Rafter Spacing

[LeeB]

Not a timber frame. Stick built. Wondering what the minimum spacing for rafter ties would be on a 12' x 30' chicken house. Snow load is 15 psi. Red oak 2x6 rafters, 9/12 pitch. Sheet metal roofing. 

[Stephen1]

You actually have a snow load down there? ;D

[Don P]

Residential building code says every rafter pair. Strictly speaking if there is something like a ridge or plate that can transfer the horizontal thrust to the tie then you can go further. The tie connection load increases.

Back in the day we would go every 4' and tie well counting on the ridgeboard acting as a beam through that 4'.

[LeeB]

Thanks Don. I already figured on 4 ft. The customer wanted to go 10 ft. but I told him no way.

[Don P]

It's small, let's see what it would take.
Total load is going to be 10psf dead + 15psf live=25psf
The ridge supports the center half of the roof so a swath 6' wide.
6'x10'=60 sf x 25psf=1500 lbs
Go here and plug in, I used full 2x10" mixed oak and passed.
http://forestryforum.com/members/donp/ddsimplebeam.html

Now, what is it going to take to resist the horizontal thrust
http://forestryforum.com/members/donp/RafterThrust.htm
144 width, 25psf load, 9 pitch, 120 spacing=1000 lbs thrust, that is the rafter to tie connection requirement.

Hmm, got another calc hidden away for this;
http://www.timbertoolbox.com/Calcs/slopedrafter.htm
15 LL, 10DL, 120, 72(span of each rafter), 9 pitch, 2x6, #2 mixed oak, I chose 16d sinkers, looks like 8 nails in each end, tie to rafter, would work. 14 if you built it like a truss, which wouldn't be wrong, just overkill here.

Your call, I was just playing with it, basically I just made the ridge into a designed ridge beam supported by trusses at 10' spacing, run a collar tie directly under the ridge on those tied trusses. The ridge beam is capable of hanging the common rafters between those trusses. Don't do this on a house without an engineers okey dokey.

[LeeB]

So, it can work with only two after ties at 10' and the end walls? Common rafters on 2' centers with 2x10 ridge board?

[Don P]

Yes

[LeeB]

That will make the guy happy. That's what he wanted. Thanks for the help Don.