Originally Posted by
FLRider
Gore-Tex is actually a multi-layer material, from my understanding (note that this may be wrong; I've not researched the material really in-depth).
There are a minimum of two layers: the facing fabric (usually polyester or nylon) and the membrane itself. Then, there is usually another layer on the "interior" of the material (the part that is used on the inside of clothing made with Gore-Tex); this is usually another piece of poly or nylon on "3-layer" Gore-Tex. If not (the "2-layer" stuff), you'll need to sew a lining fabric to the inside of the fabric to prevent abrasion deterioration of the fabric against your body in fabric applications. Not sure if that's true for a tarp, though.
That being said, tarps don't really have to be brethable. If you were going to make a waterproof sock, it might be worth looking at. However, a tarp doesn't really need to be breathable.
As MAD777 said above, sil-nylon is cheaper by the yard and lighter; it'll function perfectly as a tarp. But, if you've got your heart set on Gor-Tex, let us know how it goes!