Rafter Thrust Question

[kmart30]

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?

 

[kmart30]

Not an insulation expert but will spray foam give you the required R-30 in the ceilings?

 

[XR250]

I believe it is R5 or R6 per inch so yes.

 

[jayrod12]

As it turns out, the NBCC doesn't specifically exclude using spray foam, however Clause 9.19 mandates that proper venting is required for roof spaces. Installing spray foam directly against the roof sheathing does not allow proper venting. Therefore, not allowed.

 

[kmart30]

Ok it appears that the geometry of this whole thing has changed. See the link below...

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?

 

[KootK]

Still negligible thrust if the left rafters can cantilever to pick up the right rafters. That said, ridge beam is a simple, robust solution.

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.