I am building a lean to on my 30x30 shop that will have 2X12 rafters hung on 6x6 posts with 2x6 purlins 24 O.C. The post spacing is 10'. I would like to maximize the width of the lean to and I am wondering how wide I can make the lean to with a 2x12 rafter with 2x6 purlins. I am hoping to have a clear span of 12' 8"
Project info:
-Roof dead load is 5 PSF
-Roof live load (snow) is 40 PSF
-Roof pitch: 4/12
-Open sides on lean to
First, What you are asking is going to have to be engineered, by an engineer. The rafter tables have been "pre engineered" as to not needing an engineer to do so.
Second, doing some basic engineering myself, you are no where close to a size of rafter to span 12'8" with an 9'5" oc. This would be in the glue-lam range or LVLs, or a half truss. Rafters on 24" centers would work well however. I would guess they method you have drawn was on an old 50's/60's pole buildings. They have much less stringent design loads then. I have not seen a building here after the 80's built like you have shown, very few of the 70's barns were either.
Third, you calculated 45 percent better cost, but you didn't know the rafter size(you assumed), It would have to be high grade wood as well which would be special ordered(i saw no mention to the grade of wood your wanting to use) and expensive. Also did you down size the "purlins" for 24" rafter spacing? You wouldn't need purlins at all, you could even get away with 1x4's, more to stabilize the rafters than provide support for the roofing with a 24" spacing on the rafters. Most used 2x4s. My lean too is made like kinglake showed with 2x4's instead of osb/ply.
Third, Why would you need a ledger? You place it on the outer truss support/plate of the existing barn. No birds mouth needed either, you use a 2x10 on the inner plate of the outer poles of the lean to, and a 2x8 on the outer plate slide up to the bottom of the rafters after they have been placed. Use a simpsion strong tie VPA clip to hold it down to the plates, stronger then birdmouth/toenail.
