It is always best to seek professional advice; the answer is not to place steel rafters.
Each loft conversion needs to be evaluated on a case by case basis.
We would strongly advise that you do not just load up the roof as you are suggesting this could lead to a structural failure.
If it would help we offer a reasonable value evaluation design service, where we can advise on the appropriate level of structural work that may or may not be required in order to achieve your requirements.
Please feel free to visit our website at www.jeremywedge.com or contact us at info@jeremywedge.com where we can advise you further.
