Low Slope Roof Detailing for PGH
This is a new SFR in PNW zone 5B. The primary roof is to be framed flat with 9.5″ I-joists, see attached SE. The final roof profile is to be 4-sided hipped, whose pitch could be as low 1:12 or as high as 2.5:12. I would like a standing seam metal roof (for durability and to attach solar) and PGH min. R60 insulation. Given the ~17ft run (edge of overhang to mid-point of roof), the height of the hipped roof would be between 17″ (1:12) and 42.5″ (2.5:12). From everything that I have read on GBR, my low pitch dictates an unvented assembly.
I am leaning towards an external-only insulation strategy and leaving the I-joist bays empty as a service cavity. This would involve tapered, rigid insulation above the primary sheathing (the sheathing immediately above the I-joists). I am also thinking that, with this strategy, I do not need to detail an air-tight ceiling (ceiling could be T&G).
1. What do you make of the plan?
2. What roof pitch should I choose?
3. Do I ned to fill the entire void with rigid insulation (all 42.5” of it with a 2.5:12 roof) or can there be dead airspace above an R60 layer of rigid?
4. What rigid insulation should I use? Perhaps a combination of Polyiso and EPS (for the thicker sections)?
5. How do I deal with the thin-end of the insulation wedge, above the exterior house walls, where there would only be 2″ to 5″ of vertical distance between the primary sheathing and the hip-roof profile?
6. Do I need to tape the joints of the primary sheathing and make this a good air barrier?
7. Do I need an additional vapor barrier? What should it consist of and where should it go?
7. How should I frame the hip roof/what should the build-up look like above the primary sheathing?
Feel free to treat me like a complete novice. Thank you for your help.
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Replies
At 1:12 water tends to run uphill from capillary action. This means any lapped seams in a metal roof will leak unless sealed with sealant which makes install quality critical. There are metal roofs that designed for such low slope but they generally are seamed and gasketed.
Unless you have a height restriction, i think the simpler option is to bump up your min slope to 2:12. This would let you use more common snaplock panels and get better venting performance.
Taking a quick look at the roof, there is a lot of structure for a simple hipped roof. I think the simplest is to switch to a raised heel truss for the roof. If you want cathedral ceilings, you can go with scissor trusses or I-joists over a ridge beam.
Generally with a vented roof the air barrier is at the ceiling plane. The simplest is to use a sheet good (OSB or quality membrane) under the drywall and connect this the sheathing air barrier on the outside walls. Depending on how you build, this can be done with plywood flanges or wide flashing tape over the top plate before the trusses/rafters are set.
For best PV performance you want to make your south and west facing roof areas the largest. This tends to favor more shed roof like structures but can also be done with a hip roof. I would lay out the PV panels on your roof and adjust the ridge width to maximize coverage.
Thank you. Replying to your paragraphs one-by-one:
P1: Yup, this has been my fear all along. You only reinforce what that nagging ‘voice in my head’ has been saying
P2: Minimum 2:12 it is, height is ok up to 2.5:12, maybe even 3:12 at a stretch
P3: Are you then suggesting a vented roof with insulation (maybe blown-in cellulose) on attic floor? I don’t need/want cathedral
P4: I understand this detail and think it is bomber
P5: I’ve been modeling solar layouts with on-line calculators. Do you mean adjust ridge width by increasing the pitch of the hips?
andrew84092,
I agree with everything Akos suggested. Trusses solve all the problems you were having, including ending up with a very thick fascia.
If possible increase the pitch to 3/12. That opens up the choice of manufacturers for the roof panels, as some don't allow installation below that - and at low pitches any increase brings a big improvement in performance because proportionally they are so large. Roofs below 2/12 also tend to disappear from the ground, so you don't get the benefit of the standing seam to cap the structure visually.
With a trussed roof and cellulose insulated ceiling what you end up with is the simplest and most resilient roof assembly you can choose. https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-the-top-of-the-house
Thank you. The concept is to give the structure a modern, flat-roof appearance, so the architect considers it a plus if the roof deck is not visible. I have fought tooth-and-nail to get a pitched roof!
In any case, let’s say min 2:12, max 3:12. To get R60, I need 17” of cellulose above the ceiling, which is detailed as the air barrier per Akos. Then we have baffles above the cellulose to vent the attic.
