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(2021) R802.4.4 says "Where the roof pitch is less than 3:12 (25-percent slope), structural members that support rafters, such as ridges, hips and valleys, shall be designed as beams, . . ." The flip side of that is that where the roof pitch is 3:12 or greater, hip and valleys don't need to be designed as beams.
So what exactly does that part in red mean? Is the "depth" referred to the depth in the usual sense, measured perpendicular to the length of the hip rafter? Or is it the vertical cross-sectional height when the hip rafter is in its installed position?
<polite applause from a fellow canuck>
I concur.
That is for nailing purposes, not sure it has anything to do with sizing for strength.......?
Seems like the Canadian code answers that question nicely. I like that.
There's also R802.4.2, which says in part "Rafters shall be framed opposite from each other to a ridge board," Are we to conclude that R802.4.2 requires a gable roof and prohibits shed roofs and hip roofs?
No, but there are options to achieve compliance prescriptively for rafter tie-in. Not so much for sizing the hip rafter. You can always use the WFCM as a referenced standard. There are significant differences between the IRC and WFCM for hip roofs.
Suppose the roof slope is at least 3:12, and hip rafter sizing complies with R802.4.3. If you want to call out the hip rafter sizing as a violation, what section of the IRC would you cite?
No responses to the above = no IRC section to cite = no violation on hip rafter sizing, R802.4.3 suffices?
Doesn't work that way in the IRC Wayne. The IRC is a prescriptive manual. If it is not in the book, then you have to go to another referenced manual or engineered design. Lack of a code section does not constitute a de facto approval.
OK, but R802.4.3 does address hip rafter sizing, so the IRC is not silent on the topic. Doesn't the statement "the size must be at least X," in the context of the IRC, mean "if the size is at least X, it is prescriptively allowed"?
R802.2
802.4.3 is a section for minimum requirements that have nothing to do with sizing for loads imposed. If you wanted to apply R802.4.3 to valley rafters, we would start having a lot of structural problems. Add areas that have ground snow loads and it gets worse.
One might look at any ridge and have the same thought--yet not every ridge is required to be designed as a beam. In fact as noted previously, R802.4.4 tells us when ridges and hip rafters have to be designed as beams--when the roof slope is below 3:12. So for roof slopes of 3:12 and above, the hip rafter does not require designing as a beam. The IRC is indicating It functions more like a ridge board. [Caveat: I have not look at an engineering analysis of how the forces resolve in a hip roof--it is fairly complicated.]
Just to clarify, the terminology you want here is "power law" not exponential. For a simple hip of length L, if it needed to be designed as a beam, the tributary area is 1/2*L^2. An exponential relationship would be something like the area goes as 2^L, where the variable L is in the exponent.
It is referring to the open cut of the rafter that rests against the hip beam. You can't have the cut longer than the face of the hip beam, otherwise you don't have full bearing.
Per IRC 802.4.4, only when the roof slope is lower than 3:12. So I agree for that case, you'd need to use something like the WFCM. For steeper roofs, IRC 802.4.4 is telling us we don't need to design the hip like a beam.
