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Selecting and installing a standing-seam metal roof

Are there any disadvantages to installing a metal roof over a single layer asphalt roof? The cost savings of removing the old roof and disposal make this option attractive.

Also, does the heavier 24 gauge metal roof have advantages over a 26 gauge roof? I've heard from other contractors that the heavier gauge is harder to work with and the finish cracks more easily.

Asked by Bill Tine
Posted Jul 17, 2012 9:54 AM ET
Edited Jul 21, 2012 6:43 AM ET


5 Answers

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Metal roofing expands and contracts daily with temperature changes and solar exposure. If the metal roofing is installed directly over asphalt shingles, the shingle granules will abrade the underside of the metal roofing every time the metal expands and contracts. That's not good.

If you want to install metal roofing over asphalt shingles, you should first install 1x4 or 2x4 purlins over the asphalt shingles, 24 inches on center. If you don't want to do that, then strip the shingles.

24-gauge metal is stiffer than 26-gauge metal; it will be less likely to deform if walked on and will maintain crisper lines than lighter metal.

Answered by Martin Holladay
Posted Jul 17, 2012 10:31 AM ET


I removed shingles from my brother in laws house and we laid 29 gauge metal panels ( not standing seam ) and screwed them down to 1X4's attached directly over the sheeting to the trusses with 3" torx head screws. With the smaller gauge metal we laid the 1x4's with 24" between them to cut the EPS foam in half and lay between the purlins. The 3/4" foam was for condensation on the under side of the metal. I am not sure if it is technically enough but it seems to help. I would assume you would not worry about the condensation if you do not strip the shingles. We stripped the shingles just to make everything neater and we did the work ourselves so cost was not the issue. If using thicker metal you could ask someone familiar with the type of product you are going to install and find out how far apart you can space the 1X's for the thickness of your steel.

Answered by Jeremiah Parkhurst
Posted Jul 17, 2012 11:25 AM ET


i'm curious as to why you installed the roofing over purlins rather than directly to the existing sheathing?

Answered by Malcolm Taylor
Posted Jul 17, 2012 8:25 PM ET
Edited Jul 17, 2012 8:26 PM ET.


The house was built back in the 60's and they we cheap cheap and cheap. They used 3/8" sheeting on the roof, had about 1-1/2" of fiberglass insulation in the walls and used the black jack fiberboard as the sheathing on the wall. We also wanted some room for a little insulation to help with condensation on the underside of the steel and noise levels when raining and hailing. We also ripped off all the fiberboard and put new insulation in the bays and resheeted with OSB and 3/4" XPS foam and are almost at a normal 2X6 wall insulation value. The studs are all 2X4.

Answered by Jeremiah Parkhurst
Posted Jul 20, 2012 11:24 AM ET


3/8" roof sheathing. What a treat finding that must have been!

Answered by Malcolm Taylor
Posted Jul 21, 2012 12:44 PM ET

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