I am an engineer and since your roof could be constructed in a number of
different ways this could be an easy problem to analyze or a difficult problem.
(I have seen roofs constructed the same way birds build nests- just nail
enough pieces of wood together that they all hold each other up and it defies
rational analysis.) If the original structure is 2x4's then it is probably a
truss.
I can't tell you if you can or cannot get by with what you are wanting to do
but I will tell you what I did. We have a 30 year old house with a 2-car
garage. I wanted to pull the engine out of my MG Midget and I have been in the
attic above my garage several times with my 200+ pound weight standing on a
single ceiling joist (bottom chord or a crude truss). I figured the engine
certainly weighed less than twice my weight so I connected between 2 trusses
with a wood beam I made by gluing a pair of 2x4's together and I hung my hoist
from the center of that. The OP has the advantage of having a long item that
can be hung from maybe 5 of his trusses. He can test each truss individually
with his own body weight and if that works then he can expect that if he
distributes the load evenly between all of the trusses then he could probably
support the entire body. The key words here are DISTRIBUTE EVENLY.
Of course without proper analysis to prove that what you are doing is totally
correct, many precautions would be prudent. First of all this should be a
temporary situation- lift the body, roll out the chassis, lower the body.
Second do not let any person be under the body while it is hanging. Wood
unlike steel or concrete does change with short time and the building codes
will allow more load onto wood framing if the load is going to be there for a
short period of time. (For instance roofs with snow loading are designed
differently from floors with permanent loading.) What you can get by with for
a couple of hours might be different from what would be safe for 6 months.
> > I want to hang a Datsun Roadster car body from the joists above my shop
> > and garage space so I can start working on a frame in the shop.
>
> For what it's worth, I lifted the body of my TR4A from the garage joists.
> I'm not sure what the stripped body weighs, but since I could fairly easily
> lift each end, I figured no more than 4-500 lbs. And since I can stand on
> one of the joists, and I'm a bit north of 200 lbs, it seemed OK to me. I
> didn't leave it to hang, I put sawhorses under to support it. I put some big
> screw eyes into the joists (one at each end of the car), and used two block
> and tackles to raise/lower the body.
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