A system of elements capable of transferring thermal energy from outside to inside a building or vice versa, has abutting waterproof trays (a) enclosing tubes (d) at their junction. Correspondingly long slotted profiles (b) enclosing the joints and held captive by them may be rotated to allow the insertion of rigid elements (c) sequentially so that adjacent pairs of profiles (b) are restrained from further rotational movement and provide a planar surface. Rigid elements (c) of sufficient density may provide a flat roof which is self-ballasting against wind uplift without penetrative fixings to the sub-surface.