Pretty Good House Low-Slope Roof Assembly
Hello,
i am zeroing in on a roof assembly for my 1000 square ft. build in coastal Maine, zone 6. I really want to do a flat cathedral roof (for aesthetics and to keep the profile low and volume small) , but for now, am settled on doing a 2/12 cathedral shed roof (or possibly a 1.5). I simply can’t abide the 3/12 look for this build (or the raised roof it would create), although I understand the advantages of being able to move water off and possibly do a vented assembly. I am settled on a double stud wall assembly, with zip sheathing on the outside of the exterior (structural) studs as my air control layer. I really want to prebuild my rakes and eaves and install them after the fact, passivhaus style. I am the owner/builder and a a contractor by trade.
I am referencing pg. 124 of PGH and have honed it down to three options described on that page:
1. Outsulation roof assembly
2. Insulation as vapor control layer assembly (using rigid against the inside of the sheathing instead of closed cell spray foam)
3. Vapor open roof assembly (using tyvek on the rafters, sleepers, strapping and a standing seam metal roof)
Here are my questions:
1. Can I use a standing seam roof on any of the above on a 2/12 pitch?
2. Assuming the roofing material is the same, and understanding that I will provide about 80% of the labor, which assembly is the cheapest?
3. If i go with the outsulation assembly, can I stick a rolled roofing membrane to the outermost layer of rigid insulation and call it good?
4. If I go with the vapor open choice, how will I protect the ventilation space from taking water (especially at the top, since this is a shed roof) or debris? Do i screen it like a rain screen or seal it and discourage air washing?
5. Which roof assembly matches up best with my wall assembly (see above) as far as air sealing is concerned?
6. On the vapor open roof assembly, there seems to be rigid wood fiberboard on the exterior of the sheathing, at least in the diagram. Does this assembly require some outsulation in order to warm the sheathing ?
Thanks, Stephen
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
What will your rafters be and how deep to span? Lumber, I-joist or trusses?
The span will be around 13. Its a 28 foot box with a center beam. I haven't checked the rafter tables but i figure it would mean using 2x10's with the snow load. I will use lumber. I will gusset down with 2x2's if i choose a roof assembly that needs to add depth for more cellulose. I can't drop a ceiling because i will have a gravity fed water cistern suspended above the bathroom that shouldn't be in an unconditioned space.
I would look at using trusses for the roof and clear spanning. For 28', you are probably looking at 16" to 18" deep trusses. You can build a decent vented assembly with those and if you install baffles the full length as it can be filled with loose fill insulation for a pretty low cost high R value roof.
If you want to stick to 2x10, you are looking at an exterior insulation option. Rigid is probably the easiest but you can also look at something like this:
https://www.greenbuildingadvisor.com/question/risk-of-rot-in-warm-roof-outlooks
2:12 is still a common enough slope for metal roofs. Most taller rib snap lock panels will work. Going bellow that reduces the effectiveness of roof vents and sealing panels becomes much harder.
Thanks Akos. I hadn't considered trusses, but it would solve several problems . It would save on rafter material, beams, posts and having to post below to carry the point loads. I guess I had it in my head that a 1.5 or 2/12 pitch couldn't be vented with baffles. .. The thermal bridging would be minimal with truss cords and if I did a membrain on the interior, I could then strap it and put up drywall. I guess we would be talking soffit vents and some sort of modified ridge vent (since its a shed roof). ?
stephenr,
I'd go with trusses. The extra depth allows you to increase the depth of the ventilation channel, which helps make a 2/12 pitched roof less risky. Let the depth of the cellulose or batt insulation you want, and the size of ventilation gap, determine the truss depth, rather than the span.
If you do this you won't need baffles, and can vent the peak the same way you vent the eaves with a strip-vent on the soffits.
Right. So , assuming the top (rafter) cord of the truss is a 2x4, I could realistically do a 2 and 3/4 inch ventilation gap using luan of some other thin plywood. This would be safer. Would that ventilation plywood need to be air sealed with caulk, etc? (I will use zip on the roof) Also, do you think this plan would work with a 1.5/12, or would that be pushing it?
stephenr,
Whenever possible, when I'm dealing with parallel chord trusses I prefer to specify at least 2 ft depth. That does a few things.
- Truss suppliers love it. It's much easier to get below deflection limits with less materials. Depending on the live loads where you are, you are probably looking at close to 2 ft anyway.
