Somewhat OT: Larger Tarps

I've heard that good old blue tarps work great, they are tough and last a long time. And they are really cheap, if one gets trashed it's not a big deal.

Maybe you could buy a roll of Tyvek from Home Depot and have the scouts make some tarps themselves? Could be a great experience for them. With one 9' x 150' roll, you can make a LOT of tarps. Probably about $200-250 for Tyvek, tape, and grommets (or whatever you choose for tie outs), and you'll get at least five 12' x 18' tarps out of that.

EDIT: As you may already know, Tyvek is pretty tough stuff, and would likely last quite awhile even with scout use. :)

Do you backpack with them ever? How much sun? How much folding?

The heavier silver tarps (like the blue tarps but more durable, used on the farm etc) are a great bargain. Folding eventually causes leaks, but they will last a long time before that.

http://www.coversuperstore.com/farm-industrial.html

Nylon is stronger, lighter, more compact and more flexible, but more expensive and is effected by lots of bright sun. The heavier the denier and the heavier the coating the longer lasting (be sure to get a solvent based coating). I had some scrap 210 denier urethane coated nylon that lasted several years covering my wood pile.

For purely sun, you can get mesh shade tarps that last a looong time in the sun and let breezes thru. Very durable. heavy. Knitted won't fray if you get a hole in it.

http://www.agriculturesolutions.com/products/agricultural-fabrics/shade-cloth-fabric

None of these is as heavy as canvas.