RSJ for 75mm Cavity Wall

I would have thought that 2 would be better - one for each skin. A single one would be a bit heavy! Is it over the garage door? If so, stick some plasterboard on the underside with gripfill (after fitting the door frame) to close the gap.

Having said that, can't you get a Catnic lintel which would do the job?

The first question is what does the spec say - this is controlled work so what has been calculated and approved?

Without knowing the loadings, it is impossible to say what sized section is required.

Generally you will use two steels, which are bolted together, but kept apart with steel tube to form the cavity.

Don't forget the padstones.

dg

Sounds to me like a heavy-duty lintel is what you want, e.g. :-