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Old 05-18-2010, 04:52 PM   #1
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metal roof over asphalt shingle roof


I will soon be installing a new metal roof (painted aluminum). Is it acceptable to install this new roof over an existing asphalt shingle roof? I was planning to secure the 1x4 nailers into the trusses with longer screws than the intermediate screws. Am I headed in the right direction or is this totally unacceptable? The current roof is only one layer of shingles so I don't see weight being a problem.
Thank you for any suggestions/guidance.

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Old 05-18-2010, 05:06 PM   #2
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Acceptable? I have seen it done.

Would I do it? No.

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Old 05-18-2010, 05:57 PM   #3
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Acceptable? I have seen it done.

Would I do it? No.
I'll second that.
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Old 05-18-2010, 06:46 PM   #4
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Could you please tell me why you wouldn't recommend this install? As you may be able to tell, I'm not a roofer, however have installed a metal roof on several boathouses (new construction). Thanks for your input.
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Old 05-18-2010, 07:24 PM   #5
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Hello,

Most roofers strip the shingles off so they can access the substrate. If they find rot, they replace the wood. Removing shingles also gives you a more flat surface to install the panels.


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Old 05-18-2010, 08:14 PM   #6
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Ozom, there is no need to tear it off as long as it's not a mfr'd home. Your thinking is sound. Just make sure you use a screw with good shear strength when fastening the battens since that will be your weak point in the new roofing system.

Been doing this a few days, it'll be fine.
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Old 05-18-2010, 08:21 PM   #7
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Thanks roofboy for the input. Would it be feasible to inspect the substrate from the attic before deciding to do a tear off. There is plenty of overhead room to be able to do that.......I'm just trying to cut down on labor and the cost of having a dumpster dropped if at all possible. However, I want a first class job that I won't have to worry about ten years down the road. Thanks again.

Mike

Thanks OldnBroken......thinking I'm probably gonna take that route if I don't find any rot when I inspect from the attic. Seems we're outnumbered so far! Thanks again everyone. I really appreciate this forum.

Mike
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Old 05-18-2010, 08:34 PM   #8
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Seems we're outnumbered so far! Thanks again everyone. I really appreciate this forum.

Mike
Yes but I take quality over quantity!

Seriously though, it is a valid engineering spec. Nothing out of the ordinary or shoddy about it.
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Old 05-18-2010, 08:36 PM   #9
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Roger that OldnBroken.......thanks again.
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Old 05-18-2010, 09:07 PM   #10
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I'm just trying to cut down on labor and the cost of having a dumpster dropped if at all possible. However, I want a first class job that I won't have to worry about ten years down the road.
We probably find more rot with the tear off tool than we do with our eyes. Stains often look like rot from inside the attic and I doubt you can see the sheathing very well where we find most rotten sheathing - out toward the roof edge. We've been finding quite a bit of de-laminating plywood in the last few years. You won't spot that from the attic.

Trueness makes for a better looking roof installation and furring over old roofing is less likely to provide straight lines unless time is spent checking and shimming out low spots.

I figure there's not a great deal of difference between stripping a layer of shingles and nailing down furring strips. The cost of disposal is the big difference and I can repair the sheathing as noted and start with a known surface.

Not saying you can't do a good job not stripping, but your odds go up if you do.
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Old 05-18-2010, 09:13 PM   #11
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I see your point "seeyou"........seems I need to give this more thought than I presumed I would have to......hmmmm
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Old 05-18-2010, 09:49 PM   #12
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Ozom, there is no need to tear it off as long as it's not a mfr'd home. Your thinking is sound. Just make sure you use a screw with good shear strength when fastening the battens since that will be your weak point in the new roofing system.

Been doing this a few days, it'll be fine.
what would the difference be if it were a manufactured home?
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Old 05-18-2010, 11:11 PM   #13
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what would the difference be if it were a manufactured home?
Only allowed one layer, regardless of product.
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Old 05-19-2010, 11:50 AM   #14
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Only allowed one layer, regardless of product.
aha,


thanks.
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Old 06-11-2012, 07:46 PM   #15
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A nailing strip perpendicular with rafters and the metal roofing ribs , is mandatory for the screw pattern . When the first purlin ( nailer) goes on the drip edge at the eve, it will tell you if your edge is rotten or not . Will it hold a screw?. Nailers to be screwed tight and EVERY screw through the steel has to hit the nailers . the deck ply is irrelevant. The old layer of shingles is a good sound barrier and insulater. A dip in the roof does not go away with shingle removal..Shim under nailer. and use an appropraite length screw. takes care of that. If the steel does not contact the aggregate anywhere then you dont need a barrier ( bubble wrap ).

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