Cutting cable chases in brick, aircrete and concrete

Chases make sense wherever the wall will be replastered or at least skimmed. A cable hidden in the wall is protected from damage, takes up no space and cannot be seen. In a new shell or a renovation back to bare masonry, chasing is the normal approach.

In some situations chasing is not a good idea:

• Load-bearing walls in blocks of flats built from precast concrete panels, where a chase would weaken the panel or hit the reinforcement
• Thin partitions, for example blockwork only about 5 to 7 cm thick, where a deep chase would almost cut the wall in two
• Finished, expensive surfaces such as tiling or decorative plaster that are staying in place
• Walls with unknown contents, where old water, gas or heating pipes may run

In these cases the cable usually takes a different route. It can run in the floor under the screed, above a suspended ceiling, behind a plasterboard lining or in surface trunking. A good electrician will offer these options before starting to cut.

Marking out the route

Everything starts with a plan. The electrician follows the drawings or what you have agreed together and marks the positions of the back boxes, the consumer unit and the routes between them. A spirit level or a cross-line laser is used, with a pencil or marker spray. Routes run vertically and horizontally, never diagonally. The reason is simple. A few years from now someone will drill into that wall and needs to know roughly where to expect a cable.

Checking the wall

In a renovation the wall is scanned with a metal and live-voltage detector before any cutting. Old aluminium conductors, water pipes or gas pipes are not always where you would expect them. The existing circuits in the room are switched off at the consumer unit.

Cutting the edges

In brick and concrete the edges of the chase are cut with a wall chaser. This is a power tool with two diamond blades side by side. Width and depth are set to suit the number of cables. The tool is connected to an industrial vacuum, otherwise the room fills with fine dust within minutes. Short runs can also be cut with an angle grinder and a diamond blade, but that is dustier and slower.

Removing the material

The strip between the cuts is broken out with a chisel in a light breaker or SDS drill, or by hand with a bolster and hammer. In aircrete a hand chasing tool is often enough, and it simply scrapes the material out. The bottom of the chase should be roughly flat, without sharp lumps that would press on the cable.

Holes for back boxes

Preparation usually also includes drilling holes for the back boxes with a core drill. This is often a separate item in a price list, so check whether your quote includes it.

Cleaning out and laying the cable

The chase is vacuumed and dampened if needed. Then the cable or conduit is laid in and fixed. It is held with dabs of gypsum plaster, fast-setting mortar or clips. The filling should hold the cable firmly and finish flush with the wall or slightly below it, ready for the plaster coat.

This page covers two price-list items: chasing in brick and aircrete, and chasing in concrete and precast panels. The difference is not only in how hard the material is. The whole method, the tools and the risks change.

Solid fired brick

Older houses often have solid brick walls. Brick chases fairly well and behaves predictably. The trouble usually comes from hard mortar joints and old plaster of uneven thickness, which tends to come away in large pieces. In a renovation you should therefore expect more making good.

Hollow clay blocks

Newer masonry houses often use hollow clay blocks, Porotherm being a common example. The walls between the cavities are thin. If you hammer them hard, whole cavities break through and you get a hole instead of a neat chase. That is why this material is mostly cut and then gently broken out. Broken cavities must be filled properly, otherwise the plaster above them will later sink or crack.

Aircrete

Aircrete blocks, often called by trade names such as Ytong or Thermalite, are soft and easy to work. Chases go in quickly, often with a hand tool and little dust. Watch the depth, though. Aircrete is brittle, and a thin partition is easily weakened. Block manufacturers usually state how deep their walls can be chased, so it is worth checking the technical data sheet.

Concrete and precast panels

Concrete is hard. It cuts slowly and wears out blades quickly. It may contain steel reinforcement, which must not be cut. Precast panels are even more sensitive. Load-bearing panels are part of the structure of the whole building, and work on them is not just the flat owner's decision. Chases in panels are therefore rare, usually short vertical runs in a thin surface layer. Most routes go through the floor, above a ceiling or in the original service ducts.

Electrical cables in walls are laid in what are called installation zones or safe zones. These are bands where people normally expect cables to run. As a rule, horizontal runs go in a band roughly 15 to 45 cm below the ceiling or above the floor. Vertical runs go roughly 10 to 30 cm from room corners and from the edges of doors and windows, and directly above or below an accessory. The exact rules come from your national wiring regulations, and the electrician should know them. What matters for you: a cable should not cross the wall diagonally or run through the middle of the wall at the height where shelves and pictures usually go.

The depth of the chase depends on what goes into it. For a single cable of normal size, roughly 2 to 3 cm is usually enough. A conduit or several cables side by side need a deeper and wider chase. The cable must be covered by a proper layer of plaster or filler, not just a thin skim. On the other hand, unnecessarily deep chases weaken the wall and use more filling material. In many countries cables buried shallow in a wall must also be protected by an RCD. Your electrician will design the circuit to suit.

Load-bearing walls and blocks of flats

Load-bearing walls need care, especially with horizontal chases, and the amount of chasing should be limited. Structurally, a long horizontal chase in a load-bearing wall is worse than a vertical one. If you are not sure whether a wall is load-bearing, look at the building drawings or ask a structural engineer.

