Filling behind back boxes

3 Jan 2021
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United Kingdom
I've been chasing out some back boxes ahead of my electrician coming in to re-wire my flat, and some of the bricks behind the chase have crumbled, leaving some quite large gaps behind where the back boxes are going to go.

I remember hearing somewhere not to use bonding to fix them in, as it can attract moisture (i'm in a basement flat so could be an issue)

Should I use a sand/cement mix to fill in behind the back boxes? And if so what ratios would anyone reccomend?
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You can buy a small tub of sand and cement where you only have to pour some into a container and add water and mix.
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I use bonding and add a small amount of cement powder in the mix. Set the box in a bed of that.
It sets solid very quickly. Then drill through the mounting holes, insert red wall plugs and fix with screw.
I've done almost the same as TTC above. Set the box in either bonding or cement, leave overnight and put a couple of screws in the next day.
Tap the spare knockouts in slightly to give a good key for the bonding
If you don't want to wait, use plastic spacers and once you find the correct ones, bed them in with adhesive, drill through and use normal red plugs to secure the box.
Bit late now but Is it ok to chase into a modern basement wall, I would have thought they may have some sort of damproof rendering these days, I quess they may now waterproof externally to the blockwork when building.
I remember one place where even drilling was banned, everything was mounted to wooden blocks glued to the walls, though it was 300 years old
I like to use sand and cement mortar. It has a long working time, so you can do a number of boxes with one bucketful, and you can spend time filling in behind and around the box, and leaving a good surface ready for final plastering. If you are a bit crooked you can scrape it out and reposition before it goes off. and is cheap.

I also think it strengthens a wall filling in where the bricks are broken and gappy around your backbox. And is unaffected by damp.

in my current area, local practice is/was that the internal walls are rendered with S&C then have a hard finish plaster skim (older houses are lime and sand) so I also fill chases the same way. It's much easier to use than plaster. It grips the box better, I think, than plaster, though it can be drilled and plugged easily.

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