max roof span for outbuilding?

Hi.

I'm currently building a wooden outbuilding approx 6m x 6m, the front and rear walls are 4 x 2, the side walls are 6 x 2 all at 400mm centres.
It will be having pressed steel roof sheets with 50mm insulation fixed underneath. The roof is a simple lean to style.
Will 9 x 3 (the largest my local timber yard can get) be capable of spanning this width unsupported other than the side walls it sits on? If not can steel purlins the kind in agricultural buidings be utilised?
I really don't want any internal supports for the roof other than the walls.
Any help much appreciated,

Zeb

It will probably be more cost effective to use 3m joists with a steel RSJ along the middle.

Long ( and thick ) wood beams are very expensive.

It all depends on if the building will be covered by the building regulations as a quite massive RSJ can be required because teh BR permitted deflection is minimal.

The reality is that a considerable deflection could be tolerated and a 4" x 3" beam would probably be fine. But a 6" x 4" would be better.

Building regs would probably need a massive 10" x 4" which is total overkill !

In my father's day they just used tables to determine what size beam was required. Now a much more complex calculation is needed.

Tony