No Sheathing for Overhangs
Hi, I am building a 20×28′ timber frame house with cathedral ceilings, and I have a question about sheathing for the roof overhangs. (Also I am open to critiques on my general design)
First, here are some details about the building. It is a saltbox style 10 pitch roof. I am located in coastal New England, and will be utilizing exterior insulation on a vented roof. My roof buildup is as follows: 6×8 rafters 3 1/2′ oc, 2×6 t&g roof decking, 30# felt paper, 2 staggered layers of of 3 3/4″ EPS foam, another layer of 30# feltpaper, 2×4 vertical flat for ventilation screwed into every rafter and one between each rafter screwed into decking, 1×4 purlins, standing seam metal roof. I’ve built a box around the perimeter of the roof extending upwards the height of the insulation. Since the large rafters do not extend beyond the perimeter of the building, I will be extending the vertical flat 2×4’s (for vents) past the building perimeter and building what I guess is called a ladder system using 2×6’s to accomplish a 2′ overhang.
My question: Since I will not be installing a sheathing layer ontop of my insulation, is it okay if I do not have any sheathing for my overhangs? Instead of sheathing my overhangs I will simply put purlins over top of the flat vertical 2×4’s and put the metal roof ontop of that. Does this sound okay…? Considering the area above the insulation will be blocked off from critters, vented and screened soffits will be installed, and given the overhang support ladder is strong enough (which I could use a diagram if anyone has one) does anyone see a problem with this design? Thank you.
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Replies
Mike,
I did exactly that on a workshop I built last summer - although the overhangs are supported by the rafters. The main problem is aesthetic. If you don't mind the look it works fine.
Thank you for your reply Malcolm, it is good to know that this technique has been and can be applied. Looking at the details, I'm assuming that the purlins sit directly ontop of the facia boards?
Mike,
No the fascias cover the ends of the 2"x4" purlins. The rafters are tapered so that the fascias cover the ends of them too.
Cool, makes sense. Looks good!
It is critical to keep the critters out, that type of roof is inviting for rats, squirrels, bats etc. The detailing is important. Ask me how I know...
You could leave it like Malcolm has shown, and see how it goes. If it is not working you could put T&G or other soffiting material on at a later date.
Thank you for your reply plumb bob, I plan on preemptively installing soffits to help keep critters out, along with hardware cloth and a fine metal mesh over each soffit vent. I've lived in the woods long enough to know keeping out critters is a priority. If you have further detailing advise, I'm all ears.
You need a better air barrier over T&G than 30lb felt. Besides leaking up through the grooves between the boards, air can also leak sideways along the T&G grooves. These small gaps times a couple of dozen boards add up to a very large air leak.
Generally the best is to use a peel and stick membrane over the T&G and carry this membrane down to your wall sheathing for air barrier continuity. Make sure the membrane has either acrylic or butyl adhesive as SBS bitumen ones can react with sap and ooze.
The nice part about the full peel and stick over the T&G is you get the place under roof much quicker and can take your time installing the rigid and metal roof.
All exterior rigid roofs don't need to be vented unless you are in heavy snow country. I'm in Toronto where we get a bit of snow, never had issues with unveted metal roofs.
Thank you for your reply Akos, unfortunately the roof assembly is already installed. My friend who's a green-minded architect had the same advise for me (peel and stick ontop of the roof decking instead of felt paper) and that was my original plan.
But when it came to budget I needed to decide between 1x T&G with peel and stick or 2x t&g with feltpaper. Remember, my rafters are 31/2' OC. The metal roof also played into the budget, standing seam vs exposed fastener corrugated metal. In the end we decided to go with the 2x, felt paper and the standing seam. I guess only hindsight will reveal what is the better choice, as we couldn't figure out a way to have it all. I taped the t&g seams on the eave and gable edges to help with air infiltration and staggered the insulation seams on the roof, so hopefully that will help.
For better or worse (again, I'll discover that later) I just wasn't able to have a perfectly air tight design because of budget reasons and i've justified it in the following ways (i know this has been discussed a million times on these forums but its still fun for me to list this out). From my limited knowledge as an amatuer builder here are my current conclusions, and before reading on please know that I am attempting to stop air infiltration every place I can, this is just my justification for not using a peel and stick. I don't think everyone's goal should be to build a home that performs 100% efficient, I think the goal should be to achieve the best performance we can afford, while taking into account lifestyle and circumstantial practicality.
From my understanding some air infiltration can harm the shelter performance in two ways: it will make the space more drafty and it will introduce unwanted moisture.
Although I am in a climate with very cold winters, I am hoping that performance loss due to drafts can be somewhat negated by implementing massive amounts of thermal mass inside the building through passive solar on an insulated concrete floor, radiant floor heating in the slab, and a wood burning massonry stove weighting over a ton. I figure I could burn a little extra wood in my extremely efficient mass heater if I need more warmth, we'll see how that works out for me. My rocket mass heater variation allows me to burn softwoods without cresote build up and we have a nearly infinite supply of them on my land. And perhaps a little air flow might be beneficial due to the fact that I will not have to rely on any mechanized ventilation system, although I am rather uneducated on this prospect and I think this could be better achieved with controllable passive ventilation.
As for the the issue of introducing moisture, I am inclined to not be concerned due to the fact that there's no attic space and no wood in the house that is not exposed to airflow on atleast one side because all insulation is fully exterior of the sheathing and I'm installing a vented roof and rainscreened walls. Also shower and bathing will happen in a separate structure, cutting down on interior moisture.
So that's my reverse justification for not installing a peel and stick membrane, haha, although the real reason was budget. Thanks for reading, and FYI - I welcome a critical response.
Does anyone out there have image details of a good way to install the overhangs at the eaves without the use of rafter tails?
I want to go 2' and we are in snow country and although we are protected in the forest, we are coastal so high winds are common. Thanks in advance.
Mike,
There are a few ways to do it. I'd suggest a Google Image search for "framing applied roof overhangs". Because they typically rely on a moment connection for their strength, it's pretty difficult to make them as deep as you would like. They often also rely on the roof sheathing being in tension for their stability, so that's another impediment you'll need to overcome.
It's something you will very likely have to get engineered, so I wouldn't get far down the rabbit hole without touching base with one.
Is this something along the lines of what you are describing? In this technique there are two ledger boards, I would consider using joist hangers also. Additionally I will have the 2x4 flat (for the vents) running the entire length of the roof and extending ontop of what is shown in the picture, which may help with the tension stability. Maybe I can scale down to 16" also.
Mike,
For some reason I didn't realize you were building flat soffits. That makes everything much easier. The detail you linked to should work fine. I bet you can go 2 feet deep.
Great, thank you Malcolm.
Using this technique, would you be inclined to suggest a 2x subfascia board or do you think I could use a 1x?
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