permaculture

[Stephen Sherman <spsherm@msn.com>]

I'd agree with the other posts regarding polycarb being the only way to 
go. How long polycarb will last varies with your UV exposure and 
weather, but most GH multi-wall panels are guaranteed against 
discoloration for 10 yrs. The GH polycarb extrusions have some other 
advantages, besides being multi-walled, such as uv coating on the 
outside, anti-condensation coating on the inside.

Cost is probably the only disadvantage. Polycarb twinwall is about the 
same price per sq ft as the standard sized double pane glass panels. 
However it is not code (or safe IMO) to put glass, even tempered glass, 
on a GH roof. You should have tempered laminated glass there, to prevent 
glass pieces from falling on people if there is any breakage (such as in 
a hail storm).  Polycarb is much less than tempered-laminated glass. And 
plastics of all sorts are much lighter and can bend without breaking, so 
the framing for them is much simplier and less expensive than the 
framing for glass,

Just some things to think about in designing your GH...