How to hold up a roof! | John Sayers’ Recording Studio Design Forum archive

Well I'm back on the case with my garage build, after a year on standby. I have come to the conclusion that my asbestos* corrugated roof is beyond salvation for soundproofing, so have decided that replacing the entire roof is the way to go. I have several issues though. a) The existing trusses are only welded angle steel, they won't take 2 layers of ply+ shingles. Which my intended roof covering. b) The existing walls are only sectional concrete panels. They only sit about 6'6" high, plus they won't take the weight of the proposed roof. I could build a cathedral style truss for extra headroom but this would mean more outward load and the walls would probably fall over. So I reckon the best bet is to build new timber trusses and raise the roof height by a couple of feet, my quandary being: How to support the new roof? Ideas, suggestions, and real life experiences please. I have a few solutions, but thought I'd see what the John Sayers forum 'think tank' comes up with before making my plans public.. Garage area is 16' wide by 20' long. (About 5x6 metres) and sits on a concrete slab which is at least 4" thick, maybe 6". It's a double garage* and is designed to hold 2 cars. Small print: *In England that constitutes a double garage. Americans please stop laughing. *Asbestos! Did someone say asbestos!! The existing panels are old but sound enough to be removed. Mask, goggles, suit, hose, poly sheeting and tape will take care of them. and my local council has an asbestos disposal facility. * Before anyone suggests, demolition and rebuild is not an option, the cost of demolishing and disposing of the old garage, plus costs and labour for new walls to be built would mean that this thing wouldn't be finished til 2076.