> It's important to get some terminology straight before answering this. Rafters
> form the pitch of the roof; joists are parallel to the floor and stop the
> walls from spreading out (and occasionally are strong enough to support
> storage up there):
>
> http://www.carpentry-pro-framer.com/images/anatomy-of-a-common-rafter.png
>
> The span of the joists is important. How long are they? 2x4s will hold a lot
> of weight but not if they're 20' long.
>
> Trusses are pre-manufactured as a triangle and might support a car body, might
> not. It depends how they are held together.
As you've noted, with common rafters the joist is primarily in tension,
and at minimum it's sized for that and a modest dead-load.
Maximum span for a 2x4 joist at 24in OC is going to be no more than
10ft, maybe less.
http://www.msrlumber.org/spantables.pdf
Trusses are a different can of worms, you'll probably have vertical
loading on the lower chord of a truss, they may or may not have enough
capacity (if the trusses were designed for a tile roof and the builder
ran out of money and instead you got plywood and composite shingles you
might have enough total extra dead-load capacity to lift a Suburban)
but, absent some real data to the contrary I wouldn't trust the nail
plates etc. that hold them together with much of a tensive load.
I'm not an engineer, but if it were my garage and if we are talking
about common rafters and a 9ft-ish-wide garage and the structure was
reasonably sound and didn't have some unusually heavy roofing or
snowfall considerations I'd probably put a few vertical 2x4s up to the
rafters (what's your rafter spacing?) to pass the load up to the
load-bearing part of the structure, in addition to doing something to
distribute the load across the joists.
John.
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