In article , Christian
McArdle wrote:
Their smallest is 200mm deep, which is too great. I'd might as well use
220mm timber. I'm really looking for sub 100mm, so it fits inside the
existing ceiling joists. Headroom is everything. I'm willing to pay 10
times as much as a simple timber solution to bring down the depth.

[for sci.eng.civil this relates to providing new floor joists in a house
loft conversion, 4m span, 500mm c/s, domestic floor load only]

On a quick check a 90x90x3.6mm hot-finished SHS (square hollow section)
Grade S355 (Grade 43) steel would do the job and weighs only 9.6kg/m.
IIRC Screwfix do self drilling chipboard decking screws to go into steel,
so decking it out would be easy. You'd need to check out the fireproofing
carefully: I suspect it would come to intumescent coating by the supplier
with any damage made good on site.

I've cross-posted this to sci.eng.civil to see whether any of the
engineers there can see any good reason why square tube cannot be used in
this situation.

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