Steel joists

If you're going to the trouble of removing all the old timbers, then why not have the walls built up and place new timbers on the top like the existing construction is now. You could probably build up another two brick course and give yourself plenty headroom and still be within regulations if done properly.

Just a thought.

There is a walkway to the attic room above, hence the thickness of the timbers and the need for an equivalent substitute. Raising this floor would make it inaccessable to normal height humans.

I don't know if I understand this

Do you mean you want to replace every floor joist with a steel beam?

What is a 300mm pitch in relation to a horizontal floor joist?

And c16 wood?

Have you considered how you will fix a floor and cieling to a steel beam? And what of the inevietable cold bridge from the end of the beams where they are built into the wall, and the potential for staining where the cold steel beams are in contact with the ceiling below?

dg

Yes - or another material that will reduce the thickness of the ceiling by a couple of inches at least.

The pitch between the centres of the parallel joists.

Strandard structural softwood. Used to be known as SS3.

Floor will be bolted from above as there will be a carpet. Ceiling will hang from the flooring probably.

Not a problem - neither end wall is external

with the ceiling below?

Good point. I'll use a sheet of medium grade vapour guard plastic.

How about an alternative strategy. Suppose you were to put a single steel beam at right angles to the existing joists across the middle of the room. Suppose you were then to use half-length wooden joists running from the beam to each side wall. [You would have to fabricate some suitable hanging arrangement in order to support the joists in line with the beam - but it's quite do-able].

You should certainly be able to reduce the joists from 5x2 to 4x2 (Imperial sizes) if they are only half as long as the originals. You may need to take advice on whether a 4" I-beam is sufficient in the other direction. How long are the spans?

Roger

Ah right ! So you'd need to work out how many lengths of the narrower steel joists you'll need to make up the gaps that the flooring boards can sit on without breaking. So if there are 5 wooden joists spaced at 450mm gaps, then you'll need the same amount of steel at the same spacing.