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?

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?

Thanks

David

In article ,
David Hearn wrote:
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?

To me, yes. So I always use 35mm for sockets. It's not that much more
work, really.

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?

It's often a little tricky drilling a hole *exactly* in the right place in
some types of brick and block work. And the back of the recess is likely
to be a bit uneven - if you're anything like me. ;-) So a dollop of quick
set mortar can be an easy way - with the box wedged exactly in place until
it sets. Then make good the plaster with that one coat stuff.

--
*He's not dead - he's electroencephalographically challenged

Dave Plowman London SW
To e-mail, change noise into sound.

David Hearn wrote:
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?

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?

25mm is typically OK; it depends most on how many cables are being
connected. Assuming 2.5T&E, a single one (end of a spur) is trivial, two
(typical case on a ring, and your specific case) not much of a problem,
three (2 x ring + 1 to a spur) starts to be a bit tight, and sinking a
35mmsq makes the job easier. One-off on plaster+brick/plaster+block,
I've never found the need for anything more than a masonry drill round
the edges and a cold chisel+hammer to knock out the Inner Bit; obviously
if you were a jobbing electrician, it'd be worth splashing out on an
SDS-fitting box sinker.

As for securing them - again there's a difference between 'pro' practice
and sensible one-off d-i-y. Many pros prefer to gash out a gert
rough-n-ready lump, and squash a backbox onto a well-judged amount of
stiff plaster mix, letting the plaster that squeezes through the
mounting holes spread out and hold the box, along with the
tightly-packed plaster around the sides. If you're rewiring a house, the
time saved doing it this way adds up (and you can production-line the
fixing - chop holes, pull cable leaving over-long lengths sticking out,
mix up one batch of plaster and fix 10-15 backboxes at once). But for
those of us whose domestic authorities require Making Good to a high
standard, a neat and well-sized recess (little bits of Polyfilla/repair
plaster round the edges if your bolster gets a bit too energetic), with
the backbox held in place with a coupla screws-into-wallplugs, is the
way to go.

HTH - Stefek

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

David Hearn wrote:
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 MK have two specific ranges: sockets which have thicker
faceplates and can be accommodated in shallower boxes, as well as
standard ones with thinner faceplates that need deeper boxes.

The deeper box the better.

The boxes should be sunk so that the edge of the box is flush with the
finished wall surface or near-as-damnit. It's not too critical with
plaster, but if a wall were to be clad with a timber-based panel a gap
between the box edge and the faceplate might mean the terminals failed
to be enclosed in a non-combustible enclosure.

And boxes are usually fixed in with a couple of nails and a prayer, but
two No 6 screws (with the slots lined up :-) makes a diypersonlike job
of it.

Owain