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vapor barrier under metal roof?

Jim Baird

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IRC2000 Section 905.10 Metal roof panels

This section calls for metal roof panels to be applied to solid or spaced sheathing, except where the material is designed to be applied to spaced supports.

A 29 ga. enameled steel panel is popular in this area for residential use. Most builders use a felt or other vapor barrier when applying over spaced supports.

One hasn't, and says he can't show maunufacturer's instruction that makes the method approvable. Without instructions we are left with the code language, which doesn't mention vapor barrier.

If he can document the spaced support but not the lack of vapor barrier, can the installation be appoved?

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Hi Jim,

Now I'm kicking myself in the keester. Last week I threw away 10-1/2 years worth of Metal Roofing Magazine issues and remember seeing an article about the use of various vapor retarders under metal roofs.

Do this, shoot Dan Perkins an email. He's the guy who writes the technical articles for the Metal Roofing Magazine website and he can be reached at DanPerkinsroof@aol.com. He's in Michigan.

Alternatively, you can review the forums at the Metal Roofing Alliance site. They're just like here and are broken up into various categories for questions about metal roofs with various metal roofing guys giving input. The primary forum URL is http://www.metalroofing.com/v2/forums/i ... Categories and the Underlayment category URL is http://www.metalroofing.com/v2/forums/i ... egoryID=38.

If you'd like to call the Metal Roofing Alliance and talk to someone directly, they will be open in a couple of hours and their number is: (360) 275-6164

Here's the URL for MRA's site: http://www.metalroofing.com/v2/index.cfm

Here's the URL for Metal Roofing Magazine: http://www.metalroofingmag.com/

While you are there, sign up for a free subscription to Metal Roofing Mag, as well as Rural Builder and Frame Building News. I always seem to learn some good stuff from these, even though they are aimed more at the rural/agricultural market.



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That section later states "Metal roofing shall be installed in accordance with this chapter and the manufacturer's installation instructions." That bounces it back to the missing instructions. I'm sure the builder can supply the name of the manufacturer and then it would be quite easy to make the call.

Over a residence, or other primary building, it is common around here to install metal panels over felt. Felt is usually not used when installed on porches or agricultural buildings. In recent years, some installers are applying adhesive backed, watertight underlayments (Ice and water shield), but I'm concerned of trapped condensation if adequate venting is not provided. I found one installed on modified bitumen, but it melted and asphalt oozed out.

I know of 2 manufacturers that "recommend" felt (a vapor retarder), but don't know that any require a vapor barrier.

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About a year ago, I did an ICF's house up on a lake the other side of Stanwood, which is about 40 miles north of here. It was a nice home, built by the owner/builder. Unfortunately, he installed a standing seam copper roof on top of OSB decking without battens and 6 mil layer of polythylene sheeting between.

I talked to the manufacturer. They said, "Ain't no way Jose!" and the proud owner wasn't happy when I recommended tearing off the roof and redoing it. He thought he'd make an end run around me and called the manufacturer to prove me wrong. Uh, uh, manufacturer told him outright that it should have been felt and that the plastic was going to turn that OSB into a big mushy biscuit on top of the house.

Well to do clients demanded tear off or and redo. It got done.

Some days you win.

OT - OF!!!


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I stand corrected in the use of "barrier" instead of felt, which is what most manufacturers do recommend.

Mike, I looked around the Metal Alliance site and found a post that described a remedy. It involved application of spray isocyanurate to the bottom of the steel roof. The metal guy said if their manufacturer would buy the idea, then it made sense to him.

This is what I presented to the owner of the subject house, who had gotten manufacturer's recs from the vendor. I told owner that if vendor/man. reps. would buy it so would I. Spoke this morning with insulation contractor who said he had done very similar remedial work with urethane foam application, and that results were positive.

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Barring additional variables (geez, that doesn't happen, does it?), going w/foam on the underside should minimize or eliminate moisture migration.

I mean, if the foam closes up the bypasses that all current thinking say are the real problem, it should work, no?

Still makes me nervous though; sticking vapor barriers in places they aren't supposed to be is the road to hell.

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  • 4 years later...

When completing a diy job on a retro fit roof how do we go about getting an inspection? I want to make sure I did a good job not only for the sake of the roof but also for the protection of my family. Is there a fee involved with this too? Thanks!

Pick up the phone, contact a few local home inspectors. Ask them how long they've been in business and how many metal roofs they've inspected. When you find one you think has sufficient experience with metal roofs, hire him or her to inspect.

Yes, there will be a fee; whatever the inspector decides to charge you.

By the way, nice job dropping that link. I nixed it though. Sorry.



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