I don't know why so much steel has to be introduced onto roofs these days, but they do, probably at the behest of 'Building Control'. Jobs for the boys, I always think. The Elizabethans could build timber framed houses that lasted, seemingly for ever without a hint of steel about except for the nails. All through the centuries thereafter steel was not used, until now. Now it seems that it is almost 'mandatory'. Stresses and strains have to be calculated by structural engineers, who (sorry structural engineers) no doubt add a bit on in case of their 'safty margin' and I suspect to 'appease' Building Control, who probably want even more. The result is that you have a structure that would probably take one of the Seven Bridge spans, but here it just supports a few hundred weight of timber and tiles...Anyway, the roof was sound, and my client should be able to build into it, subject to planning consent and, wait for it - Building Control...