I did this a while ago and just realized I forgot to post it here. That is kind of rude, since this site's ingenuity made it all possible. Basically, I found some fabric at Wallyworld for 1.50 a yrd. It looked like 1.1 oz, but now I think it was 1.9. It was uncoated. I was foolish and only bought enough for a speer style. I wasn't planning on using for a tarp initially, but later decided to because of thread about silicon coating it. So, chock this up as another success in DIY Silnylon. Once the tarp was all sewn together, I set it up in my yard and mixed 100% clear silicon with mineral spirits. Then I took a sponge and scraped it all over the fabric. I did this twice all over, and a third time on the ridge line. A few days later, I set it up again and laid under it with my wife sprinkling it with the hose....NO LEAKS. I was very surprised and very happy. The final weight is 26 oz. It is a little heavier then I expected. I was expecting 16 oz. So, maybe it isn't 1.1 oz per square yard ripstop.
Here are some pictures of me using it in October 2009.