permaculture

[Stephen Sherman <spsherm@msn.com>]

I am in the design phase for a solar heated greenhouse. Dimensions 
roughly 20' x 40' lean-to format on the south side of a barn. The 
location has high wind and snow loads, which need to be handled in the 
structure. Several options for the roof trusses/joists come to mind, but 
none seem like an obvious choice:

Simple bent steel tube rafters (commercial GH and high tunnel types) . 
Relatively inexpensive, long-lived, but structurally not sufficient for 
the loads I need, without several mid-span support beams. (may still be 
the most cost effective and minimal shading choice)

Custom steel trusses. Long-lived, quite strong but fairly expensive.

Steel channel trusses. Moderately expensive, quite strong, but seem like 
they may pose problems with cleaning and longevity (basically galvanized 
sheet metal, rolled into various U or hat shapes. Should be rust 
resistant, but the various faces may tend to collect dust/dirt and be 
hard to clean).

Wood 2x or truss.  Moderately expensive, (getting hard to find good 
lumber), not as long lived, although easy to work with. Trusses have 
cleaning and painting issues, will case a bigger shadow than the metal 
alternatives, and may require mid-span support.

There may be other choices as well, but these seem the be the most 
common ones, at least that I could find. I was planning on having some 
animals in the barn which attaches to the GH, and so corrosion and rot 
resistance are important factors, I believe. [As a side topic, does 
anyone know the current "wisdom" of having enclosed animal quarters in 
air circulation with a GH? I have heard/read conflicting reports on this.]

Thoughts/ideas on the truss/joint design and material? Any experience 
with good or bad choices here?

TIA