Is sheathing a metal roof necessary?
user-433254
| Posted in Energy Efficiency and Durability on
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.
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Replies
Terrell,
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.
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?
Terrell,
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.
I disagree that it would be unnecessary to add a top pulin running horizontal to eave as it would allow in summer extremely hot air radiated from metal to rise and escape via
air flow from eave to ridge vent besides adding a safety ladder of sorts to climb and stand on while installing the roof. Not as important on less than a 6/12 pitch but there is a reason cost of roofing increases in proportion to the increase in rise.
I recall installing cabinets to 2 story apartments in Sacramento during the summer. We always finished upper story in the morning, the lower in afternoon as by then the upper story was 20 degrees warmer. Typical production build vented attic with r38 insulation above ceiling. Why for my build it'll be a conditioned attic with spray foam between rafters with well vented roofing.
Terrell,
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
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.
Maybe I'm mistaken...
I think Terrell might be planning a vented "attic" - with eave and "ridge" vents (and deep parallel chord trusses).
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.
If you use purlins, how would you ever step/walk/climb on the roof to repair or modify it? 26 gauge metal will not support any weight, even on 12 inch centers.
Mettle,
I've roofed a lot of buildings using standard 26 gauge metal roofing over purlins installed at 24 inches on center, and I can assure you that it is possible to walk on these roofs.
At 12" oc I doubt you could tell where the purlins were when walking the roof.
The roof sheathing is a structural diaphragm that distributes the various load on the building to the walls. Using the purlins as the diaphragm element would lead to deflections and make the ceiling drywall the structural diaphragm element. Do you really want structural drywall? Get you design engineered.
You can get around sheathing and still have a functional result, but speaking from personal experience - I too set out to reduce the use of sheet goods or planking to save $ - but I found the added expense of the decking would have by far outweighed the extra work and difficulties with working without a deck, not to mention the big advantage that decking offers in sealing off the space and insulation below. There's other places to save money and the environment, not the roof deck.