I have a roof terrace on a flat roof, which I'm planning to take apart to clean the boards themselves and the roof underneath, which is currently full of moss and other junk preventing rainwater from running off. In a builders report it was recommended that the roof be insulated on the outside to prevent the wood from rotting due to condensation coming off the cold roof.
Since the terrace is coming off anyway I figured I'd place some XPS insulation on the roof before putting the terrace back on. As far as I can tell, you are supposed to put the terrace right on top of the XPS tiles, but are those things really strong enough to carry the load of a terrace + furniture + several people? I'm kind of hesitant of putting all that stuff on a block of foam.
The terrace, by the way, consists of boards screwed to three perpendicular beams which rest on some rubber tiles, on the bitumen. So technically the entire load would be upon the areas directly beneath those rubber tiles.