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I think the trick is to use an ultrafine spray and have some type of valving system that only comes on when the roof surface exceeds 120 degrees or so,
otherwise you could waste a lot of water. I don't know how much water you'll need, since I don't know the square footage. original article asserts it's pretty small if you run it for 2 minutes each time, typically 4 times an hour.
I've read about commercial systems that do this. Imagine cooling a 40,000 sq foot warehouse! A case study I read last week said their A/C bill was reduced from 60,000 per year down to 28,000 per year, which gave the system less than 1 year for payback.
other stuff to ponder:
stolen from roofmister.com:
Q) What are the physical properties that support roof cooling?
A) Roof cooling is based on the fact that water, as it evaporates, absorbs large quantities of heat. For example, one gallon of water at 100°F will, in evaporating, absorb approximately 8500 Btu's. In other words, the evaporation of less than 1-1/2 gallons of water over an hour’s time will absorb 12,750 plus BTUs more heat than a one ton mechanical air conditioner.
other links
http://txspace.tamu.edu/bitstream/handle/1969.1/6787/ESL-HH-85-09-14.pdf?sequence=1
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