In article ,
"David Hearn" writes:
Can anyone advise what depth backbox I should use? I'm adding a 2G MK
socket, and was planning on using 25mm - however after reading the
instructions it said that for = 9mm face plates, use 25mm, and 9mm use
35mm. I think the 2G MK socket is about 9mm - but I just wondered what the
benefit was with going with 35mm, and whether 25mm is very cramped (2x 2.5mm
T&E). Its an internal block + plaster wall (1930's block), so I don't want
to go too deep (not to mention that the deeper I go, the more work it is!).

Is 25mm a bit shallow?

I use 25mm, but it normally ends up being set back further than the
front of the plasterwork, so it's probably more like 30mm when fitted.

One final thing - when I fitted some before, I set them too deep as I
thought they should be plastered into place. I think they should be screwed
into place and made good with plaster (rather than 1/2 - 1" of plaster being
the thing to hold it in place) - am I correct?

You can do either. Plaster will hold it much firmer than a screw,
particularly in a crumbly wall, but it sounds like you are doing
it wrong. Make the hole slightly oversize (better with rough sides
and back too), paint it with diluted PVA (if very absorbent, repeat
after first coat dries), fling in a dollop of bonding coat plaster
(or sand and cement with PVA added to the mix), and push the back
box into the hole so the plaster ozes round the sides and just
round to the front. With practice, you will get the dollop size
just right. Wipe away any excess which comes through the holes
into the box and make sure the screw lugs are clear. Leave to set.

--
Andrew Gabriel