"marvin" wrote in message
...
I'm putting an 12ft RSJ in, one end into a cavity wall.

What sort of cavity wall. RSJs require strong mounts.


When I have opened the wall there is the end of a wood lintel that is
supporting the inside wall over a window.
If i cut the wood so that the RSJ sits on brick that would only l;eave about
3-4 inches of the wood left sitting on brick.

Not allowed. Must be 6 inches minimum.


Alternatively can I sit the RSJ on the wood beam (with the packing
slate in between), but even then I would have to take some of the wood
off for it to fit.?

Probably not. Your Building Control Officer has final say so see what he
wants.