Best Material for DIY Plastic Tarp?

Polycro.

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=59450

I read most of that thread and am still confused as to what specific mat'l to buy. It appears that some folks use polycro as a generic term. It also appears that some of the mat'l is window film, as as stated, I heard it shrinks in the sun. I don't want to size the tarp and then have it shrink.

When using it on a window, you would blast it with a hairdryer to get it to tighten up. My understanding is that you can leave it out in the sun before you make your tarp to achieve the same thing. Probably wouldn't want to leave in on the grass though.

Window film will shrink a bit in the sun, but it won't shrivel up into a ball. It will be trickier to work with and more delicate of course. Dan Johnson has done some amazing work with it. See http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=59450

I would buy the cheapest blue poly tarp, with the assumption that the cheaper ones are thinner and lighter. I found ones that were green on one side and brown on the other, which I think is much more pleasing to the eye and an 8×10 was about $4 on sale. That will give you good tough stuff to experiment with, and you can still use it as a… tarp!

FYI, the cheap poly tarps are quite useable just the way they are and they aren't terribly heavy. In fact the cheap ones are quite close in weight to a PU coated nylon tarp. They are more bulky to pack. They can always use more tie-outs, which you can add with a grommet kit or some creative sewing. Check the library for sail-making books to get good tips. There are all kinds of gizmos sold to clamp on the edge of a plastic tarp or sheet/roll plastic like Visqueen.

You can learn just about anything you need to know about pitching flat tarps by practicing with a blue poly tarp. Silnylon will slack some when it gets wet, but the theory is pretty much the same otherwise. If I were on a strict budget, I would have no problem using one for a shelter– it would beat staying home!

You can pre-shrink polycro sure. But if this is just for prototyping…don't leave it in the sun?

Just buy the cheapest painter's plastic you can find. Sub 2mil stuff is pretty cheap for a lot of "fabric." It'll let you mock up any shape and design with some gorilla tape nice and quick. Durable enough to experiment with if you realize it's thin plastic and not actual silnylon or cuben.

3 mil polyethelene plastic tarp from home improvement store intended for protecting things when you're painting. It's cheap

for example $16 for 10' x 25' (this is 4 mil which would work too) http://www.amazon.com/Warp-Brothers-SP-4CH10-C-Coverall-Sheeting/dp/B000WJVVF2/ref=sr_1_1?ie=UTF8&qid=1368744058&sr=8-1&keywords=polyethylene+sheeting

if you want it to be lightweight, then 2 mil, but it's not going to last a long time. I've used tarps with this in the wilderness for a few nights

polycro http://www.amazon.com/Duck-Brand-1297119–120-Inch-Shrink/dp/B000NHW2Z6/ref=sr_1_1?s=hi&ie=UTF8&qid=1368744235&sr=1-1&keywords=polycro

$6 for 84" x 120"

might be a little lighter for the same strength as ployethylene

Coghlan's tube tent made from 2.5 mil polyethylene .Very tough and handles edge stress well. Less static cling of dirt than other plastics.