GH problems

We are in the final stages of trying to finish this. It measure 12x12. We used 6mm (?) clear plastic, double layered. The opening faces west, and the east side is a solid wood wall.
Couple questions??
We were going to use pvc pipe to elevate the ceiling (left extra plastic to do that)....well after we got the pvc up, it rolled to one side...we cant seem to figure out how to keep it from rolling and how to keep it in place. Any ideas?
Another thing, how would hallogen lights work as a heating source? We live about 7 miles fromt the Gulf. We do get freezing weather, but very seldom does it last very long and even less seldom does it last all day. We bought a commercial size hallogen work light that pushes 15000 watts.
Thanks
JEn

MSJen, I'm having trouble envisioning what's happening with the PVC pipe. When you say you wanted to use it to elevate the ceiling, are you meaning you're using a piece to push up (a vertical support for) the highest point - kind of a straight up-and-down brace like in the center of a tent?

Can't speak to the halogen lights as a heat source (I guess I'm batting a thousand here!) but 15,000 watts (if I read that correctly) should be aplenty. My electric 1500 watt heater pushes enough heat (similar size GH) on most nights - the only time I break out the propane heater is when the temps threaten to head for the 20s or colder OR I'm trying to maintain 40 F or higher for baby seedlings. Especially if you've double-layered your plastic, you will get a lot more bang for your buck with your heater than I do.

Terry,
There is a ledge on the wood wall and a 2x4 across the front (opening side). we were trying to elevate the ceiling (give it a "rain run off" angle) by taking the pvc and mounting it to the 2x4 and then also mounting it to the wood wall.
We mounted the ledge side, used a board to put enough tension on the pvc to get it to bend, and then mounted it to the 2x4. We we took the support that i was holding down, the pvc rolled to one side and the ceiling drooped back down again.
How do you get the pvc to stay in place and not roll of to one side. There is not a center support. We were trying to mount it horizontally .
We may end up putting a center support in there, although i hope we can find a way to not have to.
Thanks
Jen