Mark

When specing stuff architects and indeed structural engineers will ALWAYS over spec items.

Simply to make sure it doesn't go wrong, the consiquences of it going wrong are not worth considering in terms of the potential human cost and for them they will have PI (Personal Indemnity) insurance which is costly to claim on and will effect their lively hoods.

Now - simply looking at the info you have given the reason for the size of steel is the span. 10m is a fookin long way.

I used a structural engineer when I did my house - he charged £50 for some similar calculations, I'll happily give you his details.

He will work from sketchs and a description of the work and produces a good report.

I was very happy - all done over email too.

PM me as I wont get back on tonight.

He will tell you what you NEED rather than anything else based on the steel I used.

Jake