permaculture

[Lisa Rollens <rollens@fidnet.com>]

Stephen,

I just built a greenhouse like you describe, on the south side of the house, 
slanted roof, just not as large as yours.  I finally decided to use angle 
aluminum for the roof structure.  Moisture in a greenhouse would cause bad 
rusting on any steel.  Galvanized steel would work but it's about as 
expensive as aluminum and the aluminum is lighter in weight, easier to build 
with.  (cut with skilsaw, drills easily )  Wood roof structure would have 
created too much shade in the greenhouse.  My roof length was around 23 feet, 
so that would require more than 2X4s, probably 2X6s.  Too much shade when the 
sun is low in the sky, and like you said, requires painting and 
maintenance...and eventually replacement due to inside moisture.

I think the recommended angle for the roof is latitude plus 10-15 degrees, 
and since I built on a hill, it turned out to be quite a steep roof.  I don't 
have a snow load situation.....max would be 9 or 10 inches at any one time, 
very occasionally, but the roof is so sloped that snow can be brushed off 
part of it easily.  If that is a problem for you, you could use wider and/or 
thicker angle aluminum.

I also used all stainless steel fasteners...screws and nuts and bolts inside 
the greenhouse.  Expensive, yes, but i didn't want to have to replace them at 
heights of 12-13 feet.  i've had a bit of experience with a nailed wine rack 
in a storm shelter with a dirt floor.  The nails rusted to the point that the 
whole thing fell apart in about 3 years.

Bottom line is that I didn't want to do this again in 5 years!  Hope this 
gives you some ideas........Lisa, in the MO Ozarks


On Jan 29, 2012, at 12:07 PM, Stephen Sherman wrote:

> 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
> 
> 
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