Christian McArdle wrote:

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

You can probably use a loading of 0.8kN / m rather than 1kN/m
Note also that a floor is a load sharing system which reduces the size
requirements a bit more.

1. Timber joists (220 x 50) (CL16)

Taking the above comments into consideration you could use something
like 150x75 C24 so that saves you 70mm as well.

Given that the steel joists would save 70mm of height, propelling the
project into feasibility, why aren't all loft conversions done with them?

Because it is a pain to work with. When you want to trim a bit of here
or there it is not just a quick cut with a saw. Fixing other beams to
stringers is not as simple as nailing on a joist hanger with square
twist nails! Also not as easy if you need to drill a hole through the
centre of a joist for a pipe or wire run.

Furthermore, the calculated deflection ended up as 3.4mm rather than 11mm,
so it would be less bouncy.

True - but even 10mm is not that bouncy. If you need to go to steel at
all - then looking at flitch beams first, gives more "workability". Most
of the advantages of working with wood, and you are still in the bounds
of manually handling the beams without needing a crane (you can lug the
three bits up a ladder separately and assemble the beam in the loft).

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.

Timber will cost you as much in all likelihood. The floor joists cost me
something in the order of 500 quid for the wood, and 400 for the flitch
plates (I used four) plus all the hangers, bolts, dogs etc. Having said
that I saved on the cost of a crane!


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/