ejb said:
I have a 32x26 one story shop/out building with an almost flat roof (2/12 pitch).
I want to change the roof to a regular pitched roof like 8/12 and add some storage space as a second floor.
There are no internal posts currently and I'd prefer not to add any, so my plan is to remove the old roof which will leave essentially a 26x32 "box" with no top and then purchase some 26' long IJoists that are rated to cover that span and cover them with 3/4 TG plywood which will become the floor of the 2nd floor.
For the new roof I figure conventional framing, 16"OC joists 2x10's or whatever is called for....
Question is, by my rough estimating in order to put an 8/12 pitch over a 26' span it means each side will end up being almost 17' long. Seems to me, with a 17' span on the rafters, a knee-wall(or other support) will be called for...so, can a knee wall put put on a framed floor if there is no support directly under that knee wall? in other words, the knee wall will transfer some of the weight of the roof to the floor joists, but not directly to earth...is that common? safe? likely to be code compliant?
Thanks all!
(Yes, I'll ultimately get a building permit but need to design my solution first)
Your plan is pretty good and should work without any complicated engineering.
Just to be sure, are your walls stick build with studs on 16 inch centers with double top plates. Do you have any windows or walls on the side walls, and do they have proper sized headers? If so, this is good. If you have pole construction, then that will need some engineering to know what the poles will support based on there size and spacing. Then the purlines need to be sized according to the load you will be putting on them. This is beyond my skills with what information you've given.
The I joists will rest on top of the walls and tie the walls together. They will suport your second story loads and become the bottom chords of your new roof. This is extreme over build, but for your purposes, an ideal way to do this.
The 8:12 pitch will give you an inside roof height at the peak well over 8 feet, so that's plenty for storage. Going with a steeper pitch will give you more height, but I can't imagine why that would be important.
You'll need 16 foot long 2x6's for your rafters and cut them down to size.
You mention a hip wall, but I wasn't clear on where you planned on placing this hip wall. My first thought was above the exterior walls to raise the entire roof. This is possible, but tying it all together is VERY involved and would be the expensive way to go. VERY EXPENSIVE.
Placing your rafter tails above the exterior walls will be the easiest, cheapest and strongest way to go. Just tie them into the I joists and exterior walls with whatever overhang that you desire. Then build the hip wall at the one third point of your rafters.
With a 26 foot span, your half way point is at 13 feet, so your hip wall should be around 4 to 4 1/2 feet in from the exterior walls. This will give your rafters the support of bracing that a truss has because of the strength of your I joists. You will literally be resting the weight of the rafters on the I joists.
Eddie