- It allows you to use a variety of types of insulation, and in most climates insulate to almost any level you would want.
- It leaves you a big enough ventilation space that you can treat it like an attic and not use baffles.
- The deep vent space makes low slope roofs function better.
What's the limit in terms of slope? I'm not sure there is a clear line. For me the cut-off is 3/12, but installed diligently a 2/12 roof can work well. When you get down to those pitches you working at the margins of a sloped roof assembly, and that means increased risk. It's worth remembering that decreasing the slope from to 2/12 to 1.5/12 is proportionally a much larger change than going from say 8/12 to 7/12, and those minor jumps at low pitches may be what tips the assembly into being risky.
Evaluating a similar design concept myself, also 2/12 roof cz6, and would like opinions on what depth of ventilation space is necessary to consider this an attic space that does not require full baffles and/or air barriers between insulation and sheeting?
Thanks Malcolm. I just want to make sure I understand what you are saying about the baffles. If I went with 2 foot trusses, and I was using cellulose, I could blow, say, 18 inches of loose fill cellulose and leave a 6 inch gap beneath the sheathing, vent it with soffit vents at the top and the bottom, and I would be good, is that right? I would blow this after I lay my ceiling, as an alternative to netting it and blowing dense pack in order to fill the space below the baffle plywood (which I now would not need) and the netting (which would not be needed either). And a six inch ventilation gap would be a lot less risky if I went with a 2/12 pitch. Correct?
stephenr,
Yes.
You would probably want one run of baffles at the eaves to stop the insulation from being scoured by any winds.
In such a confined space it might be tricky to maintain a consistent gap above the cellulose. You might want to plan on a larger vent space to leave you a margin of error. The ones I've done have all used batts.
I think a lot of this depends on how much visibility you get. Trying to fill a 2' space to 18" is going to be hard just for access, getting a sense of how much insulation is installed is even harder.
Some of the options:
-minimum baffles, fill form the 2' attic
-full length baffles, fill from outside through the soffit area on the upper side
-full length baffles plus mesh. Install from the inside, no need to dense pack as you are not worried about settling
-frame in mini attic near the peak for better access, fill from there
-insulate with batts
I think all options equally doable. If you are dense packing the walls, I would go for mesh. Otherwise for DIY and smallish space, batts are probably the simplest.
Following this discussion, as your project is similar in a number of ways to mine... :-)
I don't have much in terms of responses to your questions, but, for whatever it is worth, I can chime in on questions ##2 and 3:
2. Assuming the roofing material is the same, and understanding that I will provide about 80% of the labor, which assembly is the cheapest?
Just recently, I was given a brochure for the metal roofs made by a company called Ideal Roofing (with which I have no association whatsoever), specifically their Junior HF metal roofs. At the store, a manager and a contractor said that installing their metal roofs is really easy. It might be worth a look: http://www.idealroofing.com
3. If i go with the outsulation assembly, can I stick a rolled roofing membrane to the outermost layer of rigid insulation and call it good?
Coincidentally, just the other day, the salesman of a roofing company told me that, with metal roofs, you can do that. I have no knowledge to share (sorry :-( ). Yet, the claim from the salesman struck me as rushed and incomplete: What thickness of insulation did he have in mind, and with what compression? How would the metal roofing panels be connected? My uneducated instinct is that a layer of sheathing would make the install of the roof easier.
I know the above does nor even quite amount to the proverbial 2 cents! Good luck with your project!
(By the way, if you happen to have any suggestions about my questions -- in a post from yesterday -- I'd be super-grateful!)
yup, I like the idea of filling from the soffit area, but I might only have 6 inches (if I fill 18) or so of space to work with. . Pulling a 4 inch flexible tube through 28 feet of truss bay could be a challenge. Once the trusses are in place, an answer in the form of limited baffling could present itself. I intend to dense pack the walls, so dense packing the ceiling area as well would make sense. I imagine 3.5 inches of vented space (the width of the rafter cord) beneath the sheathing would be sufficient, since 2 inches is usually more than sufficient for a steeper sloped roof. That would give me roughly 20 inches of dense pack at an r value of 76. Yikes. More than enough. If I would save money, reducing my trusses to 20 inches instead of 24 would give me the r-60 I am looking for and I wouldn't have to work too hard up in the trusses to lay luan perpendicular to the underside of the roof truss cords. Thoughts?
stephenr,
That sounds like a good plan to me.