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I'm planning a permanent greenhouse structure that will be attached on one wall to my house. The estimated size will be about 16ft wide, 45ft long, with the lowest wall being approximately 9ft tall and sloping up to about 15ft. I will be fabricating/assembling the structure myself. I'm looking for advice on material choices for the frame.

There are several material options I've considered for the frame:

  • Wood (Cedar/Redwood)
  • Fiberglass Reinforced Plastic (FRP)
  • Rebar
  • EMT Conduit
  • Square steel tubing

The roof and walls will be tempered single pane glass.

I've mostly disregarded wood and FRP as viable options. Rot-resistant wood species like cedar or redwood are too costly, and in order to make a structure that is strong enough, the framing members would need to be large and bulky, which causes excessive shading and blocking of precious sunlight. FRP is stiff and durable, but is difficult to work with and is also very expensive.

Metal framing members, in the correct configuration, can be very strong, durable, and do not cast a lot of shadow. I believe the most efficient way to implement roof supports is to use a series of truss beams running the short direction of the structure. They would look something like this: truss beam

These truss beams could be manufactured easily and cheaply from either 1/2" EMT conduit or 3/8" to 1/2" steel rebar. EMT conduit is lighter weight, and slightly cheaper, but is galvanized which makes welding more dangerous and more difficult. Rebar is heavier but does not have any coating which makes welding more successful. Another option might be small dimension thin wall square steel tubing, but I don't know how easily available this would be for me.

Which material would be best suited for this application?

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    Have fun fabricating. It would almost certainly be less costly (at least in the long run, and probably in the short run), particularly for a glass-glazed house, to purchase a frame from a greenhouse supplier. Which will in almost all cases the galvanized steel (greenhouses are not kind to bare steel.) Trusses are rarely seen for such a short span, since the cost of tube stock that will simply take the load is considerably less than the cost of fabricating a truss. – Ecnerwal Jul 22 '16 at 13:50
  • Frames for small permanent attached greenhouses (10 x 12, for example), can easily run over $10,000. For a freestanding structure, you're probably correct though. – cathode Jul 22 '16 at 14:57
  • Sure, whatever the market will bear, and some ends of the market will bear a lot, under the guise of "conservatory pricing" .vs. "greenhouse pricing." I happen to have an active quote for a 30x72 (that could be attached to another building) and the frame for that is only ~$14,000. – Ecnerwal Jul 22 '16 at 15:16
  • Unwise for you to dismiss wood framing. IMO it would be the easiest, cheapest, and most durable option. The fact that practically every home is of wood construction is not a coincidence. Tough to convince me that welding up a truss is less cost, time, and trouble than using an easily cut and fitted 2x8. – Jimmy Fix-it Jul 22 '16 at 16:11
  • Rebar would be a bad choice because it is junk metal. Conduit is not a structural material. 16' of glass will be heavy and break if not supported – Ed Beal Jul 23 '16 at 10:19

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