Backbox depth

[David Hearn]

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

[Dave Plowman (News)]

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

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.

[Stefek Zaba]

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

[Andrew Gabriel]

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.

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.

[Owain]

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