Take a look at the attached, which is a plan view of the roof hip corners we are stick framing now after having erected trusses.
It is a 4-1/2 pitch roof, with a 13-inch HAP everywhere. The truss engineer has sized two-ply hip trusses, and we have 40-1/2 inches from studline to inside face of subfascia. There is one pair of hip jack trusses at each corner, but from there on out we are sticking it.
The rafters we’ve to cut and hang will hang on the hip truss and subfascia only. The large HAP prevents any bearing on the wall below, and besides, we’re so far out we’re mostly off the wall.
Our design snow load, per the ASCE calcs, resolves down to a 65 psf loading, and I have shown in the diagram the tributary area and live load that is being picked up by the subfascia and rafters.
We have put on a single 2×6 subfascia everwhere, but in coming into the corners, I want to back up the 2×6 with a 2×8 behind it, just between the corner and the first inboard hip jack truss. My framing partner says no, let’s just do the single 2×6.
What would you do?
Gene Davis, Davis Housewrights, Inc., Lake Placid, NY
Replies
Gene, some quick engineering gives an approximate *eqiuvalent* distributed load on the beam (subfascia) of 250 lbs/ft. Considering the 8' span and a deflection of 1/4" you would need a beam 6" x 7.25". That is much bigger than I would have guessed, and I may have made some errors as my scientific calc batteries are dead so I'm using the freebie that came with my computer.
The formula is (width of beam) x (height of beam^3) = 2211. Technically, it's a moment of inertia (I) of 184.
Insert standard disclaimer here.
Mike
I'd do the single.
Is this a closed soffit system, or open?
If it's closed, you have the opportunity to run some cantilevered lookout block to help hold up the corner, but that might be overkill.
Build it, then jump on it. If it holds, it's good enough. If it breaks, then put the double in.
blue
Use 2x6 subfasia all the way. Use long boards and start them from the corner - in other words, no splices near the corners. You said >> The large HAP prevents any bearing on the wall below, << Just pad up the wall cap plate with multiple 2x4, or whatever so that the wall can give some support to the jack rafters.
Just pad up the wall cap plate with multiple 2x4, or whatever so that the wall can give some support to the jack rafters.
I'm with Matt, or stud up to them, one under and one along side would make a nice stable feature.
Wish I could have stuck around, looks like a fun project you have going! Of course you know I realized that you did not need another cook to stir the pot so I high tailed it out of there!!
Good luck Gene!
EricI Love A Hand That Meets My Own,
With A Hold That Causes Some Sensation.
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Here are a couple shots of the job from last week. At the one showing the hip truss, that is a 58 inch cantilever you are seeing, and the truss engineer tried sizing it with 2x6 top chords (note it is doubled) but reverted back to 2x4s after figuring he could get triangulation in the drop.
Bottom face of drop equals face of soffit framing, so the drop will all be buried in the soffit.
In the upper right quarter of the shot, you can see the first inboard truss tail, that of a hip jack truss, about six feet, or three rafter bays, away from the corner. By the time we are framing our two outboard sticked-in rafters, there is no wall plate below.
Gene Davis, Davis Housewrights, Inc., Lake Placid, NY
Yea - I understand about the "flying jacks"... :-) as a I did from your earlier drawing. I guess that one of the liabilities of those really wide eaves. Maybe it would help some if you were to shim solid between the lookout part of the hip truss and the side of the house wall.
Sorry to change the subject, but what brand of house wrap is that you are using? Is it the kind that is like a very thin foam? Just curious. Matt
That was basically what I was envisioning Gene. I didn't count on such a beefed up hip though.
Just run the subfascia as a single member. Take care to run a 16' section starting from the hip. Then install the jacks- I would use 2x4's and triangulate them too.
After completing the corner, including the plywood, you'll find that the jacks are strong enough.
Don't try landing any 747's on it though.
blue