Framing deep overhangs air-sealed single pitch roof
I’m building a small studio for my wife. I’m in North Idaho, Zone 5, 40 lb snow load, we also occasionally will see some high 50-60 mph winds, though rarely. The structure is 12’x16’, with a 3:12 single pitch (shed) roof and cathedral ceiling. The rafters are 2×12, 16” o.c. I plan to plumb cut the rafters flush with the plates, and attach with truss screws, then wrap the whole structure solitex mento wrb as the primary outboard air barrier. Planning to strap out the tops of the rafters with 2x4s on the flat top create a vent space and provide nailing surface for the 5/8” cdx roof sheathing. I’d like to have as deep overhangs as possible while keeping the roof profile as slim as possible, for aesthetic reasons. The soffits will be enclosed with T&G cedar. I’d like to have 18-24” overhangs on the sides and low end and a 3’ overhang on the high side if I can do it. My main concern is with the high end of the roof and the corners. I was thinking of running the 2×4 strapping out long and scabbing on 2x4s on edge beneath them and running back to the wall to support the cantilever. Think that would cut it? Maybe 2x6s?
I guess I’m just looking for some advice on how to achieve this detail without having to add on craftsman style brackets or an exposed beam that I’d have to air seal around or something…
And a sub-question: Should I be concerned about outlookers running perpendicular to the vent cavity disrupting air flow/venting. I’m not super concerned about it, since I’ll be air-sealing with Intello Plus on the interior as well, but still… should I cut some v grooves in the outlookers or would that compromise their structural integrity?
Thanks in advance for replies!
Adam
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Replies
Adam,
I know this approach was popularized by a few builders a few years ago, but it has so many downsides as the primary air-barrier I'm not sure it makes much sense any more.
First, exterior sheet membranes are a poor choice as they are vulnerable to damage, but more importantly it just doesn't makes sense to prioritize continuity over the small area interrupted by roof framing over structural integrity. Rather than go through all the gymnastics, why not tape the exterior sheathing, leave a membrane over the top plates to transfer the air-barrier to the inside of the ceiling and be done?
To maintain continuity of the outriggers, drop them 1 1/2" and strap with 2"x4"s parallel to the rafters.
Malcom,
Thanks for the reply. Leaving a strip of membrane over the top plates and transferring the air barrier to the ceiling membrane was my original idea, actually. Then adding a layer of house wrap to the tops of the rafters to retain the insulation, then strapping out the rafters to create a vent space. I think I’ll go back to this plan, sister on 2x4s to the rafters to create the high side overhang, and just rip the tails on the low side.
I think I got a little turned around worrying (perhaps unnecessarily) about windwashing through the insulation. Got afraid of taping around rafter tails (which I know see transferring through the top plate solves), and thought “why not just wrap the whole thing and frame the roof on top?”
Any advice on the 3ft cantilever on the high side? Was thinking of sistering on 2x4s to the rafters. Would maybe go to 2x6, but with 2x4 on the flat on top I’d be getting into a much larger fascia than I’d like…
Thanks again!
Adam,
I like to taper rafter tails, so you get the full benefit of the framing member at the wall, and a thinner profile at the end of the cantilever. The disadvantage is you need longer rafters.
If you don't like that idea, why not use sistered 2"x4"s on each side of the rafters? It keeps things shallow, but also looks good.
Unfortunately, my wife doesn’t like the look of tapering the tails… I tried that already. I think sistering 2x4s to each side will be the ticket. And dropping (as in lowering, not eliminating)the outlookers. Thanks again!
Adam,
Might be worth using the same detail on the lower side for continuity - and think of the points you will pick up from your discerning wife!
I agree with Malcolm, the details for air barrier continuity are well worked out for the transfer from exterior walls to interior ceiling. With the transfer through the top plate, there is the opportunity to have several air tight layers for redundancy, if you wish. Exterior sheathing, exterior WRB, and interior vapour barrier or GWB can all be detailed as air tight. Then the ceiling can have the VB and drywall detailed as air tight. I see buildings hitting passive house levels of air tightness with a single, well detailed air barrier layer, let alone multiple systems.
Then the roof can be framed conventionally with the front and back having the rafters extend past the plates, and the sides can have ladder framing. I would recommend parallel chord trusses or Ijoists for the roof framing over dimensional lumber.
As for venting the ladder framing, good question. Codes have allowances for notching and drilling of structural members, planning for ventilating these rafter spaces is a good idea if done in a way that will not compromise the structure.
Thanks for the reply, Plumb Bob. I actually like Malcom’s idea of dropping the outlookers 1 1/2” and strapping out on top. Seems kind of obvious now 🙄.
I think it’shelpful (for me) to think of these vented (unconditioned) and insulated cathedral ceilings as very small attics, where it’s ok for outside air to touch the insulation.
I’m still fairly new to this whole construction thing, and am self-taught, so I greatly appreciate all the wisdom and help I get from this community.
Thanks again for your reply!
Malcolm or ar_lily
In the proposed solution, would the soffit look like A or B in the attached image. I would really appreciate learning from this thread and making sure I understand the details. These are images from the internet so I know they will not be exactly the same but thought they would server the purpose.
Hugh,
The choice is almost entirely an aesthetic one. You have three main options. The most common is the horizontal soffits you see on most houses (your first section). You can also enclose the underside of the rafters, but follow their pitch (the second one). The third option is to leave the rafter tails exposed and vent thorough the blocking at the wall. The complication of that one is you can see the sheathing above, so it is often replaced by T&G at the eaves. I have been assuming Adam is using the third option.
Thank you Malcolm, I omitted an image of the 3rd option being certain that it was not the one being recommended... but like is funny that way. Much appreciate the reply you have provided.
Hugh,
I use it a lot. The sistered rafter tails get dropped 3/4" or 1 1/2" depending on how thick the t&g is. The blocking between the rafters is cut down 2" and a piece of U-shaped perforated metal flashing forms the vent.
You can see the detail in post #5 here: https://www.greenbuildingadvisor.com/question/keeping-bugs-and-birds-out-of-vented-attic-without-soffit-but-with-accuvent-or-other-baffles
My preferred method is to cut the tails of the rafters plumb and level as in A (called a planchard cut I believe?), but follow the roof pitch with the soffits as in B. I like the rafters to have a minimum 30" overhang, this typically means keeping the rafters full size to keep the proportions looking right. The vent holes can be put in the piece that covers the level cut on the bottom of the rafter.