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I've made them and used them for years. Here's a few lessons I've learned over the years with them. Don't use tarps. Use plastic film. Plastic film lets snow slide off, a tarp holds the snow on. The weight of the snow will make it collapse. Plastic films you buy at Home Depot and such degrade to nothing in a year or two. You'll get more if you use the expensive good greenhouse plastic. Black plastic films last longer then the milky white ones do. PVC works well, but after a few years it becomes brittle and will no longer flex and buckle if overloaded, but will crack and splinter. Using black plastic skins probably would prevent this. I've used 20' lengths of 1.25" pvc on hoops 12' wide. That's about the limit you can get that has a chance at standing up to snow loads. Next time I wouldn't use 20' hoops, it creates a large level area on the top of the rounded roof. I'd use 10' and a 90 or 45 degree elbow to make the roof peak. I've done this a number of times over the years. Each time was cheap, but I had to do it many times. My in-laws have a metal piped greenhouse with commercial plastic over it. It's been there for something like 8 years now, without maintenance. I think they passed me on cost effectiveness a few years back. Especially if you consider your time and effort worth something.
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