I am framing a 5-1/2 in 12 hip roof on a 28'x35' building where the hips will be almost 25'long including the overhang. The ridge is supported, so I'm assuming the hips will act as an inclined beam. My question is what material must I use for these long hips? LVL's? Doubled and spliced lumber? What if I ran diagonal ties across the wall plates? How about using a 2x10 for a second top plate, to resist the outward thrust of the rafters? How is this usually dealt with without using costly engineered lumber?
Andy, all good questions. If you say the ridge is supported this means there is a bearing wall below that it will be braced to. Possibly try and get an angled brace from that wall to the hip, keep it close to a 45* angle. If that is not possible you will have to use doubled up 2 x 10 or larger,(no splices unless you can get a supporting bracing directly under the splice) or an LVL or MicroLam. This should be sized of course by an engineer. You will have ceiling joists that will tie from common to common, splicing on the center bearing wall. When close to the hip you will have to return the ceiling joists the other way, make fingers that attach from plate to the closest joist you can get to the hip. On the top of the main ceiling joists run a strong back down the center,one board flat and one nailed to it, on edge. This should tie from king common to king common(the ones sitting on the 28'wall). Set up your joists like that and you will stiffen up the walls from outward thrust. Using diamond ties( the diagonal braces) will not be neccessary. And since you mentioned them I suppose your ceiling isn't being drywalled, otherwise they would be exposed. Two by Ten would stiffen the wall but not, alone, prevent outward thrust.
Thanks for the speedy reply. It all makes sense, but after thinking more (probably
too much) I come up with another question. If the walls are adequately tied to
resist outward thrust, the hip (theoretically) doesn't bear any load, and
therefore shouldn't have to be very big, just as the ridge in a gable with joists
perpendicular to ridge can be a piece of 3/4 stock. Can this same logic be applied
to a hip? What I am getting at is can I use a spliced doubled 2x8 (without
support under the splice) instead of ordering a ridiculously expensive LVL? (I
know the final answer comes from my bldg inspector, but it's interesting to know
if anyone else out there thinks like I do).
Thanks for the input
I think you are asking if the hip acts as a post rather than a beam.
On a steep roof it will act as a post. On a flat roof as a beam.
I read recently that to size a hip you use the horizontal projection of the structure to compute the loading and bending. (I looked for the link but...)
So the loading on the hip runs along the following lines. The hip projection is (14*1.4) 20'. The roof loading might be 30#/sq ft projected in the horizontal plane. The loading on the hip runs from 0#/ft at the bottom to (14*.5/1.414*30) 150#/ft at the top (plus something for the eaves).
(I sure hope I did that math right.) You solve the beam sizing problem.
I have problems with this method of solution (it gets the wrong answer for the work I do), but it is acceptable to the code authorities here.
Andy, depending of course on pitch and length hips can be very strong and self supporting. But when they are long and of a low pitch such as your situation they will need support or be calculated for size and load. I remember building a 4/12 hip roof with a mission tile roof. The plans went through plan check and the frame through inspection and was passed. However when the roof was covered with the tile one hip sagged a good 2 inches. The hip was not exceedingly long, 14' 9, 2x 10, unsupported.(Ok don't worry my memory is not that good, I just happened to have those old plans nearby and looked it up). So even if it passes plan check and frame insp it is no guarantee it will be strong enough.