Chasing cables and pipes into walls.
When running cables or pipes up (or along) a masonry wall, the neatest method is to bury (or, in builders terms, chase) them in the wall surface.
There are a few guidelines to be followed when chasing cables or pipes into walls:
- Chase cut-outs should always be vertical or horizontal between start and finish on the wall – never cut a chase at an angle between these two, nor step the channel.
- Vertical chases should be no deeper than one third of the wall thickness – with standard 100mm bricks and blocks, that going to be 33mm, which is quite deep anyway – that’s not allowing for any plaster coating which could be 10mm so the maximum depth then works out at 36mm from the front face of the plaster.
- Horizontal chases should be no deeper than one sixth of the wall thickness - with standard 100mm blocks, that going to be 16mm which is usually quite sufficient – that’s not allowing for any plaster coating.
- Chases on opposite sides of a wall should not be in line, i.e. ‘back to back’.
- If chasing in a cable, it should be noted that any new wiring circuit falls under Part P of the Building Regulations
The thinking behind the above guidelines fall into two areas:
- The person cutting a chase will probably not be the same person who drives in a nail in future years to hang a picture – so knowing that wires/pipes have only been chased in either vertically or horizontally will reduce the likelihood of him/her driving a nail through them.
- Cutting away masonry weakens a wall structure, more so with a horizontal cut than a vertical.
Most professional builder will have a chase cutting tool - basically an angle grinder with two parallel cutting discs with a depth setting. The builder simply sets the depth gauge, then runs the grinder up/down, or across the wall making two parallel cuts, the waste between the cuts is then knocked out used a cold chisel. Even with the usual dust extraction fitted to these chase cutting tools, they produce a fair amount of dust and appropriate precautions need to be taken to protect both the property and person.
For the DIYer:
- It may be possible to hire a chase cutting tool;
- The next best thing is to use a normal angle grinder with a stone cutting disc to cut two separate, reasonably parallel, cuts and then knock the waste out between using a cold chisel.
- The final option is to mark the sides of the chase on the wall, then use a power drill to cut away most of the ‘waste’ by drilling numerous holes to the required depth, and finally use a hammer and cold chisel to remove the waste and trim up the edges.
Whichever method you use remember to take the appropriate precautions:
- Check the wall for cables/pipes already chased into it.
- Use protective eye goggles/dust mask to reduce.
- If using power cutter, try and use one with a dust collection system; even with that, everything will end up covered by dust.
Putting the cable or pipe into the chase
Cable should be run either:
- In a conduit – both plastic and metal conduit is available, but for a chased run, plastic oval conduit is best as it’s fairly narrow so will sit in the fairly shallow depth and avoids the need to earth bond the conduit (as would be required with metal conduit).
It’s usually easiest to run the cable(s) through the conduit before you fix it in place. Once the cable is through the conduit, place it at the back of the chase and use a few large head nails on both sides of the conduit to hold it to the back of the chase.
- The alternative to conduit, is pvc channelling (metal channelling is available, but again should be earth bonded). With channelling, lay the cable down the back of the chase, position the channelling over it, and nail the side flanges of the channelling to the back of the chase.
Copper pipework is very rigid so can usually just be laid in the chase and secured using a few large headed nails on each side.
Filling the chase
Once the cable or pipe is in place, use a paint brush to wet the sides and back of the chase using clean water, then apply a coat of neat PVA adhesive to the sides/back of the chase and conduit/pipe, this will aid the adhesion of the filler.
As a filler, use:
- A strong sand/cement mix (3:1:1 soft sand:sharp sand:cement).
- One Coat plaster or patching plaster.
- A standard decorator's filler.
Both the plaster and filler may sag if applied to a deep chase, so they may need to be built up in layers.
The chase needs to be filled from the back and around the conduit/pipe to the front – a small trowel will help push the filler into the back corners and behind the conduit/pipe – use the trowel across the sides of the wall surface to cut off the filler to the line of the wall. If channelling is used, there’s no need to try to fill the chase behind it.
If the wall is to be tiled, there’s no real need to give the wall a fancy finish, but if the wall is to be painted or papered, cut back the surface of the filler by about 3mm before it fully hardens. After the filler has gone off, fill the front of the chase using a skimming plaster to give a smooth finish.