The plastic on it right now is just 3-mil clear plastic sheeting (such as you would find at Lowe's or wherever, usually in the paint dept.); but I bought enough 6-mil greenhouse film to do both my new GH structure and the hoop house, so once our weather warms up a little bit (assuming it ever does) I'll be putting that on -- with new lath strips, which I think I'll either paint or coat with some leftover deck stain, because they haven't held up to the weather very well.
The doors themselves were made from 1x2's (cut by ripping 1x4's); the actual "frame" that the door goes into is 1x4 lumber.
True confessions: I came up with the plan for the hoop house at a time when DH and I were in, shall we say, a "less than cooperative" period of time, and I was planning to do all the work myself... as it turned out, we did manage to work together on it
For our larger (16x20) new structure, though, we have 2x4 framing for the doors. Depending on how large yours is, you might want to use 2x4's; my hoop house is only about 6-1/2 feet tall and I felt that using such heavy lumber was going to be total overkill.
