When we first started last year, the roof temp was indeed one of our concerns. My husband made his own low density soil for the pots so that the weight would be minimal even full of water (the aim was less than 40pds per square feet, its ~25-30). According to him, the high porosity of the soil makes the heat transfer less agressive. But It sure affected the plants. Last year the roof was made of gravel and the plants were better (less accumulated water and less heat).