> I'm designing a tarp canopy for our camp, a la creativeshelters.com.
> I'm shooting for a 30ft wide by 100ft long rectangle, flat roofed.
I hasn't rained in eons on the playa. Louise and I stopped using flat-roofed tarps when it rained rather heavily while we were out somewhere. We came back to our camp and found the water pooled in the middle of the tarp, our EMT bent from the weight, etc. It didn't burst the tarp, thank goodness, but it was a mess when we stood under the water and pushed up to drain it - lots of splashing all around onto our stuff under the tarp, on close neighbors, etc.
I believe our switching to a gabled roof has kept it from raining ever since. I'm confident, nevertheless, that if you put up a flat-roofed tarp 30x100 feet, you will cause it to rain heavily once again.