In a block of flats, contact the building management or the owners' association before any work on load-bearing walls. Whether work on the structure needs a notification or approval depends on local building regulations, so always check with your local building authority. Do not rely on simply "letting someone know". Ordinary chases in partitions during a rewire are a different matter from cutting into a load-bearing panel.

The job is priced per linear metre, but not every metre takes the same effort. The main factors are:

• Wall material: aircrete is quickest, brick is moderate, concrete and panels are slowest
• Condition of the wall: bare masonry is easier to chase than a wall with thick old plaster or tiles
• Width and depth of the chase: a chase for several cables or a thick conduit means more material to remove
• Height: runs just below the ceiling and ceiling chases are done from a stepladder or scaffold tower and take longer
• Access and furniture: an empty room is very different from a lived-in flat where everything must be covered and protected
• Dust control: working with extraction and sheeting takes longer but saves a lot of cleaning
• Unknown services: older houses often hide old cables and pipes that have to be worked around
• Scope: a few metres for one socket is a different job from a whole flat, where the tools are set up once and the work runs systematically

Check exactly what the item includes. Some quotes cover only cutting and removing material. Others also include filling the chase after the cable is laid, taking away the rubble or drilling the back box holes.

What should be ready before the electrician arrives

• A clear idea of where sockets, switches, lights and appliances will go, ideally drawn on a floor plan
• Knowledge of the furniture layout, especially the kitchen units and beds
• An emptied or covered room, with anything sensitive to dust taken out
• Access to the consumer unit and the option to switch off the relevant circuits
• In a block of flats, agreement from the building management and a word with the neighbours, because chasing is noisy

Order of work

Electrical chases usually come after the masonry is finished and before plastering. In practice their positions are coordinated with the plumber and heating engineer so that routes do not cross and no extra chases are needed. In kitchens and bathrooms it is important to know where the units, tiling and mirror will be, because that is where sockets are moved most often.

Once the cables are laid and the chases filled, the electrician inspects and tests the installation, and then the plastering or skimming follows. It is a good idea to photograph the finished routes with a tape measure in view before the plaster covers them. Anyone who drills into those walls later will be grateful for those photos.

How long it takes

In aircrete you can get through quite a few metres in a day, fewer in brick and far fewer in concrete. Chasing a whole flat during a renovation usually takes from one to several days, depending on the material and the scope. Gypsum plaster or fast-setting mortar used for filling sets in a matter of tens of minutes. Before plastering, though, allow the drying time stated by the manufacturer.

• Diagonal routes: the cable crosses the wall at an angle, someone later drills following the usual zones and hits it
• Chase too shallow: the cable sits just under the surface and the plaster above it cracks or shows a line
• Chase too deep in a thin partition: the wall is weakened and may crack later under load, for example when a cabinet is hung
• Broken block cavities: cavities that were not filled properly lead to sunken or hollow plaster
• No dust extraction: dust spreads through the whole home, into the ventilation and into electronics
• Chase not cleaned out: the filling bonds poorly with the base and may work loose
• Cable not fixed: during plastering it slips out of the chase and ends up almost at the surface
• No record of the routes: nobody knows where the cables run, and every later change turns into trial and error
• Cutting into reinforcement: a cut bar weakens the concrete structure and may start to corrode

A handy homeowner can cut chases in a brick or aircrete partition, provided you stick to the zones and work carefully. This does not apply to concrete, precast panels or load-bearing walls, which are work for a professional. Laying and connecting the cable, connecting it to the consumer unit and testing are always the job of a qualified electrician. Agree the routes with the electrician in advance.

1. Agree the box positions, routes and chase sizes with the electrician.
2. Switch off the circuits for the room at the consumer unit and check that they are dead.
3. Scan the wall with a metal and voltage detector and mark any services found.
4. Mark the route with a spirit level or laser, vertical and horizontal only.
5. Cover the floor, windows and furniture, and seal the door with plastic sheeting.
6. Put on safety glasses, a dust mask rated for fine dust, ear defenders and gloves.
7. Cut the edges of the chase with a wall chaser connected to a vacuum, or use a hand chasing tool in aircrete.
8. Break out the material between the cuts with a chisel, gently and without heavy blows in hollow blocks.
9. Check the depth and the flatness of the bottom, and remove any sharp lumps.
10. Vacuum the chase and leave it ready for the electrician to lay the cable.
11. Photograph the routes with a tape measure before they are filled.

Look at the chases together with the electrician before plastering. Afterwards nothing can be corrected without more cutting.

• Routes run only vertically and horizontally within the agreed zones
• Box positions match the plan and the furniture layout
• Chases have an even depth and no cable sticks out above the surface
• Broken cavities in hollow blocks are filled
• Cables are fixed in the chases and show no mechanical damage
• No reinforcement has visibly been cut in concrete
• The filling is firm, without voids or loose pieces
• Rubble has been removed or collected as agreed
• You have photos of the routes with a scale