Well, I finally hit the Wal-Mart Lottery last night. I've been checking the fabric dollar bin pretty regularly and have found some nice stuff. Recently I bought a bolt of nylon in a neat digital camo pattern. It's not ripstock but it's a pretty tight weave. I'm planning on making a hammock and tarp out of it, mostly just for the practice.
Last night I checked and found a bolt of light gray silnylon. Of course I bought the whole bolt of 15 yards. It's only 48 inches wide and obviously factory seconds. Doing the blow through test I think it's about 98% waterproof. With a coating of silicon spray it should work pretty good.
My plan with the other material, which is 60 inches wide, is to make a McCat style tarp with an 11 foot long ridge and 7-8 long foot sides. I wanted to make the sides 5 foot from ridge to edge, so I guess the original square dimensions would be 11X10. To do that with the narrower material I'd have to sew three strips of material together. But that would mean there would not be a seam along the ridge but would have seams about 2 feet down the sides from the ridge. Is this workable? Or do you need to have a seam at the ridge? Would a strip of grossgrain ribbon need to be sewn along the ridge to provide extra support if there was no ridge seam?