I am building a new double garage - single brick skin with a 4.6m wide door. There will be an apex style (ie 4 sloping roofs coming to one point) trussed roof above. I have planned 500mm square piers either side of door. I assume I need to allow a minimum of 150mm either end of catnic/rsj. Can someone please advise me if spanning this length can be done by catnic or do I have to get an rsj? In either case will they come with the spec and calculations that they are able to support? Many thanks