Ok ... I sent a question to the seller, and here is more info:

It is 30D ripstop nylon, 1.1 ounces before coating, 1.3 ounces after silicone treatment.

He has tested it for waterproofness, and said it is very good. He draped the fabric over the opening of a bucket to make a bowl. Then filled the fabric with water, then rubbed it hard and let it sit for a day ... then rubbed it more and sit more. Then took the fabric off and it was completely dry on the inside of the bucket and the bottom of the fabric.

So ... It sounds good. I went ahead and ordered 10 feet to test it out, and I'll share my personal impressions when it arrives.