Making Good Walls After Electrical Installation

Chases left by an electrician and those left by a plumber look similar, but they differ in size and in the room conditions. That decides the material, the method and how much work is involved.

• After electrical work the chases are usually narrow and shallow, because a cable or flexible conduit takes up little space. Openings for socket, switch and light boxes come on top of that. These are mostly dry rooms, so a gypsum-based or fibre-reinforced filler is enough.
• After plumbing work the chases tend to be wider and deeper, especially for waste pipes and insulated pipework. Larger pieces of wall are often broken out around outlets. Bathrooms and kitchens are damp, so a lime-cement or other moisture-resistant mortar is the usual choice.

If the same wall also has pipes in it, one rule matters. Pipework is only covered after the plumber has pressure-tested it. If a leaking joint is plastered over, repairing it means cutting the wall open again. Hot water pipes should also be sleeved before covering, because they expand and contract with temperature and can crack the plaster above them.

The method is basically the same for all chases. What changes is mainly the layer thickness and the material.

1. Check the cables. Cables must be firmly fixed, for example with clips or dabs of plaster, and conduit held in place. Take photos of the routes with a tape measure against the wall. They are very useful later when you drill for shelves or pictures.
2. Clean out. Brush or vacuum the dust from the chase and remove any loose bits of plaster. The base should be sound, dry and free of grease.
3. Prime or dampen. A primer strengthens the surface and stops the masonry drawing water out of the filler too quickly. Aerated concrete blocks and concrete react to this more than brick does.
4. Fill in layers. A deeper chase is not filled in one go. Press the first layer in well, let it firm up, then add the next. A thick layer applied at once slumps and cracks.
5. Reinforce. For wider and deeper chases, roughly over 1 cm, and where two different materials meet, embed fibreglass mesh or reinforcing tape in the filler. It spreads the stress and prevents hairline cracks.
6. Level and sand. Leave the final layer slightly proud, sand it with fine paper once dry and check it with a straightedge.

• Gypsum-based fillers and bonding plaster dry quickly, trowel smoothly and sand easily. They suit dry rooms and are the typical choice after electrical work.
• Fibre-reinforced fillers and plasters are more flexible and stronger. They work well on difficult backgrounds and in spots prone to cracking.
• Lime-cement and cement mortars tolerate moisture and greater thicknesses, but dry more slowly. They belong in bathrooms, kitchens, cellars and anywhere tiles will go on.
• Lime and clay plasters are used in older houses with the same original plaster, or where the owner wants a breathable wall. The filler should match the original plaster, otherwise a crack forms more easily along the joint.

Plasterboard walls are not plastered. Openings and cuts are made good with jointing compound, with tape where needed.

How much work the job takes, and so what it costs, depends on several things:

• Length and depth of the chases and the number of back boxes
• The background: a newer building with flat plaster is simpler than an old house with crumbling plaster and mixed masonry
• Whether mesh and several layers are needed
• Access: work behind fixed furniture, in tight spaces or near the ceiling goes more slowly
• The finish required: a base for tiles only needs to be flat and sound, while a smooth painted finish has to be sanded perfectly

If chases cover a large part of the wall, or the original plaster is cracked and sounds hollow when tapped, filling alone is not enough. Then it is worth considering a full skim coat with mesh or replastering the whole wall. In a block of flats, work in shared walls or on communal wiring is agreed with the building management or owners' association.

You can handle smaller cable chases yourself once the cables are in place and the electrician has checked them. Don't touch the wiring itself, and don't cut new chases or move cables. That is a job for a qualified electrician, and in many countries the work must also be certified.

1. Check that the cables are firmly fixed and sit below the wall surface.
2. Cover the back box openings so no filler gets in.
3. Vacuum the chase and remove loose pieces of plaster.
4. Apply primer to the chase and let it dry as the instructions say.
5. Mix only as much filler as you can use before it sets.
6. Press the first layer deep into the chase with a filling knife.
7. On a wider chase, embed mesh in the next layer, overlapping onto the surrounding wall.
8. Strike off the final layer slightly proud of the surface.
9. Let it dry completely, going by the manufacturer's drying time rather than by touch.
10. Sand smooth and check against the light with a straightedge.

• A single skim of filler. In a deeper chase the filler shrinks and cracks.
• Skipping the primer. The filler loses water too quickly, ends up weak and flakes off.
• Forcing it to dry. Heating on full or a strong draught causes cracks. Most manufacturers ask for at least around 5 °C, and room temperature is better.
• Painting a damp surface. Patches and blotches appear on the wall.
• Too little sanding. After painting, every edge shows in side light.
• Burying faults. Cables that are loose or poking out, or pipes that haven't been tested, get covered over and cause trouble later.

Before painting or tiling, look over the wall yourself:

• Repaired areas are flush with the wall and a straightedge does not rock on them
• Side light from a torch shows no waves or steps
• There are no hairline cracks and no hollow sound when tapped
• Back box edges are clean and the boxes are not filled with plaster
• The material is fully dry, with no darker damp spots
• You have photos of the cable routes, and for any pipework, confirmation of the pressure test