I don't have any experience with the weight of snow on a roof, But I would expect if the greenhouse is heated, and the roof is pitched sufficiently, that you would never really get a buildup because the heat from the greenhouse would melt the snow to fast. But here's a link to the greenhouse I'll be building. Not as big, but the same basic idea. For some reson I cant post the exact link, so you'll need to fallow the directions to get to the page.
Horticultural Engineering - Rutgers University
First Scroll down the page and click on "
2003-2007 High Tunnel Project," then scroll down the page again and click on "
Pictures of High Tunnel Construction and Preparation"
Here are some other links about greenhouse strcutures I had bookmarked
FREE plans of PVC pipe structures, greenhouse, cold frame, furniture fittings
Arched Greenhouse
Greenhouse Presentations
Reference Sources
GPN Magazine structures zone
Greenhouse Energy Cost Reduction Strategies
Missouri Alternatives Center, Alphabetical Links - AgEBB (click the big yellow "view now" button)
P.S.
An important factor in how a plastic covered greenhouse holds up in wind, is how tight the plastic is stretched. If it's loos it flaps in the wind and tears/rips easily. If it's tight fitting, it wont flap in the wind. Then you mostly just need to worry about things being tossed around by the wind hitting it.