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Is sheathing a metal roof necessary?

I'm a carpenter here in Asheville,NC zone 4/5 and trying to design a home for myself so I need to make it as inexpensive and green as possible. I'm thinking of a shed roof using flat trusses with blown cellulose to R-60 and a metal roof. Of coarse, there will be eave and "ridge" vents. So do I need to sheath the roof for metal or will just using purl-ins work. My main concern in condensation. If you dont think this would work could you suggest other systems that would be inexpensive labor is less an issue since I will be doing most of the work myself.

Asked by terrell whitworth
Posted Feb 20, 2013 5:51 PM ET


7 Answers

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Personally, I would use plywood or board sheathing, asphalt felt, 1x4 strapping then steel panels.
Make sure the fasteners for the steel panels are long enough to go through the 1x4s and the sheathing material.

Answered by Lucas Durand - 7A
Posted Feb 20, 2013 9:08 PM ET


Fully agree on the solid sheathing and felt, not as sure about the purlins. We use a lot of snap-lock screwed to the sheathing. What roof pitch are you thinking?

Answered by David Meiland
Posted Feb 20, 2013 9:50 PM ET


On top of your insulation, you need an air barrier. Under your metal roofing, you need roofing underlayment to catch the dripping condensation. The easiest way to do this is to have solid sheathing with taped seams -- that's your air barrier -- covered with roofing underlayment.

Next, purlins parallel to the eaves, and then roofing. If you really want to get fancy, you can install two layers of purlins -- the layer first perpendicular to the eaves (for drainage), and the next layer parallel to the eaves (so you have something to screw the roofing to). I think that's unnecessarily complicated, though, since the condensation moisture will evaporate before there's enough of it to require drainage.

Answered by Martin Holladay
Posted Feb 21, 2013 5:25 AM ET
Edited Feb 21, 2013 5:26 AM ET.


You can build a roof without sheathing. Do need to 'rack' the house with sheathing on walls or cross bracing.

Attached a solution. Use the reinforced SOLITEX MENTO Plus roof underlayment on top of your rafters. Add purlins on top of this for venting and drainage, then add horizontal purlins to mount the metal roof to. Since your subroof will be waterproof, well drained and airtight this will be a cost effective solution to build your roof (saving the cost of plywood). For more questions contact me at, info (at) foursevenfive.com

Vented roof without sheathing 475 SOLITEX MENTO PLUS.jpg
Vented roof without sheathing 475 SOLITEX MENTO PLUS.pdf 175.53 KB
Answered by floris keverling buisman
Posted Feb 21, 2013 10:33 AM ET
Edited Feb 21, 2013 10:35 AM ET.


Floris and Terrell,
As I mentioned the last time this topic came up, the chief disadvantage of the "no sheathing" method is that it isn't very buildable. There isn't much to stand on when you are installing the membrane across the tops of the rafters.

Answered by Martin Holladay
Posted Feb 21, 2013 10:38 AM ET


Maybe I'm mistaken...
I think Terrell might be planning a vented "attic" - with eave and "ridge" vents (and deep parallel chord trusses).

Answered by Lucas Durand - 7A
Posted Feb 21, 2013 12:23 PM ET


Other advantages of sheathing are that it provides backing for fasteners at roof penetrations and flashing, makes for a walkable roof for maintenance purposes, allows a thinner gauge of metal to be used and is more resilient to damage from falling debris. I prefer to install directly against the sheathing on the assumption that eliminating the air space between the purlins will also eliminate much of the condensation.

Answered by Malcolm Taylor
Posted Feb 21, 2013 12:26 PM ET

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