A modular green roof building element has peripheral walls 2, 3, 4, 5 which contain a growing medium in which vegetation of the green roof can grow and a floor comprising a membrane 10 which is water permeable but impermeable to the growing medium so that water can drain through the growing medium out and out through the water permeable floor. The roof building element has sub dividing walls 6 so as to define cells 7 for retaining the growing medium. The water permeable membrane may comprise a geotextile of non- woven polyester or polypropylene. The sub dividing walls may have flanges 8 at the base, to which the floor is attached, so that the cells are effectively cop shaped with a bole 9 in the bottom underneath which is the membrane 10. To make a green roof, a number of modulargreen roof building elements are positioned next to each other on the roof the modules may have legs attached so that the module is spaced away from the roof.
