Do I need purlins if rafters run perpendicular to roofing?

Guest User
9/25/2017
Hi, I'm building a boat shed, 16' wide x 28' long, with a monoslope roof pitched 1.5/12. It's an add-on to an existing shed that has that same pitch, so it's an extension. And the pitch runs the 28' length. And of course, as a boat shed, there aren't any posts supporting the center. Since it's an add-on I'm running a double 2x8 set on top of the existing shed wall and poles (utility poles) as an end beam on one side, and another double 2x8 on top of the new poles as the other beam. And I'm planning to have the rafters running the 16' width of the shed at 16" OC . We get quite a bit of snow and I've run the snow load calcs for 50/sqft, so those rafter should be 2x12's for that 16' span (if I did the calculations right!). So can't I just attach the corrugated roofing (26 gauge) directly to the rafters? I guess I just don't see why that wouldn't work well, since the strength of the corrugated panel is lengthwise anyway, and it's just 16 OC. But in this situation most folks seem to run the rafters lengthwise, along the 28' pitched run, then do 2x4 purlins. But that doesn't seem as strong to me, I'd get a ton of sag over 28', and it would require a lot of extra dead load. Am I missing something? Thanks!
Info @windowsonwashington.net
An informed customer is our best customer.
9/25/2017
Opening up for comment. As far as the load calculations go for your region, best bet would be to check with permitting or a local engineer. Once you have the snow loading totals, you can call the panel supplier and see what they recommend for compliance.

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