Even with a 2:12 and hipped roof this will work? 2:12 is enough of a pitch for ventilation? The hip framing wont get in the way of the ventilation channels?
Andrew,
You decide how high the raised-heel on the trusses will be, and that will dictate how far you need to run the baffles up the roof sheathing before you can terminate them.
2:12 is definitely at the low end of pitches for effective ventilation, but that is made much easier by having an attic, rather than individual rafter bays.
Two ways you can increase the effectiveness of the venting are by strapping the trusses with 2"x4"s @ 2'-o" oc, and by detailing the caps on the hips as vents, much as you will do at the ridge.
P3. A vented roof with lots of loose fill insulation over a flat ceiling is by far the cheapest, most energy efficient and robust roof in pretty much any climate. I don't have as much precipitation as the PWN, around me vented low slope/flat roofs work without problems.
P5. Yup the ridge width is up to you (and the architect) sometimes adjusting it a bit lets you get a couple of extra panels on the roof without changing the look much.
Since this house is still in just a plan consider rethinking the plan.
What is the appeal of such a low sloped roof?
That low slope forces you into a risky unvented roof with expensive and ungreen foam insulation.
Why a budget busting metal roof?
Consider shingles on 5-12 hip roof trusses with raised heals to make room for cheap fluffy insulation. My guess is if you put a value on your time the trusses and the crane to set them would cost less than stick framing your roof. My wild guess is this roof at R60 would cost 25% less and be less risky.
It may not be the look you had in mind and I could understand.
Walta
Thank you Walta. The maximum 3:12 pitch is dictated by maximum building height. I cannot change this aspect of the design at this stage. So given this, a metal roof seems like the best option. I hear you about using foam, I don't really like it either ...
andrew84092,
Unless the construction budget made it impossible, I would never use asphalt over metal. Amortized over the life-span of the house, you make the initial outlay back anyway. The big difference for me is never having to worry about it again.
Good point. This will be built as a forever home. I view the construction budget not as an upfront amount, but as a set of discounted cash-flows (upfront and ongoing maintenance) over at least 30-years. The greenest houses are those that stand for a long time, 100+ years.
I agree with Malcom, a asphalt shingle roof is almost always more expensive if you go are thinking past 10-20 years. In our area its not uncommon for asphalt roofs to fail in 10-15 years. According to one metal roof manufacturer, the cost to re-roof a house doubles every 10 years. If you are somewhat in control of the design of the house, and keep the roof lines simple, standing seam is the best choice.
I also agree on the pitch, the steeper the better. If you do go with a shallow pitch, limit your protrusions to the bare minimum. I am confident in double lock standing seam panel on a 1/12 pitch, less confident in my ability to flash any protrusions in.
If you can find someone who likes to do mechanical lock standing seam, hire that guy. He will like mechanical lock for its unmatched ability to flash any protrusions into the panel. It is definitely slower and more labor intensive then snaplock, so if he wants to make a quick buck he will push the snaplock. Check out this guy on YouTube, to see what all mechanical lock is capable of. https://m.youtube.com/@asm101/videos
Thank you. I’m working on keeping protrusions to an absolute minimum. Maybe even running the plumbing vent(s) up the side of the house. How difficult are hip ridges to detail on a low slope metal roof (realizing that a hip ridge will have a lower slope than the roof decking itself)?
Thank you for the link. I’ll take a look
andrew84092,
Ridges on hips are are detailed almost identically to gable ridges on metal roofs. Although the pitch is less on the hip ridge itself, the slopes on the roof panels leading up to it are the same as the rest of the roof, so they are no more risky.
What is the max height requirement 18 feet?
Has anyone else won an exemption?
How sure are you that this is the right place to build?
Walta
Walta, it’s 28 feet. We started with a blank slate, but after 2 years of work on this we are pretty far along with certain elements of the design, which include the footprint of the house and number of stories (2+basement for storage/mechanical). I know it might seem strange, and perhaps putting the cart before the horse to some people, but what we are left with is a 3:12 max pitch for the roof
How about this?
andrew84092,
A plenum truss may work depending on if it takes the ducts to where you want them.