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Old 12-15-2011, 08:38 AM   #1
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What's the RIGHT WAY to build sheet metal roof and cathedral ceilings?


New home construction question, guys.
I see a lot of discussion about building or covering a roof with sheet metal panels and not correctly venting or insulating.

What is the right way to build a house with 5:12 metal roof (center beam with post support) and have cathedral ceilings inside for usable loft space?
How's a roof like that vented? How would it be insulated? No cold space????

Thanks much,
MG

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Old 12-15-2011, 09:58 AM   #2
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What's the RIGHT WAY to build sheet metal roof and cathedral ceilings?


My log house has a metal roof and cathedral ceiling (12/12 pitch). It is constructed like this:
  • Rafters are 2x10, 2 ft on center
  • Decking is 3/4 plywood
  • Vapor barrier is Titanium
  • Metal roof attached directly to the decking (it's a standing seam roof, so the fasteners are actually hidden
  • Ridge vent
  • Air baffle in each rafter bay
  • R-30 fiberglass insulation
  • Ceiling is tongue-and-groove 2x6
  • Ventilated soffits

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Mustang Gregg (12-16-2011)
Old 12-15-2011, 10:28 AM   #3
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What's the RIGHT WAY to build sheet metal roof and cathedral ceilings?


The guy who put our metal roof on in the renovation told me he had talked with various different metal roofing makers and they all gave him different "right" ways to do the job. On first blush you'd probably think -- well, just do it how the manufacturer says. But then there's no real difference between what any of them were selling -- cold formed steel, painted.

So I guess the point is, it's kind of like a lot of things, there doesn't seem to be a single "right way" -- there are many ways that are acceptable. There are, however, wrong ways, and you want to avoid them.

For example, fasteners won't hold as well when screwed into decking, as they will when screwed into furring strips. Especially if the deck is 1/2" OSB. More of a problem if you're in a high wind area. If you do build with furring strips, then you have an air space between the roof and the decking, which reduces the need for venting, especially if there are ridges in the roofing.
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Old 12-15-2011, 11:33 AM   #4
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What's the RIGHT WAY to build sheet metal roof and cathedral ceilings?


md2lgyk has a good method. An alternate is to have the ventilation above the sheathing like so (edit: same as pyper's suggestion)
  • Vapour barrier
  • Rafters
  • Insulation to fill the rafters
  • Decking
  • Breathable vapor barrier (#15 felt or a breathable synthetic)
  • 2x2 strapping
  • Metal roof attached to strapping
  • Ridge vent, soffits
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Mustang Gregg (12-16-2011)
Old 12-16-2011, 09:02 AM   #5
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What's the RIGHT WAY to build sheet metal roof and cathedral ceilings?


Shazapple:

Excuse me. But what is the 2 X 2 strapping you mentioned?
Is that the sheathing boards that the metal sheets are screwed to?
And they are 90 degrees to the rafters?

Thanks,
Mustang
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Old 12-16-2011, 07:53 PM   #6
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What's the RIGHT WAY to build sheet metal roof and cathedral ceilings?


spray foam
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Old 12-16-2011, 11:01 PM   #7
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What's the RIGHT WAY to build sheet metal roof and cathedral ceilings?


Forresth:

You mean spray foam on the rafters after everything it put down?

MG

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