Going back to the original question by JEmH, if the roof framing system is NOT designed as a truss, you can not have just the ridge plate, two sloping roof rafters and a horizontal ceiling joists on bearing walls. Oversimplification is absolutely necessary in wood design as there are lack of homogeneity in the makeup, many different species that are most commonly visually graded, no ways of determining any splits or checks inside the member, direction of cut and many other issues. RE: Rafter stresses in hip roof construction SacreBleu (Structural) 12 May 05 18:28 Needless to say, it probably drove the framer nuts. It was his new house and had to be right. Once, he even brought some tools and added a few struts and braces where he thought there should be some, climbing all over the roof framing to do so. He would write all sorts of notes on the framing, instructing the contractor to do this and that, or he would call. During the carpentry framing, he would spot all sorts of things that 'just didn't work' in terms of good engineering design and proper load path. He would leave work every day to go inspect the progress. some axial perhaps is produced.but it may be that the amount of axial is negligible with regard to the stresses induced by the flexure.just a thought.Īs a side story to this, I once had an engineer on my staff have a new house built. But this is usually taken out by the roof sheathing in my opinion. Hip and valley beams taking compressive loads from intersecting rafters - the fact that the rafters are at 45 degrees (or whatever) to the supporting ridge/valley beam does cause axial force to flow into the supporting beam.

The idea that a ceiling joist and a rafter somehow form a truss, and thus add compressive load into the rafter would only be accurate if there was some mechanism to develop the horizontal shear through the 'truss' - without any diagonals, the two would tend to act more independently with no truss action. I do believe you are fairly on-target with the idea that the simplified analysis is just trying to allow some expediency in design.and also allow for long-term experience. Rafter framing, in actuality, has all sorts of complex load paths running through it. (At least here is Pennsylvania, where, I should point out, a statewide building code is fairly new.)Īnyone have any insight or observations, or should I just slink back into my world of concrete? RE: Rafter stresses in hip roof construction JAE (Structural) 12 May 05 16:08 I think that residential plans that wouldn't stand up to engineering analysis are being reviewed and approved by code enforcement officials. Apparently, the P in PE stands for paranoid. (Every other roof nowadays.) Or am I missing something? Yet I can find close to no code requirements to 'design' these beams, even for a complicated roof, where the load paths might be nonobvious and circuitous. Am I to assume that neglecting this is an expediency that the codes permit based on experience? If so, I can buy that.īut here's where I'm really troubled: it appear to me that hip and valley beams will have to resist the end reactions of all the rafters framed into it, which again might be combined bending and compression. Two rafters and a ceiling joist are acting like a truss, so there is also axial force in the rafter. This seems fine except that I'm not seeing the rafter as a simple span. I note that with few exceptions, the spans seem to be calculated by treating the rafter as a simply supported continuously laterally braced beam using typical NDS Fb values with appropriate C's for duration, repetitive use, etc.

Tops of rafters must be fastened to each other or a 1' ridge board, hips and valleys must have a 2' board as deep as the cut ends of the rafters, and the free ends of hip and valley beams must somehow transfer load down to a wall or something. There are few caveats on their use: The whole system must sit on joists with sheathing, ties, or something that resists horizontal reaction at the lower rafter ends. These list the maximum horizontal span for a roof rafter given species, grade, dead and live/snow loading, and spacing. In almost any residential construction code or publication, (I'm thinking of the IRC, specifically) there are rafter span tables. This question has bothered me for some time, and it seems to be coming up more often, so I guess it's time to delurk.

Rafter ties in hip roofs