I'm re-wiring a 1930's bungalow which must have been rewired in the 1960/70's(?).
I've come across a old double socket backbox, which is 60mm deep and was mortared in - took the best part of a morning to get out!!
I now have a deep hole (75 x100 x 150mm) in a single brick wall. How do I fill this so that I can get a modern backbox to fit?
You are lucky to have any brick left at the back. Is it *really* a single brick? What's behind it: cavity, another room, the outside air, something else?
Probably best not to try to use screws and plugs into the back, but you should be able to "glue" it in place with all sorts of things. Mortar, plaster, car body filler? Probably not expanding foam, but you could glue it to the back with a dab of that, then run the wires, then fill the gaps with plaster or other filler to add some strength. Remember that ordinary plaster shrinks a lot, so if there is much thickness, do it in thinner layers letting each dry in between. Try to keep expanding foam or car body filler away from electrical insulation.
Unless you are going to save it a cold chisel and lump hammer should be enough to bend the sides in to ease removal within a few minutes
Bonding plaster. First paint the opening with a 50:50 water:pva mix and let it dry. This will help prevent the brick or existing surrounding plaster almost instantly sucking all the water out of the new plaster and it cracking up. Before applying the plaster paint the opening again and let it dry to the sticky state. Apply the plaster in the hole and insert your new back-box using wooden wedges/spacers to position it whilst the plaster dries. You are not trying for the finished result with this initial fitting. I recommend always fitting a 35mm deep back box where practical and don't forget to fit the rubber grommets in the holes you have knocked out in the box.
If chasing any wall for the wires also thoroughly seal with the 50:50 water:pva mix before filling.
If it's internal, I would think about putting something heavy behind it where you can. Voids behind sockets can be a route for sound to travel from one room to another, compromising your soundproofing.
Theo
Can't disagree, but the OP might not want to buy a whole sack for that job. Patching plaster is available in smaller quantities, both dry and pre-mixed. Also, if mainly in contact with brickwork, PVA is perhaps a Rolls Royce option, but I agree that it is worth moistening the hole with water first.
It really depends on how much "bulk" filling is required for the whole job. A 12.5Kg bag of undercoat/bonding plaster is only a few quid more, although expensive compared to the price of a 25kg bag.
I tend to always have PVA to hand so not problem with using it for sealing existing brickwork/plaster before any filling. In my house the
100 year old, one inch thick, plaster does act as sponge and the dried PVA acts as an effective temporary waterproof barrier long enough for plaster/filler to go off.Tip: if buying a one off bag of plaster put it in a plastic rubbish sack before you load it into the car - this will prevent the plaster dust getting on the mats in the boot of the car and on your cloths when subsequently off-loading and carrying into the house :)
Unused undercoat plaster can be used in place of lime to improve your soil if you are on heavy clay. Just chop the sticky clay into slices with a spade and sprinkle it in then mix and chop it in well.
Over the years I've sprinkled unused undercoat plaster on my lawn (when rain has been expected) with no adverse results.
Thanks to all. The one problem that isn't clear is what I then attach the box to. I had thoughts that I would somehow wedge a piece of wood in and screw into that, but will I be able to use the plaster to hold that bit of wood in place?
To answer the question about the wall this is in, yes it is single brick so there may only be a long inch a brick left !! It is between the hall and a kid's bedroom so thanks for the comment about noise transmission, but probably not a problem.
Old[1] bonding plaster is ideal for this, mix some up partially fill the hole, and push in the new box, level it, fill flush around the edges and leave it for a bit.
[1] Old plaster sets much faster than fresh stuff, so for patching mall areas like this is ideal since it will go off in a few mins.+1 That footnote is true for bonding but, AFAIK, not for multi-finish. Mixing the two seems to give a good filling plaster with a reasonable set time.
Wood and No More Nails, then back box, then repair plasterwork?
You can't screw into an inch of brick and wood buried in the wall is not going to be fire-risk, without anywhere to spread.
You can, but perhaps best attack from the other side of the wall!
I keep and out-of-date part-used bag of Wickes fine surface filler for jobs like this. Sets in a few minutes and can be scraped and sanded too unlike gypsum plaster.
Plaster of paris is handy for jobs like this too.
IME multi finish also gets quicker with age, just not such a large change in speed as you get with bonding.
Fair point about the finish though - if you are coming flush to the plaster and are not planning to add a fine filler as the top coat, then bonding on it's own will leave a slightly rough finish that would not look good painted.
With 25mm+ of plaster at the back of the box and the bit around the sides it will be firmly held in place once the plaster goes off - it will not move!
but will I be able to use the plaster to hold that bit of wood in place?
Don't use wood behind but you will need some temporary wedges/spacers to hold the box flush with the surface of the wall and top horizontal until the plaster goes off (insert the box into the wet plaster). Filling of the gaps when the wedges/spacers are removed will further strengthen the fix. Just make sure that you leave space for the cable when plastering in.
wouldn't it be easier to re-tap the fixing holes to m3.5 on the existing box?
Out of date gypsum plaster can be used in the same way if you can work quick enough before it sets in a few minutes. You may have to very quickly mix it at the site of the fill to maximise the time you have to apply it :) While in that still wet but gone off stage it can be easily scraped away and once dry sanded with a coarse grit sandpaper. Again I would only use it for the initial rough fix and use something finer for the final surface finish. I'm not sure I would risk an well out-of-date plaster product for the OPs case where maybe some time will be required to position the socket square and flush before the plaster/filler goes off.
I current have some left over plaster in a a few airtight containers of various sizes for quick/small jobs.
Well my house was rewired in 1973 and I don't have a single box that deep though sometimes I wish they were!. Sounds a bit odd to me. I'd be pretty sure the wall must be thicker than one brick or the original fitting would have never been able to be fixed in. Brian
For small quantities of PVA I dilute some wood glue, usually the waterproof sort, and use that. Too expensive for anything big. Never needed enough PVA to be worth getting more.
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