What is the best method of cutting out for back box in blockwork?

[Notch7]

Ive Got a couple of sockets to fit in my spare bedroom, I wondered whats the neatest way to chop out?

The house was built in 1959, it’s some sort of blockwork, darkish and crumbles quite easily - it’s not the good old bluish breeze block


Do I drill loads of holes, use an angle grinder, or bolster?


PS: the only existing socket is a spur that is fed from an original JB. I was going to replace existing socket with a dual box with FCU, then spur off that (after fuse) to another socket on opposite wall. I hope Im allowed to do that?

 

[sparkwright]

I would mark out the box, and then 'chain drill' round the marks, the holes fractionally deeper than the depth of the box.

Then carefully break through the holes with a hammer and bolster. You don't have to go full depth, just enough to neatly cut through the plaster.

Then you can think about going deeper round the edges with the bolster, and also chip away at the main area with either a bolster, or a cold chisel or a comb chisel - which ever makes it easier.

Keep checking the depth by inserting the box into the recess. No point going too deep, it only makes it harder to make good.

Be mindful that some blocks have hollow sections - if you go too crazy you will have nothing to fix to.

 

[flameport]

as above, but before drilling, score the plaster on the outline of the box using a knife, which will mostly avoid the edge of the plaster cracking into a jagged mess.

 

[fidom]

Sometimes if plaster is stale it sets more rapidly than normal. I don't throw it out, I keep it for jobs like this, where you want the box to be firm before completing the wiring.

 

[pcaouolte]

If the blockwork is fairly soft a multi tool with a carbide blade can cut a neat edge to the box, use a bolster to take the meat out of the middle of the square and then use the multi tool again to grind the blockwork flat at the back.

 

[Mottie]

A sparky mate showed me this years ago when I was helping him out on a rewire. After you've chased a hole in the wall for the back box, flop a dollop of cement/plaster/filler in the hole and then set the back box to the depth and position you want by squeezing it in and trimming off any surplus that oozes out. Leave it a day (providing you have time) and stick a screw in it when the filling has gone off. You get a nice solid fixing doing it that way.

 

[Harry Bloomfield]

The house was built in 1959, it’s some sort of blockwork, darkish and crumbles quite easily - it’s not the good old bluish breeze block


Do I drill loads of holes, use an angle grinder, or bolster?

Some blockwork of that age can be quite thin, the risk is of blasting straight through to the far side, with direct blows with a hammer and chisel. I would try to direct any blows - rather than at right angles to the wall, angled down, up or sideways.

 

[johnny2007]

Use one of these and you won't destroy the wall.

RS Electrical Supplies

RS Electrical Supplies

www.rselectricalsupplies.co.uk

Cheaper versions available.
I usually drill a hole in an ice cream tub and catch all the dust after having started the hole.
If you only need it for 1 cut, maybe you could rent it out (not sure about this though)
Or, as others have suggested, set the depth on a 6mm drill bit and drill a grid of holes in the square you need.
Then gently break it all up with a chisel.
Sometimes with soft material like breeze blocks you can just grind in between the 6mm holes with the drill bit and get a neat finish.
Use a collar on the bit so you don't go deeper than needed.

 

[Notch7]

Use one of these and you won't destroy the wall.

RS Electrical Supplies

RS Electrical Supplies

www.rselectricalsupplies.co.uk

Cheaper versions available.
I usually drill a hole in an ice cream tub and catch all the dust after having started the hole.
If you only need it for 1 cut, maybe you could rent it out (not sure about this though)
Or, as others have suggested, set the depth on a 6mm drill bit and drill a grid of holes in the square you need.
Then gently break it all up with a chisel.
Sometimes with soft material like breeze blocks you can just grind in between the 6mm holes with the drill bit and get a neat finish.
Use a collar on the bit so you don't go deeper than needed.

I saw those, a bit dear for a couple of sockets.

Do they work well?

 

[Notch7]

Thank you all so much for the great advice.

I think I shall buy a comb chisel - seems ideal for any chopping out.

And I will prob get a bag of bonding, as Mottie mentions - it’s an easy way of getting the back of the hole flat.

 

[crystal ball]

Thank you all so much for the great advice.

I think I shall buy a comb chisel - seems ideal for any chopping out.

And I will prob get a bag of bonding, as Mottie mentions - it’s an easy way of getting the back of the hole flat.

I used to knock the knockouts in slightly to give the bonding a key

 

[johnny2007]

I saw those, a bit dear for a couple of sockets.

Do they work well?

Yes, but not worth just for 2 sockets.

 

[chivers67]

Like PacaouolteI use a multi tool for the box outline (with a tile cutting bit) and an Angled masonary chisel bit. Less than a tenner on Amazon.

 

[markyd1]

Addition to a circuit...Minor works cert and RCD will prob need upgrading