amount of pre assembly is possible in an embodiment incorporating metal components metal straps j incorporate holes at modular intervals through which screws may be driven to attach the element to the structure prior to attachment rigid elements c formed from metal tubes or channels are threaded onto the straps and located between fixing holes a series of these rigid elements c may then be attached to the structure with the strap j acting as a hold down mechanism shims of an appropriate thickness are located at the fixing holes allowing the rigid elements c to slide along the strap j all of the straps j and rigid elements c may be attached to the structure and aligned before final fixing the trays a profiles b and tubular elements d as a separate sub assembly may be brought to site as a coil not unlike a roller shutter the dove tail profiles b can be located in the spaces between adjacent rigid elements c after which the metal tubes or channels can be slid sideways on the strap j to key the elements together final tightening of fixings may be carried out after all elements have been located to their permanent positions a sub assembly of elements a b and d is not essential and a different procedure can be used to advantage in an embodiment where.