On Wednesday, October 17, 2012 11:49:06 AM UTC+1, GMM wrote:

I have to build a floor in a loft.
I’d like it to be robust enough that it doesn’t all wind up
in the bedroom below.



At present, there is a lath and plaster ceiling, with rather
wimpy-looking ceiling joists,

So question 1: To work above the ceiling, I’m thinking of supporting
it with boards on acro jacks, possibly moving these according to
where I’m working as I go and putting some boards on the ceiling
joists to spread the load (mostly me!).
Does this sound like a sensible thing to do – ie any better
suggestions?

I'd put at a beam at right angles to the joists in the middle of the ceiling, supported by acro-props. This will halve the effective length of the joists, and make them plenty man enough for the job.

You'll then need boards on top of the joists to stand on.

Lastly, to get decent access into the loft space I need to create a
new doorway. The only way I can approach this is from the inside
of the loft. Normally, cutting a new opening would be
best done using strongboys to support the triangle of brisk above
until the lintel is installed but I’d
hesitate to jack against the top of the ceiling and getting them
through the hatch wouldn’t be easy.

So question 3: Instead of using strongboys, would a board (4x1 or so)
fixed to each brick (multimonti into the brick centre) above the lintel
do an adequate job of stopping everything moving while I get a
lintel in?

Sounds like a reasonable plan. I'd probably just use a rawl-plug
and No 10/ No 12 screw. It doesn't have to take much weight, and it
doesn't have to take it for long.