I like to protect that part of my tent floor that takes my weight. I don't like the idea of sand, small rocks and the like grinding away at the floor under-surface while I move around in my sleep. To that end, I take a plastic leaf bag, cut the sides off to open it out and then trim to about 20" x 70"; weighs about an ounce. After a few days of use, they get small holes in them. But, I don't have it there to keep water out, just to protect the fabric floor. It is surprising how well one of these will hold up. Also, they don't seem to have the same degree of static cling as Tyvek or sheet polyethylene. Since it is much smaller than the tent floor there is no problem with water collecting on the plastic sheet.