Hi Nick, This will depend on the type of rig you are looking at. LOLER may well apply in some cases of use of the rig you are looking at if visiting companies attach motors and trussing to it or there is any sort of flying system (winches,counterweights or simply hemp fly systems) If you are looking at a fixed grid of 50mm ali pipe attached to RSJ's or similar then a structural engineer will be your best course of action. They will likely be able to give you a WLL and SWL from a very simple inspection. 50-100kg per metre is common (but i couldn't possibly commit to that without seeing it.) and this would be sufficient for generic theatre lamps. Automated lamps are much heavier. Overloading of these systems is possible but usually unlikely in any professional venue (or professionally installed) and depends on the distance between fixing points, guage of ali pipe used and nature/size of lamps in use. With this type of arrangement the work at height to focus the rig is the main hazard. The security of the lamp hanging brackets is second and electrical issues third PM me with pictures if i can be of any more help Best Mike

Nick, There is wealth of information available from Association of British Theatre Technicians (see link below) http://www.abtt.org.uk Thay might be able to help or point you to one of their publications which deals with this. They have a number of codes which deal specifically with lighting and safety Regards Colin