I might actually try this during our next heat wave. All the other tips and tricks do absolutely nothing for the upstairs in many UK properties in my experience (I'm talking indoor temps of 23-29c at midnight, when the outside has cooled to 20c, from a daytime peak of 35c). It's like the whole house is turned in to a radiator and there is nothing you can do

Hello, I tried the experiment a few years ago briefly and it did work. Some back of the envelope calculations show that it is cheaper and more energy effiecient. However there is a risk that lichens and or moss will thrive on a moist roof. So could be more trouble than it is worth.

The roof is a fragile thing. Fool with it at your peril! Folks throw ideas around casually - "Put a garden up there! Use solar shingles! Spray water on it day and night!". Most will end in disaster.

The roof is designed to hold itself up, keep out occasional rainstorms, and not blow away immediately in a storm. Nothing else. Pushing it way past its design parameters will end in grief.

I'm rather confused by your reply and the reply to it. It rains a lot in England. Why would spraying it with water be any different from night upon night of heavy rain ?!

I don't know actually. I had not thought of about places with constant rain. Anyway does it work? If you spray your roof on a super hot day does it feel any better indoors?

Have the same problem in the upstairs of my house (also in UK) - still unbearably hot at night, even when it's cooled significantly outside.

It seems houses here are built only with heating in mind, rather than cooling. I get that in much of the UK they need heated a lot more often than cooled, but it makes for pretty horrible summers.