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Rafter Thrust Question

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kmart30

kmart30

Structural
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Apr 28, 2016
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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?
 
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It would reduce the lateral load seen by the outside walls yes, as the rafters would no longer be relying on each other for support (like a three piece truss). Depending on the end condition of the rafters at the outside walls there still may or may not be lateral load induced but it may be significantly more manageable.

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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Ok good to hear. Yes the interior wall will now receive the roof DL & LL and is supported on a 8x8 sill beam spanning about 6 feet in between piers.

 
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But can the beam support the new loading? It never saw it before, it could not be sufficient.
 
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Yes I need to double check this as well. Will probably have around 500-600 plf on it from the new loads and would need to back out half of the original ceiling load. Also, its resting on 100 year old brick piers that appear to be in fair condition (not crumbling or cracking but some weathering from being in a crawlspace). Any ideas on how to approach determining weather a 100 year old pier is suffice? Obviously run calcs to see but how do you put a fluff factor on old piers? Might just recommend new CMU piers and be done with it.
 
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The other thing to consider is that the span/deflection ratio requirements are different for rafters with a sheetrock versus no sheetrock (L/240 versus L/180)
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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Those are good points that I missed as well XR.
 
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Yes thanks for pointing this out. We dont need to worry about that on this one.
 
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also ventilation issues...

Dik
 
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Kmart30:
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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I have taken this exact approach in the past. Thoughts so far on load path to the foundation via the central wall are good ones...definitely need to convince yourself you've got adequate support there (this wall now sees half of the roof load). In the past we have abandoned the old piers and gone in with new 6x6 posts on new concrete piers.

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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FoxSE14 said:
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.
Click to expand...

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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XR250, I am having a hard time picturing what you mean by

"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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The full height studs extend up to the roof diaphragm, negating the need for the double top plate at the ceiling height. I've done this detail lots. Although sometimes I'll provide built-up full height studs at a spacing that works for the top plate allowable spans and then connect the top plate to these studs. Minimizes the number of cuts to make through the existing double top plate.
 
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Good point on the insulation, building code says you need a min. R-30 insulation in the attic. Im not sure how thick a batt of R-30 is but I know its more than a 2x6 or 2x8. Im thinking I need to double check all my field guy measurements on this one. The location or this interior wall is essential and I dont want any oopsies!
 
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For R-30 in Batt you'd likely be looking at R-36 (an R-14 and R-22) and that would be 9". So you'd need at least 2x12 to have air gap.
 
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jayrod12 said:
Although sometimes I'll provide built-up full height studs at a spacing that works for the top plate allowable spans and then connect the top plate to these studs. Minimizes the number of cuts to make through the existing double top plate.
Click to expand...

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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There's a clause in the code somewhere about spray foam not allowed to be used directly against the underside of wood sheathing. .. at least in the nbcc
 
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Its done here all the time - must be different code provisions
 
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