How to compute a roof's load capacity?

[rekees4300]

I have a 40' X 80' shed that has a metal sloped roof supported by wooden trusses spaced 5' apart. The span is 40'. The shed was built by a previous owner and unfortunately I don't have the shed's construction specs. I would like to know how much weight the roof can safely support in order to determine when it's necessary to remove snow/ice using a roof rake & ladder (a PITA task). How is a roof's load capacity computed?

I realize the depth of snow isn't enough to determine it's weight because of its moisture content. Wet snow weighs much more than fluffy snow. It would need to be collected from a measured area, melted, then weighed. One square foot of water one inch deep = 5.2 lbs.

 

[beenthere]

If me, I'd take the design (pics, drawings, pitch, lumber dimensions, and grades stamped on the lumber plus roof makeup) to a local truss builder, and ask them to run some numbers.

They should be able to do that, and hopefully would do it gratis.

 

[Tig]

It should be specified in your local building code although I've never tried to figure out specs of an existing roof. If you are talking about engineered wooden trusses they may have a manufacturer's stamp on one of them. The manufacturer would have the details.
Another thought is that around here, a copy of the truss specs must be submitted during the framing inspection part of the building permit process. Call you building inspector and find out if they do this.

 

[Egon]

You may also have to account for wind loading.

 

[rekees4300]

Okay thanks for all the replies. I know there are many variables involved. Hopefully I'll be able to determine the roof's load limit.