My dad had a greenhouse business and had a lot of hoop houses. He got me connected with a place to by UV treated special plastic (4 mil). It warranties for 4 years. I have had one GH last over 8 years with the same plastic. The windstorm took out mine at the 4 year mark. I use 3/4 or 1 inch steel conduit that electricians use. I tried PVC the first time and then switched to the conduit. During the heat of the summer I have shade cloth to cover the greenhouse and ease up on the intensity of the sun through plastic.
All that counts is that it works. ;0)