is not permitted and a roof is un-flat, does metal roofing become non-viable?
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no. If thatr is what Florida requi4res and the customer has such a poorly framed roof, then just like with shingles, to make it get straight again, you have to peel off the sheathing and sister or shim the rafters right.but if it is only of by an eighth or a quarter here and there, the metal can still disguise that.
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Strapping is not strictly verboten, but I believe most manufacturers are now requiring against it in high wind areas. I think stripping the roof and sheathing to sister new rafters/re-sheath would be way above and beyond the budget of most I deal with. Nevertheless, as you say, it is the RIGHT way. Thanks for your insight.
In the early seventies, I learned roofing in Gainesville Fla. I can imagine the sort of roofs you are referring to. Matter of fact, I recall falling through one of them. The lady of the house was somewhat suprised when I walked OUT of her bedroom, having landed on her bed after fitting neatly between rafters and knocking some sheetrock down...LOL
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LOL...
You too huh?
When I had the crew of Los Guys do my roof, and the OSB was shot, Chileo about went through, I went up to see what they were crying about, and sure enough, put my size ten, right through the bathroom cieling...
All my secondfloor cieling sheetrock nails popped, cuz the rafters are 2x6 at 14' span....why is it everytime I try to fix something, I create more work?
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Gainesville, huh? I do some work there now and again. (I'm in Colombia county). While there are still some of the old cracker house roofs you're referring to, things have changed quite a bit since you and Tom Petty were hangin' around!
Yeah, I drove through three years ago chasing old memories. Must be 2-3 times as big a town! Could barely find some of my old haunts
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Pull off the old shingles & tar paper. Put down a heat resistant adhesive backed underlayment like Elk brand HT Shieldguard. Then put on the metal roof. If the decking is in bad shape, then redeck after taking off the old shingles and tar paper.
All the metal roof specs I've seen for houses say to apply it directly to the underlayment with no mention of strapping. All also recommend a high temp underlayment.
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Metal roofing of the kind most places use was first intended for metal buildings and attachment top purlins. They definitely have plenty of specs available for wind load uplift in a strapping situation.But I can imagine why the AHJ officials in Florida are leary...The problem is not with the attachment between metal and furring strap, or the void between strapping layout. it is probably with the attachment of the strapping to the rafters. Hard one to be sure of, and harder still to inspect!
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