Wall Rendering (Cement-Lime Roughcast Plaster)

A good render starts long before the first trowel of mortar goes on the wall. A typical job looks like this:

1. Preparing the background: the wall has to be sound, clean and free of dust, grease and loose material. On old walls, anything that does not hold well is knocked off, and larger holes are filled beforehand.
2. Spatterdash coat or primer: smooth concrete gets a thin cement slurry thrown on (a spatterdash or splatter coat), which creates a rough "key" for the render to grip. Very absorbent backgrounds, such as aerated concrete blocks, get a primer or a key coat as the render manufacturer recommends. This stops the wall from sucking the water out of the mortar too quickly.
3. Fixing beads: plaster beads (or dots of mortar) are set on the wall to fix the plane and the thickness of the render. Angle beads go on external corners so the edges stay straight and resist knocks.
4. Mixing: a bagged dry mix is mixed with water exactly as the instructions say. If the mortar is made on site from cement, lime and sand, the ratio depends on the purpose: more cement means a harder render but less breathability.
5. Applying: the render is thrown on by hand with a trowel or sprayed with a rendering machine. One coat is usually around 10 to 20 mm. Thicker build-ups are done in two coats, and the second only goes on once the first has started to firm up.
6. Ruling off: a straightedge resting on the beads takes off surplus mortar, and hollows are filled in. Once the surface has firmed up, it is worked with a float or sponge, depending on whether it stays as a textured finish or gets another coat.
7. Curing and drying: the render should dry naturally, out of direct sun, frost and strong draughts. In hot weather it is lightly misted with water when needed. Heaters or blowers should not be used to speed it up, or the render will crack.

Hand application suits smaller areas, repairs and walls with lots of reveals and recesses, or places where a machine cannot be set up easily. It is slower, but the plasterer has full control over every part of the wall.

Machine application pays off on whole houses and large, open wall areas. The mix stays more consistent and the work goes much faster. It does need a water and power supply on site, room for the machine and careful masking of windows, doors and floors.

The right mix depends on the background and the location:

• Living spaces indoors: a standard cement-lime base coat, later finished with a skim or thin-coat finish
• Aerated concrete and lightweight blocks: lightweight renders made for these materials, so the render is not harder than the wall behind it
• Damp cellars and old masonry: renovation (salt-resistant) render systems, because ordinary render on a damp wall soon comes away
• Exterior walls and plinths: weather-resistant renders that can cope with splashing water, used as part of the facade manufacturer's system

Two jobs with the same wall area can differ quite a lot in effort, and so in cost. The work goes more smoothly with:

• Straight walls built from precision blocks, where a thinner coat is enough
• Large, uninterrupted walls without many windows and corners
• Easy access and water and power ready on site

The work takes longer with:

• Old, crooked or damp masonry that needs a thick coat to straighten it
• Removing the old plaster and repairing the background
• Lots of reveals, corners and chases cut by the electrician
• High ceilings or stairwells that need a scaffold
• Working in an occupied home, where everything has to be covered and cleaned up

If you live in a block of flats, agree the timing of the work, removal of rubble and protection of shared areas with the building management or owners' association beforehand. Rendering interior walls usually does not need permission from the authorities. If the work affects the facade, load-bearing walls or forms part of a larger renovation, check with your local building authority what the local building regulations require.

• Rendering onto a dusty or bone-dry background: the render does not bond and after a while sounds hollow or falls off.
• Skipping the spatterdash on concrete: smooth concrete gives the mortar nothing to grip.
• Too thick a coat in one go: the render sags and cracks as it dries.
• Rendering in frost or full sun: below roughly +5 °C the mortar does not set properly, and in the heat it dries out too fast.
• Skimming or painting too early: in practice it often happens that the next coat goes onto damp render, and a few weeks later stains or cracks appear. A rough rule of thumb is about one day of drying per millimetre of thickness, but the manufacturer gives the exact time.
• No reinforcement where different materials meet: for example brick next to a concrete ring beam. Without mesh embedded in the render, a crack almost always appears there.

You can handle a smaller area in a utility room yourself if you have some patience. For a whole floor of living space or a facade, it is more sensible to call a plasterer. Leave any work at height or on scaffolding to professionals.

1. Knock off any loose material and brush the wall free of dust.
2. Apply a cement spatterdash to concrete, or a primer to absorbent masonry, following the instructions.
3. Set plaster beads plumb and level with a spirit level and let them firm up.
4. Mix the dry render with water exactly as the bag says, only as much as you can use within its working time.
5. Throw the mortar on with a trowel, working from the bottom up in small sections.
6. Rule off the surplus with a straightedge resting on the beads and fill any hollows.
7. Work the surface with a float once it has firmed up.
8. Protect the render from draughts and frost and let it dry fully.

• A long straightedge held against the wall shows no obvious hollows or bumps; the permitted deviation is set by the relevant standard or by your contract
• Corners and the edges of reveals are straight and plumb
• The render sounds solid everywhere when tapped; a hollow sound means it is coming away from the wall
• There are no cracks, crumbling patches or blotches from uneven drying
• Render is neatly finished around sockets, pipes and door frames
• Windows, floors and services have been cleaned of mortar after the work