huangyhg

gambrel roof framing
when framing a gambrel roof we always use a load bearing ridge beam with load bearing purlins to support a gambrel roof structure or use collar ties on each side of rafters at the hip joint between the upper and lower rafters. i was recently talking to a builder that had told me that he was going to use a load bearing ridge beam without the load bearing purlins or collar ties, or gusset plates at the hip joint of the upper a lower rafters, i told him i didn't think that was a good idea not support the joint between the upper and lower rafters. has anyone ever built or seen a gambrel built using a load bearing ridge beam without the load bearing purlins or collar ties or gusset plates support the hip joint?
this sounds to me like a bad idea. each rafter pair is unstable under lateral or unbalanced gravity forces. it needs a horizontal reaction at the ridge as well as at the plate to prevent collapse. it might be stable if the ridge beam is adequate for the horizontal load, or if the roof sheathing can transfer this reaction to the gable walls. there are many buildings built as you described, though with gussets of negligible moment resistance at the knee joint, which have performed well for years, but if you were to analyze them you would find them grossly inadequate. also, they sometimes become misshapen over the years, or even collapse. it's best to use gussets adequate to make each rafter pair stand on it's own.
is this a wood framed roof? if so, gusset rarely ever work. the alternative wood be to cantilever the outer rafter over a load bearing knee wall to pick up the interior rafter load. if you do this be careful of the uplift force at the exterior end of the outer rafter.
the only way i've seen this done successfully is to create a bearing wall close to each of the pitch breaks, supported by either a bearing wall below or beefy floor joists in the attic.

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