If you toss your tarp over a rope and it rains, water will follow the rope down into your "home". There are ways to stop that, but if your tarp had ties and you use them, you can avoid that little problem...
It should probably be noted that I have never used my tarp as a stand alone shelter, but rather as a rain fly for my Byer Amazonas hammock, hence the relatively large 10' x 12' size.
The Equinox siliconized tarp that I have is very light (the 10' x 12' size weighs less than 19 oz) although it is a bit pricey at $85.00 US. These are available from Campmor out of Mahwah, NJ. Also be aware the newer style tarps are a dark green, not the older bright green version shown on the website.
Jim