1. The live load must be taken from your local code requirements, I suggest you get a copy of the code, and discuss your project with the local building inspector.
2. I don't know how you developed a dead load of 20 psf, that is relatively high for residential. You may be confusing dead load with live load, or perhaps you are not working on a residential project.
3. I am not sure what exactly you mean by a composite joist, if you mean you will nail two 2x10x24 ft joists together, is there a question somewhere in that sentence?
I hate people who do not put a queston the right way. I just did the same, I am sorry.
Myself: previous experience - building a 8x12 shack, participating to building of a large garage (28x28 foot print), gambrel roof (ha, ha).
The house that I have bought, L shaped, with a garage under the same roof, but with only two walls. The "third wall" is, in fact, three 4x4 posts, supporting the roof.
All the house has 10 ft tall rooms, including the garage.
The roof above the garage has 8:12 slope.
I intend to build a bedroom on the first floor, and a second bedroom, in the attic. The size of the garage is 24x24 and there are ceiling joists, spaced 16 inch, 2x10 of 24 ft.
Because the attic space is not proper for the room, I will lower the joists by two feet. There will be an inner adjacent frame, 8ft tall, that will be the load bearing. The existing 10ft frame will be a small suport for the roof rafters - the bird mouth in the rafter is set on the uppper frame plate (sill?). The real support for the rafters will be a V support between the rafter and the joist and the upper ceiling joists of the attic room.
Right now, Architectural Graphic Standards by Ramsey & Sleeper mentions, for a span of 24 ft, joists of 4x10 24ft for a 60 pfs live load. For residential a dead load of 20 pfs is for heavy snowing areas but, I survived two hurricanes right over my house and, I am sure that the house is calculated for more than 20pfs (is still upright)
I know that it might not be a liniar relationship between the size of the joist and bending stress resistence or deflection, but doubling the joists (right as you said, nailing two 2x10 24ft as a 4x10 24 ft) will do the trick. I was looking for a confirmation, taken from some detailed tables.
Thank you for answering my post and I would be grateful for any help.
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