Seeking Advice on Low-Slope Shed Roof
baking_fool
| Posted in General Questions on
I am planning a post and beam house with a 1/10 shed roof for climate zone 5 in eastern PA. The beams are on 5′-4″ centers across the 42.7′ width shown below, and the house is about 25′ in the other dimension. The stock plan has 12″ rafters on 24″ centers, likely for standard insulation. A span calculator shows that 2×8 rafters on 24″ centers will handle a snow load up to 90psf – where 50psf is the requirement in my area.
*After* researching low pitched roofs here on GBA and coming up with this EPDM solution, I discovered that metal roofs can handle a 1/10 pitch.
There is a R-49 requirement for roofs here, and I discovered that polyiso’s R-value decreases to about 2.8 per inch in cold weather. However, polyiso can withstand EPDM adhesives, whereas EPS and XPS can not. A mimimum ratio of exterior R to total R of 41% is required. I am proposing therefore 2″ polyiso on top of 4″ EPS outside, and either 4″ of spray foam (R-28) or possibly R-23 batts somehow attached to the underside of the sheathing (it is recommended here that the interior insulation is in contact with the sheathing). The batts would technically satisfy the overall R-49 requirement, but at R-2.8 per inch estimated for the polyiso worst case would result in a total just below the requirement.
Am I anywhere near close to something viable? I tried to distill what I saw in different threads here for my circumstances.
One question I have no clue the answer to is what happens along the four sides of this giant sandwich – how are they structurally finished? I see that this kind of roof relies heavily on proper sealing along the edges, between the membrane and the drip edge, and drip edge to deck. But what will the fascia attach to where the foam boards are? Perhaps 1 by material around the perimeter?
Is having the rafter space partially filled by insulation below the structural sheathing ok, as long as it is in contact with the sheathing?
Now that I see some kinds of metal roofing could be used, are there other pros and cons than cost and expected life?
Thank you in advance for any assistance. Obviously I will have to retain a professional contractor, but I am trying to gain an understanding before taking that step.
Don
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Replies
Plyiso doesn't de-rate that much. Only the very first layer looses significant R value and there are now even formulations that don't loose any R value at all. If you look at the cold climate R value data average, for a thick roof, you are close to R5/inch and that only on the coldest days. The rest of the time you get the full R value. I'm in zone 5 and have no issues with polyiso on roofs.
As for your specifics, I would see if you can reduce the purlins to the required insulation thickness. Even if you have to go to tighter spacing, it will make the insulation install much easier. There are insulation wires also you can get to hold the batts up but adds labour and cost.
Most codes allow for a full assembly performance on U factor basis. Since you have continuous exterior insulation, this generally means around an R38 assembly. Much thinner roof and easier to detail.
For the edge of the foam, I prefer to picture frame the roof with 2x lumber on edge cut to the foam height. This not only protects the foam from critters but also gives something to screw trim into.
Roof to wall air barrier transitions are never easy and there is not one that works for all roofs. A post+rafter+purlin type roof makes it even more challenging, sometimes, the easiest is a bit of SPF around the perimeter. If you are going with SPF instead of batts, you can go with open cell foam. This will still work to air seal, use significantly less polymer and generally much cheaper.
If you can get a metal roof that will work at your slope for a reasonable cost, go for it. Much more durable than any of the membrane products.
Akos -
I did not understand a few of your points. First an easy one - what I call rafters is that what you call purlins? The baseline construction of this roof is rafters placed perpendicular to the beams, which are exposed in the house when looking at the ceiling. I believe you are suggesting to reduce the rafters to just allow the required insulation thickness.
I did not understand your point of R38 and "full assembly performance". Are you saying that with continuous exterior insulation, I don't actually need R-49 calculated by adding each individual material?
Thank you for your comments and suggestions.
I just logged in here after a week of "critical error" failures in getting this to post. There was never an indication my attempts may eventually post. I was told web support would contact me but they didn't, and now days later several copies suddenly posted. I apologize profusely for the copies. I very much appreciate your reply, and my first task is to go delete the copies of this post. *edit* All the copies of this post including those under "Related articles" are not connected to any account (nor is this one). Thus, I sadly can not delete them.
Here are the images that should be attached to this post:
Don,
The reason membranes are usually used on low-sloped roofs rather than metal is primarily because it's much easier to deal with the edge conditions and penetrations. The big disadvantage metal has is the flashings typically used rely on gravity and can fail when capillary action drives moisture upward, or ice-dams allow water to pool.
I agree with Akos that metal is a much better roof if you can find a way to detail it to overcome those problems. However when you use any material at the limits of it's specifications (like going to a 1:10 pitch) the result will be less resilient than one used closer to the normal circumstances. Designing potential problems into any building assembly is never a good idea. I'd spend a lot of time seeing how details like the ridge, eaves, standing seams and gables will work before committing to any manufacturer's metal roof system.
Good luck with your build!
Interesting points, Malcolm. I would not want to "design in" a weakness for sure. It sounds like either a membrane or metal solution needs to be very carefully chosen and implemented. My initial reaction is that these replies give me pause.
Don,
I can only see the one elevation you've posted, but I doubt the design would suffer from having the roof pitch increased to 2/12. You could then safely use metal roofing and ventilate the roof too.
Most metal roof manufactures I've seen have many options for that kind of pitch. Some even hold their warranty at even lower pitch. Most are a mechanically fastened, taller seam. And these days the roofers will likely put a peel and stick barrier down first.
Lot's of valleys and dormers I would think twice. But if its a simple roof without a lot of obstacle then metal will do just fine.