My house is tall and narrow and lacks an attic. In the summer the bedroom gets pretty hot - because the black rubber coating of the flat roof got really really hot. I could not touch it on a sunny day without burning myself. So I have painted it white using Henry Acrylic Enviro-White Extreme Elastomeric Roof Coating.
I have used less than 5 gallons of paint, and have covered the roof with three coatings. The paint is extremely thick, almost like liquid rubber, yet it rolls easily using a regular roller. It is elastomeric - meaning it stretches without cracking as it heats.
The difference was immense. On a hot day as I left a small unpainted patch, which I could not touch with my hand, the rest of the roof was absolutely cool to the touch - warm, but fine. We will see next summer how the hot days feel.
PS: I have heard the argument that black roofs soak the sun in the winter better. I don't put much stock into that argument - the house should be well isolated, and I am suffering from the heat a lot more than from cold temperatures.
PSS: I wish I could make a roof garden, but my roof is too small and hard to access, so I picked the white paint instead.
I have removed the roof A/C units and replaced them with heat pumps.