A quick play with SuperBeam appears to indicates for my proposed loft
conversion floor (4m span, 2kN/m2, 500mm spacing), I need either:

1. Timber joists (220 x 50) (CL16)
2. Flitch joists (147 x 50) (10mm plate, not full depth)
3. Steel joists (127 x 76). (UB 13mm thickness Grade 43)

Given that the steel joists would save 70mm of height, propelling the
project into feasibility, why aren't all loft conversions done with them?
Furthermore, the calculated deflection ended up as 3.4mm rather than 11mm,
so it would be less bouncy.

A quick phone up of the local steel merchants gave me the chance to sound
all professional by reeling off the SuperBeam specifications (they seemed to
know what I was saying!) and gave me a total cost of around 700 quid plus
VAT to joist out a 8m by 4m total area, making the cost only a small
proportion of a loft conversion anyway. It's not like the timber would have
been free, either.

Is there something preventing the use of steel for joists? I realise that
they need to be fireproofed, but this can't be that expensive can it? Isn't
the fireproofing just squirty foam?

Christian.

P.S. By not restricting to standard sized UBs, further fiddling shows that a
76 x 76 x 12mm RSJ section would also suffice, giving a clear 144mm
clearance over the wood. In fact, slightly less, as this RSJ would actually
fit within the current thickness of the 4" ceiling joists. Coupled with
installed an unventilated roof (as per Kingspan's recommendation) with 75mm
between joists, 25mm underneath and no ventilation space, I would only lose
25mm of headroom. Not bad for a building regs compliant conversion, don't
you think? What's the catch?