Thread: need advise on roofing structure

hi guys,

im building a second story on top of my existing home.
the issue i have is the roofing.
its a flat roof (well not flat, its got a 7 degree slope east to west) the span from east to west is 10meters and from north to south its about 7metres. the slope will be visible from the inside. so i dont want trusses like a normal roof and there are no internal walls for structure either.

a few questions.
i was thinking of putting engineered 250mm I beams (as the roof needs to be 250mm anyways) for the span, but would they bow in the middle with a 10 metre span unsupported? or are they really strong??
if they bow, i can put a support beam spaning from north to south so it takes the load in the middle and plaster around it like a bulk head.

my other issue is cost. i know for floor joists they are to be spanned every 450mm if using yellow tongue. but what could i span them at for a metal roof? if i could do a span of 900mm or more that will save me a lot of $$$

any other suggestions would be great.

thanks