I would hold off on the Sunbrella, it is good fabric but has two drawbacks, doesn't handle chaffe and can leak. This is why you don't see it on any of folding tents, it could work on a tent like a Maggiolina with a solid roof and the Sunbrella is on a vertical wall and well protected from chaffing.
Sunbrella is a superior fabric for the sun. Works incredibly well but you have to be careful in area where it can rub against itself. It generally is very water repellant but I was in a hail storm and the hail beat against it and caused the fabric to be just like a mesh, leaked like I couldn't believe.
I think the best material for your needs would be a blend, part acrylic polyester (Sunbrella) and part good old fashioned cotton. The cotton will swell when it gets wet and be waterproof, Sunbrella needs a treatment to shed water, otherwise it is just a fine mesh. Good old canvas is great but, from what I hear, the quality isn't very good these days, you can't get the old Egyptian cotton canvas. I've talked to guys that have canvas rooftents from the 30's that are still in good shape. You might want to talk to the guys at Seattle Fabrics, they have rooftents and can help you, I send some of our clients that way
http://www.SeattleFabrics.com
The other challenge you will face is the thickness of the fabric. Rooftents take much more abuse that a standard ground tent so the fabric needs to be thicker and stronger to last. The trouble is, most standard sewing machines can't poke their little needles through. I would also recommend getting GorTex thread if you are going to the trouble, last much longer and is very strong. The machines we use have what is called a "walking foot" keeps everything straight. Even still, we have had a few customers comment on how some stitching isn't exactly straight always. When you get four layers of heavy canvas, it is pretty hard to control, so if you are using a standard sewing machine, get a sample of the fabric you choose first to test on your machine. You may need to go with a lighter weight fabric.
Rich @ AutoHome
www.AutoHomeUS.com