Cutting and Filling Chases in Brick and Concrete Walls for Wiring and Cables and Filling for Plastering

Warning: To complete electrical works you must comply with Electrical Regulations. Click here for more information.

Cutting chases in walls is not just a question of putting them where you want in any haphazard way. There are guidelines to follow to make the installation of electrical cables as safe as possible for you and anyone else entering your home. It is as well to note at this point that if you are chasing walls for the installation of a new circuit that this work may be covered by Part P of the building regulations – see our project for more info.

The Simple Rules for Cutting Chases in Walls

• Vertical chases should not be any deeper than one third of the total wall thickness
• In a cavity wall, vertical chases should be no deeper than one third of the thickness of the skin they are being placed in
• No chases, horizontal or vertical, should be made back to back
• Horizontal chases should be no deeper than one sixth of the thickness of the wall thickness
• In a cavity wall horizontal chases should be no deeper than one sixth of the thickness of the skin they are in
• Chases must go from a to b in a straight line and diagonal chases should not be used

Common sense really – If the chase is too deep it will impair the stability of the wall. If chases are back to back, there is no wall! It must always be remembered that the person doing the electrical work may not be the same person who wants to hang a picture on that wall or put a serving hatch or window in it. All chases therefore should be sensibly placed to give everyone a chance of working out where they are in relation to lights, switches, sockets and appliance points.

How to Chase out a Wall

Cutting a chase is a simple job and most electricians will own a chasing tool. The chasing tool seen in the image below is basically an angle grinder with two parallel blades. The depth of the blades can be set, a hoover hose plugged into the end and off you go. They are still dusty, even with the hoover, but the time they save is worth the cleaning up afterwards.

Make sure you set the depth of the cut to a little more than the depth of the conduit you will be running in the chase to allow you to fill over the top of it once it’s fixed in place.

You may also find it easier, in order to keep your chase line straight, to mark your lines on the wall with a pencil so that you can follow them.

As ever, always think safety – ensure that you are wearing protective goggles and gloves beofre you start.

Chasing tools in the above image:

1. Chase cutter – blades set to single chase
2. Chisel
3. Lump hammer
4. Large bolster
5. Medium bolster

You can see the blades for the chase cutter in the image above set to a single chase as are most cutters. When the chase lines are cut you will need a hammer and bolster to chop out the middle but this is really easy as long as the chase blades are not set too far apart. You can also see a hammer and an assortment of chisels in the image. It is now possible to buy a chasing tool which fits onto your electric drill.

Once you have cut your chase you can now run in your cables and conduit (ensure that your conduit is the correct width!). Remove the front cap by prising it off and then fix the conduit in place and run your cables down. With the cables in place replace the conduit cap by pushing it on until it clicks in place.

Filling Chases

Filling a chase when the conduit and cables are in is a matter of personal taste. Because of the depth we prefer to use a mix of sand and cement to make the chase really hard when it is set. This gives any future DIY enthusiast or builder a clue something is there should they try to hammer nails into the wall.

We mix the mortar at 3 soft sand to 1 sharp to 1 cement giving a strong mix which is still a little flexible. We then paint the chase with undiluted PVA adhesive to give the mortar the best chance of adhering to the conduit in the chase. The mortar is pushed well into the chase and the chase is filled. A trowel is then placed on both sides of the wall, straddling the chase and this is pulled upwards to cut off any surplus mortar.

The chase is then left for 30 minutes or so, when we use a pointing trowel to cut the surface back about 3mm to allow it to be finished off with skim plaster (information on plastering can be found in our skim plastering project). Skim plastering is not necessary when filling chases on walls to be tiled after fitting pipes etc for concealed showers.

Chases can also be filled with a plaster called patching plaster or One Coat plaster. This can usually be applied in coats up to 50mm thick but despite what it says on the packet, it will sag at this thickness so if you have a deep chase, build the fill up in layers. If using One Coat plaster make sure you wet the chase really well before applying it. The plaster will dry out really quickly and crack badly if its moisture content is not maintained.

Once the plaster has been laid on and allowed to dry for the required time to can decorate over with a finish of your choice and your done!