How much load can this support?
 

My garage is 20x20 concrete circa 1930's and the roof is pitched in the traditional triangle shape.

The garage as it is laid out now has a loft and the roof structure is supported with 2x4s. The wood beams lay on top of the concrete.

How much weight do you think i can put up in the loft area before I should worry about it?

It does not have beam support in the center of the garage

Thanks!

So what you're saying is... You're garage has concrete walls with 2x4 rafters and 2x4 rafter ties and the span is 20'?

yes, It may be 18-20 ft. def. not more

In that case, it probably can't support any weight. The 2x4's are only meant to keep the walls from spreading and that's it. Anything you put up there will cause the rafter ties to sag.

but its a concrete block structure so i dont think it is going to do much

It's not about the foundation, it's the 18-20' 2x4 that can't handle the weight.

Look up the "yield strength" for the wood you are using.

Just from the weight of the wood alone you should have a 0.4" to 0.8" sag in midspan.

For L/180 and 24" OC you can support a dead+live load of 7# PSF.

Its the 2x4's that hold most of the weight, not the concrete block
A 2x4 spanning 20' is not designed to carry ANY weight, not even a sheetrock ceiling
They are there to hold the walls from spreading apart due to the downward/outward force of the roof

Ok, I had a chance to look at it better last night. The Beams that go across the span are def. 2x4. Then there are 1x4 on every beam that tie into the 2x6 of the roof. It def. feels strong enough to support my 200lbs when Im up there.

Basically, what I am trying to do is make sure it is strong enough to support 4-500 lbs spread out across the entire span. Do you guys think it will be ok?

Thanks everyone.

How much does it deflect with this concentrated load? This value can be used with a formula

http://www.engineeringtoolbox.com/be...on-d_1312.html, see
Beam Supported at Both Ends, Load at Center

to give the modulus, E, of the wood. With E the rest can be figured out.

There is a big difference of 200 lbs Temp, & the weight being on there all the time
Time is what causes joists to sag & fail under lighter loads
What size are the rafters ?
Are you sure you do not have a truss system ?

PICTURE Please!

Be safe, Gary

Sounds like a truss. 2x4 joists would not last on a 20' span.

It's hard for anyone to tell you that you're OK to start stacking up a load on a system that's not designed for it. However, there are methods to increase the load bearing capacity of a truss. One strategy would be to take plywood sheets and attach them vertically to the truss. The top of the plywood needs to be cut at an angle equal to the roof pitch, and butted up under the roof decking. The bottom of the plywood would attach to the horizontal 2x4 joist. Somewhere in the middle, the plywood will attach to the 1x4 strut. Use lots of nails and liquid nails if you got it to attach the plywood. Do this on each side of the room and for every truss in which the new load will lie and you will have greatly increased the load bearing capacity.

Well here is a pic. You can see the rafter (2x6) way at the top then the 1x2 goes down to the 2x4 span

http://uploader.meinwirt.com/files/78/IMG00771.jpg

Where are you located ?---snow load ?
How far apart are the roof rafters ? 16", 24" ...other ?