Framing exposed rafter tails, sistered to common rafters
I would like to have exposed rafter tails on our addition. But, I don’t want to expose the spruce common rafters I’m using to the elements. I have some full dimension 2″x6″ red cedar I would like to sister on to the common rafters, so that the exposed tail will be rot resistant.
I attached a drawing. The tails are ⅔ in, ⅓ out. They are dropped 1 ½” to allow for t&g car deck below the roof sheathing (nicer to look at from below). There’s blocking to accept the end of the sistered rafter tails. This is all in an unconditioned attic space.
Some potential issues: the seat cut is the full 5 ½” of the top plate. This normally wouldn’t be allowed with the 8-12 roof pitch, because the birdsmouth would be more than 25% of the rafter depth. But is that an issue in this context? Shouldn’t the common rafter still have it’s full strength?
Other issue: the wall does not have a double top plate. The sistered rafters won’t bear directly on the studs.
The height of the overall assembly also makes it easier for me to match the eave line of the house we are adding on to.
There are no building codes here in rural Alaska, but of course I still want it built correctly.
thanks for taking a look.
Justin
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Replies
Justin,
It's a common detail here done in a very similar way to what you are proposing. A few suggestions:
- The strength of the connection to the common rafters and the applied tails is in the nails used to fasten the two pieces. The blocking at the butt ends of the 2"x6"s adds nothing. I wouldn't bother.
- The small birds-mouth is an advantage. As you say, you already have sufficient bearing on the comm0n rafters. The smaller the cut on the 2"x6"s the better.
- Depending on how much shear you need for your roof, you may just want to drop the applied rafter tails 1" and stop your sheathing where the t & g ends, rather than sheath over it too.
- Think through where your blocking between the rafter tails will go carefully. I typically like to cut it to that it leaves a 2" gap to the t & g above, which I fill with pieces of perforated U-flashing. If you angle the blocking at 90 degrees to the bottom of the rafters it makes things much easier. The blockimg should be far enough out from the sheathing so that it can cover the top of the siding and any trim.
- Carrying the detail up the gables means either the last common rafter, or the gable wall, should be also built 1 1/2" lower. As you go up the gable, run each third piece of t & g back to the first rafter in from the wall, which will provide strength for the overhang.
- Watch the nails your roofers use on the overhangs so they don't show from below.
Thanks, Malcolm. This is so helpful. I was just thinking through the gable end considerations. I'll follow your advice.
Justin
Have you thought of false rafter ends? Cut short pieces and attach them to your trim board spaced down for the t&g. Just like a ladder frame on a gable end. Makes the spacing easier and there is less cutting and fit, I think I saw something like this on This Old House, Westerly RI project.
>"the seat cut is the full 5 ½” of the top plate."
This, and Malcom's subsequent response, is a bit unclear to me. If it's like you have in the drawing, all seems great. But it sort of sounds like you're cutting into the tails more than is shown in the drawing? As Malcom says, less is more there as far as material removal.
Is this structure already built? You didn't really ask... but I am wondering about the insulation and venting details surrounding what is drawn. It appears there won't be much room for attic insulation above the wall top-plates as drawn, and how are you venting? Just something to think about if you're still in design phase.
Tyler,
Yeah -sorry, my response wasn't clear. Justin was worried about too small a seat cut on the rafter tails. Since they aren't load-bearing, the only reason to cut them is so they fit over the top-plates.
Tyler,
It looks like I'll be able to stick to a 1 ½" heel cut in the tail pieces, and leave 1 1/5" for car deck below the sheathing. (My lumber was cut locally, and they did the 2x10's at 9 ½" instead of 9 ¼" for some reason, so that buys me a little room, too.)
There's actually plenty of room for insulation. The walls are 10'3", to match the house we're adding to. But the ceiling height is 8'9". The ceiling joists will be a lot lower. They aren't shown in the drawing. Venting will be soffit to ridge. It's an unconditioned attic.
thanks for looking.
Justin
In seismic zones (much of AK), it’s usually required that the roof diaphragm be tied to the wall diaphragm.
So, the blocking should be full height and receive boundary nailing from the roof sheeting, and the blocks tied to the top plate with something like a Simpson A35 or LTP4. Once upon a time the blocking could hang below the top plate, angled as mentioned above, and face-nailed to the plate.
The idea is that lateral loads transfer across every transition: roof, wall,floor,wall, foundation, with loads accumulating from top to bottom.
Drilled, wire-mesh-backed blocking for eave ventilation is common, since the top edge seals against the roof ply. And ridges often need to be boundary nailed as well, so no leaving the ply short for a ridge vent.
The details on this site, and conditions often discussed here, are often based on places other than the west coast.
Just encountered this wall/roof diaphragm connection on my own build. Curious, how would you achieve the same thing with a raised heel truss where solid blocking wouldn't be tall enough to span from the top plate to the roof sheathing?
Truss blocking panels can be provided by the truss supplier, or, in effect “mini walls” built between truss heels: a four-sided 2x4 frame with a ply face boundary nailed all around. Either use a sill (sole plate) nailing schedule to anchor the bottom of the frame to the top plate, or some kind of surface hardware (like an LTP4) applied across the joint. Uplift from overturning moment should be considered, since these are probably near-square in aspect ratio? If truss heels are held down with some kind of uplift anchor, that should suffice.
Some situations have the heels without rafter tails (perhaps added later, or with plans for a mono truss added for a porch...), and there the “little walls” can be sheathed with continuous sheets, or the heels themselves used as the studs for the wall extension- just add blocking as a roof-level top plate.