Rafter Thrust Question
Rafter Thrust Question
(OP)
I have a 4/12 roof conventionally framed with 2x6 (actual) rafters at 24" O.C., 2x8 ceiling joists, and a 1x6 ridge board. They want to make one side of the ceiling vaulted to follow the pitch of the existing roof rafters so they would have to remove the ceiling joists on one side. Obviously when removing the ceiling joists on one side you increase the thrust to the wall from the rafter. There is an interior wall about mid span that was supporting the ceiling joists and still is supporting the o=one side of ceiling joists to remain.
If I design a pony wall to extend above the interior wall and support the ridge (or very close to it) will this greatly reduce or even eliminate the thrust from the rafter at the top plate? The way I look at it is if you had a ladder against a wall and you permanently supported or fixed the top end then the bottom end wouldn't be able to slip out right? Am I missing something here?
If I design a pony wall to extend above the interior wall and support the ridge (or very close to it) will this greatly reduce or even eliminate the thrust from the rafter at the top plate? The way I look at it is if you had a ladder against a wall and you permanently supported or fixed the top end then the bottom end wouldn't be able to slip out right? Am I missing something here?
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One concern for me, what is below the interior bearing wall for support? What happens now that it sees roof live and dead load as opposed to just ceiling dead load?
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Also, if the vaulted area terminates at an exterior gable wall, the double plate will no longer be braced out-of-plane by the ceiling diaphragm.
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Dik
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Convince yourself that you have vert. reactions at both ends of the vaulted rafters, by analysis and by bearing details, and you should be o.k. Be careful not to overcut horiz. seat, bird mouths, at these bearing points. Several of the suppliers have rafter/joist bearing hardware to accommodate this condition. If you have any vert. settlement at the ridge reaction, on the vaulted rafters, this will translate into a, funny unequal (indeterminate), thrust or geometric movement out at the exterior wall bearing points, the rafter doesn’t change length. The roof framing on the other half of the bldg. has to be trussed in some way, or you have an unresolved rafter thrust there too.
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As for the rafters on the vaulted side: typically insulation/ventilation requirements set forth by local code/architect will govern rafter depth. We normally are forced to use 2x10 or 2x12 based on insulation/ventilation alone. Nice and stiff before we even put our calculator to it...we sleep nice knowing we shouldn't get call backs due to cracked drywall ceilings.
XRs second point is the hidden thorn on these vaulted ceilings. In the past we've had to get creative to solve that problem...we try to get the double top plate to span horizontally, many times that's not enough or we can't trust we'll find a lap-free length of double-plate over the desired span. Glad you don't have to wrestle that one.
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If I can't span the dbl. plate horizontally, I usually have them sister each stud with a full height 2x6 (cut thru thru the dbl. plate) which converts it to a balloon framed wall.
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"I usually have them sister each stud with a full height 2x6 (cut thru thru the dbl. plate) which converts it to a balloon framed wall."
How does this solve the dbl. top plate spanning horizontally issue?
Thanks,
S&T
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Good idea.
As far as insulation goes, you can always use spray foam if you do not want as much rafter depth.
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Where is the thrust now? The wall is offset and not directly under the ridge but it is fairly close. The wall is still supporting a good portion of the roof system but there still is potential for the ridge board to deflect causing thrust right? Does the ridge board now act like a "beam" in this situation? What is the best way to analyze this situation...Im thinking scrap the pony, add a structural ridge beam and call it a day...thoughts?
RE: Rafter Thrust Question
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.