What could I use? My kitchen has a flat roof with black rubber on it. I think if I can put a white coating on it, it will surely make it cooler. At least not so hot?
Be sure that any coating applied to "black rubber" roofing does not contain any petroleum-derived substances. The black is typically EPDM which is damaged by petroleum products.
I have no idea how well coatings will, or will not, bond to EPDM.
I am curious as to why you care about the temperature of your roof.
I am concerned about the temperature of my kitchen, directly under the roof.
I'd suggest checking to see if your existing roof could be ballasted:
Here is a link that might be useful: Ballast
Trebruchet, do they install or only sell? There are two companies listed in Iowa who have the material. I will email both of them. Can you make a guess as to cost for a roughly 12x24 roof? I recently got sticker shock for spray foaming my well pit, 3 walls 4x4x5 tall! Apparently, getting here was most of the cost. Most of the time living in the wilderness is fine until I need something I can't buy at Menard's and haul in my Ranger.
I have no idea if they install or sell. I wouldn't ballast a roof for less than a couple thousand dollars, I'd guess.
Ouch! Well, it was a great idea, anyway.