In article ,
Christian McArdle wrote:
Wow, what was the name? I'd really like more information on
this one.

I'm currently considering steel to get a 4m span under 100mm.
A wood based product would probably be easier to install than
steel, provided it can be approved for 30 mins fire resistance
with only plasterboard below

It wouldn't help you on height. IIRC the Catnic ones I referred
to had top and bottom chords of something like 75x38 flat (one of
selling points was lots of nailing room for floor deck and
ceiling) and the overall depth was about 300mm. So it had the
stiffness of a 9x2 but weighed half as much.

With timber joists deflection is nearly always the limiting
factor, and stiffness is proportional to the cube of the depth.
Keeping the width and joist spacing the same, if you reduce the
size from 6" to 5" the deflection goes up by (6/5)^3 = 1.75 and
from 5" to 4" by (5/4)^3 = 1.95 - i.e. 2 x 50x100 = 1 x 50x125

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