GB wrote
usenet2012 wrote

What's the best ways of going about it? Where could I find some plans?

A ---- Can you use that as a plan?

Seriously, I think that your biggest issue is making sure that the new
roof is sealed round the edges, so as to keep the rain out and is
adequately ventilated. That's assuming there is a parapet round the flat
roof. If no parapet, just make sure there are adequate eaves with
guttering.

I detest flat roofs. They are never built with enough slope to ensure that
the water runs off, rather than pools.

Doesn't pool on my metal decking roof.

I have no idea why not.