My experience is that PVC fittings break. PVC pipe bends in ways you don't want it to bend. If you're determined to use PVC anyway, study how metal pipe frame greenhouses are built and try to do something similar on a smaller scale. Make bracing connections to more PVC (or wood) using metal plumbers' strap and nuts and bolts. This will allow more flexibility in the joints. I've got plenty of broken pipe and twisted carriage bolts to show what a direct bolt hole through the pipe will do. If you have to put a bolt through, glue in wooden plug reinforcements to drill through. If you're going to sleeve PVC over rebar, sleeve a smaller bit of PVC (longer than the rebar stub) inside first to reinforce and spread the load at the pressure point. If you want advice on your build, post pictures of what you have in mind. It's hard to envision exactly what your structure will look like from just "a PVC greenhouse". Tell us about size and height. Tell us about shape (gothic arch, gable, round arch, low arch on kneewalls like Jay Slick's, etc). Tell us about frame spacing and what you'll have for purlins. I personally like wooden strapping and wood block gothic arch frames. They're easy to build, cheap, shaped to shed snow and wind, and they're very strong. You can fasten to it easily. In my limited experience, PVC is only suitable for very small houses, or for houses that will be disassembled and moved periodically on the shoulder seasons and left uncovered (no snow/wind loads) in the winter. Here is a link that might be useful: Gardening The Hard Way

Whatever you will not have to replace anytime soon! ;~) Seriously, if I thought that I could afford it, I'd use insulated glass. I do have a fantastic local building supply store. I've been drooling over some of their 'Gigantinormous' insulated windows, etc. If I thought that I could get enough of them, I'd... What ever I end up building, I'll probably use multi-layer polycarbonate panels. Triple-layer ones look to be the best in the long run. I'm thinking about just making one as a summer shadehouse. In that case, I'll just use a decent quality shade cloth. Hopefully, I can keep out bugs, dogs, cats and the stray sheep! ;~) The challenge with a dome would be the cutting involved. You have to watch out to avoid wasting material. I wouldn't consider using anything less than the best that you can afford. You'll end up paying for it anyway. I've had hoophouses with cheaper plastic fall apart within months. All my tomatoes...gone in one night! As I now have thousands of dollars of orchids that I'm protecting, I'm being very catious about everything! ;~) It all depends upon your needs, intentions and budget! Scott

Ok, now I understand. You're going to have posts that form 4 foot high side walls, and considering separating each pair by more than 12 feet, which will lower the peak of the roof but give you more greenhouse area. This would give your curve an ellipse shape. There are formulas for calculating these shapes, but they are a little tricky. I would suggest two things: 1) Make your side walls about 6 feet tall, that way, all points on the roof curve are more than this and you don't have to worry about bonking your head when you are working inside near the side walls. 2) Figure out your curve by experimenting. Install one post, and then have someone hold a second one at varying distances to give you an idea of the roof curve with a piece of PVC in place (this may require two assistants). Doing this experimentally rather than trying to calculate it and then imagine it will be much more useful. Just be aware that, as you go wider and the peak gets lower, you will likely start compromising strength and have more trouble making the roof stable.

Mine are 17.5-18 feet wide, I think that is about the widest you can go. If you get some PVC with bell house, you could join two pieces together. If you join two pieces, DO NOT USE COUPLERS! The belled housing is much stronger. Do share pictures when finished. Jay