Don't know how the wind is in your area, but where we are, those south winds can really be a booger. I'm a "first-timer" too, but think putting rafters at least 24" apart would be structurally more sound than farther apart. We plan on putting ours 18" apart, which probably is really over-kill, but both of us feel more comfortably doing it that way, unless however, someone can convince us to put them farther apart than that. Good luck and send some pics as you go.

What happens is that the snow or ice will bow inward and create stess to the screws/nails weakening the connections and possibly even breaking away the ends of the panels at the screws. You might be thinking of attaching to the wide face of 2x4" framing but you'll get better support and more light by framing as in a house with 2" edges out.

Your delema is one reason why wood framed greenhouses are not often built. With an arch metal structure 4 ft spacing is quite typical and the poly film stretches and gives with a reasonable snow load. I insert 2x4" props under arches at a few points of my 30' GH just as insurance to prevent the structure colapse but the plastic film (the replacable part) usually never fails first.

I built a 12x12 wood-framed GH with twin-wall polycarbonate on the S and W walls and 8-on-12 pitched roof. I used treated 2x4 rafters on 24" centers because of concerns about wind. My county bldg inspector required this and a carpenter I hired to help (who wanted 16" centers) reluctantly agreed. I used doubled 2x4s on gable ends and galvanized brackets to reinforce attachment of rafters to ridge and eaves. I used aluminum T-bar extrusions to hold polycarbonate edges to 2x4s and 9" spacing on screws on T-bar, middle 2x4, and ridge and eave beams. In ~20 months I have been happy with the results.

Don't skimp on the type of wood you use on your rafters. You can use regular pressure-treated pine elsewhere on your structure, but for your roof, select something like Douglas Fir or Western Larch. These woods have superior span strength.

-Bruce

Hi
Have built structures from both aluminum and PT wood .The aluminum is almost 20 years old while the wood is long gone after 10. All things considered including price don't think I'd use wood again.
I have no snow loads but do have high humidity,intense wind and best of all Carpenter ants lol. My two cents gary

Just an FYI about pressure-treated wood: the stuff you buy from Lowe's or Home Depot is not of a grade meant for contact with the ground, or below ground. On the yellow tag stapled to the end is a tiny number beside ACQ (alkaline copper quaternary compound) which will read .25 or .40. The .25 treatment is only for above-ground, and .40 for ground contact. There are also .60 and .80 treatments, but those are more difficult to find and quite a bit more expensive, and are meant for marine use.

In the years I've used PT wood in my gh structures, first in Vero Beach, and now in the Panhandle, I've never had a carpenter ant or termite give the wood a second look. (As a matter of fact, I once had termites destroy a piece of untreated plywood attached to a PT 2x4...ignoring the 2x4.) The termites are thick here, and will quickly make short work of untreated wood. If it's treated to a sufficient degree, the copper acts as an effective detterent. I have seen ACQ .25 2x4s begin to swell and weaken when half-buried, after perhaps 7-8 years.

BTW, when using PT wood, you must use either stainless steel fasteners or ones that are coated. The ACQ will cause ordinary fasteners to corrode.

Depending upon how you've designed your roof, the span, and the pitch, you may or may not need to have it strong enough to support your weight on the rare occasion you need to climb up there. That's why the selection of denser wood grades may be important.

-Bruce

Bruce
You must have those sissy type carpenter ants lol I can't really blame them for the destruction of my wood structures. Dry rot or whatever .My aviary made of PT is still holding up but is sure looking sad.
have been thinking of expanding the shadehouse and had pretty well decided on aluminum since the attached one has stood up so well. What would be the advantage of wood??
thanks gary

You are right -- the carpenter ants up here are nowhere near as voracious and aggressive as the ones in south Florida! I can remember my arm covered with a swarm of them, on more than one occasion, before I knew what was happening. If those soldiers get you in the right spot, they can draw blood. Still hate fire ants more, though! (I'm just now recalling a couple of encounters with those nasty south Florida bull ants too -- sting is 10 times worse than a yellowjacket!)

The advantage of wood is that, if it might be necessary, can be easily redesigned and/or retrofitted. If you're satisfied with what you've got, and no need to change or add something, then there's no advantage.

For someone considering a ground-up, do-it-yourself gh, wood is the clear winner. Cutting metal, drilling holes, covering mistakes without throwing away whole pieces, expense if it's a larger structure, sealing cracks in colder climates, etc., etc., makes aluminum a poor choice for most folks.

-Bruce

Check your local building code before spacing the rafters that far apart. Good luck.