Helpful video reference. This UK tutorial "How to chase a wall and fit a metal back box - Chasing a Concrete Wall" walks through the full process of drilling, chiselling and fixing a back box into solid masonry. The technique shown - close-drilled perimeter, then bolster to clear the centre - is the standard method used by UK electricians.
1. Scan for hidden cables and pipes
Run a cable avoidance tool or a combined cable and pipe detector over the area where you plan to chase. Under BS 7671, cables should run in defined safe zones - vertically above switches and sockets, or horizontally from them - but older wiring in pre-1970s properties often does not follow this convention.
Mark any detected services on the wall with tape before you pick up a drill. If the detector shows something you cannot explain, stop and investigate before going further.
2. Mark out the back box position
Hold the back box against the wall at the height you want. Standard socket height in a lounge or bedroom is 300 mm from the finished floor to the bottom of the socket; kitchens often use 900-1000 mm to clear the worktop. Check the box is level with a spirit level, then pencil a line around the outside. Add 3 mm clearance on all four sides: the box must drop into the recess freely, not be forced in.
3. Score the outline
Score firmly along your pencil lines with a sharp bolster chisel, or use an angle grinder fitted with a masonry or diamond disc. Scoring first cuts through the plaster or render skin and prevents it from spalling back beyond your lines when the drilling begins. Keep the scored line vertical and square - it will show around the faceplate if it wanders.
4. Drill the perimeter and base
Fit a 6 mm or 8 mm masonry bit in the SDS drill. Drill closely-spaced holes (10-15 mm apart) around the inside of the scored outline, going to the required depth. For a standard 25 mm single-gang box, drill to 30 mm. For a deeper 35 mm box, drill to 38 mm. Once the perimeter is drilled, add a grid of holes across the full base area of the recess - this makes the chiselling much easier.
In very hard engineering brick or dense concrete block, a diamond-tipped core drill the exact size of the back box can be quicker than drilling and chiselling, though hiring one is rarely worthwhile for a single box.
5. Chisel out the masonry
Start in the centre of the drilled area and work outward to your scored lines with a bolster chisel and club hammer. Remove the masonry in stages - do not try to lever out large lumps at once or you risk cracking beyond the recess. Check the depth regularly by dropping the back box in and feeling whether it sits flush or proud. The base should be as flat and level as possible.
A wall chaser (a specialist disc-cutting tool) makes the surrounding chase for the cable much faster and neater than a bolster, though it produces a large volume of dust. Wet the wall surface slightly before chiselling to reduce fine dust, and always wear a dust mask and safety glasses.
6. Chase the cable run
Chase a vertical run from the top of the back box position up to ceiling level, or drop vertically to the floor and run under a floor covering. If routing horizontally, keep to within 150 mm of the ceiling or floor, or directly from one accessory to another - these are the safe zones defined in BS 7671. Make the chase just wide enough for the cable (a 2.5 mm twin-and-earth needs about 50 mm wide) and 25-30 mm deep.
Line the base of the chase with a thin bed of bonding plaster or press oval PVC conduit into the channel before running the cable through it. Conduit provides mechanical protection and makes future cable replacement possible without re-chasing.
7. Fix the back box and make good
Dry-fit the box in the recess, mark the fixing hole positions, remove the box, drill and plug the holes (6 mm masonry bit, red wall plugs for most brickwork). Thread the cable through the appropriate knockout entry, screw the box firmly into the wall, and check it is flush and level.
Fill around the box with bonding plaster or a repair filler, then apply a finishing coat once the bonding has dried. Leave the filler to fully cure - usually 24-48 hours - before painting. Fold a generous loop of cable slack inside the box before fitting the faceplate so the cable can be pulled out for future maintenance without disturbing the plaster.
When to call us
Chasing a recess and fixing a back box is practical work a competent DIYer can handle. The electrical connection - extending a ring final, running a new radial, or fitting a spur - is where Part P rules apply. Richard can survey the wall, check the cable route, carry out any notifiable electrical work and provide the certificate. Small jobs in Sandwich from £10 per 10 minutes.
Need a socket added in Sandwich?
Richard can assess the wall, chase the route, and connect the new socket - all notified under Part P where required.
Contact Richard