Sinking boxes into hollow concrete blocks

[wannocks]

Hi Chaps, I've agreed to sink some double back boxes into the block work to make life easier for the chap who's coming to extend the ring into a new bedroom. The walls are hollow concrete block so will be a royal pain to sink the boxes into. I plan to dot and dab plasterboard onto these walls but I don't think this will provide enough depth for the boxes.
My question is, what is the minimum depth that will allow a standard (plastic) double socket front to be surface mounted on the block? If not some advice for sinking a 35mm into the block would be appreciated.

 

[PrenticeBoyofDerry]

Is the wall being plastered afterwards.
If so you'll get 12mm from board something the same from the dot'n'dab(maybe a little less and about 5mm from the skim.

 

[JBPElectrics]

Hi, It is usual to get away with a 25mm metal back box.

Cut the hole by hand and just chuck it in! Works a treat!!!

CBW

 

[wannocks]

25mm surface mount it is then! All the original ones in the house are 16mm in the plaster finish and are very tight. No metal fronts here

 

[PrenticeBoyofDerry]

If the fittings are flat plate, flush fitting you will need 35mm box. If standard stepped fittings 25mm will be okay.
You don't want surface mounted boxes as you are sinking, metal ones need to be used.

 

[wannocks]

Oops, better make myself more clear
Plan is to use 25mm standard steel installation boxes surface mounted on the block work, then dab and dot plasterboard and skim to 25mm depth.
No chance of ever using flat plate accessories as the rest of the boxes in the house are only as deep as the plaster (16mm)

 

[PrenticeBoyofDerry]

If you do that, when you fix D'n'D boards, bring them out as much as you dare as you don't want to be filling out with skim(3-5mm under) and two coat skim too or batton/timber out wall and screw fix boards. Still 3-5mm under though for skim.

 

[mikhailfaradayski]

Surface mount the 25mm b/boxes direct to the block.
When installing cables and clipping, use 10mm T+E clips as the first clip adjacent to the box.
These should be thick/deep enough so that when the boards go up, they can press against the clips and stop the boarder from pushing them on too far, stops them leaving the front edge of the box proud