INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

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?

RE: Rafter Thrust Question

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?

RE: Rafter Thrust Question

(OP)
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.

RE: Rafter Thrust Question

But can the beam support the new loading? It never saw it before, it could not be sufficient.

RE: Rafter Thrust Question

(OP)
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.

RE: Rafter Thrust Question

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.

RE: Rafter Thrust Question

Those are good points that I missed as well XR.

RE: Rafter Thrust Question

(OP)
Yes thanks for pointing this out. We dont need to worry about that on this one.

RE: Rafter Thrust Question

also ventilation issues...

Dik

RE: Rafter Thrust Question

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.

RE: Rafter Thrust Question

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.

RE: Rafter Thrust Question

Quote (FoxSE14)

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.

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.

RE: Rafter Thrust Question

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

RE: Rafter Thrust Question

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.

RE: Rafter Thrust Question

Thanks for the clarification Jayrod. makes sense.

RE: Rafter Thrust Question

(OP)
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!

RE: Rafter Thrust Question

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.

RE: Rafter Thrust Question

Quote (jayrod12)

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.

Good idea.

As far as insulation goes, you can always use spray foam if you do not want as much rafter depth.

RE: Rafter Thrust Question

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

RE: Rafter Thrust Question

Its done here all the time - must be different code provisions

RE: Rafter Thrust Question

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

RE: Rafter Thrust Question

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

RE: Rafter Thrust Question

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.

RE: Rafter Thrust Question

(OP)
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?

RE: Rafter Thrust Question

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close