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5.2 Rendering a wall

A two-coat sand, cement and lime render on brick, block or concrete walls, inside or out.

Cross-section through a rendered wall: masonry, a 12 mm base coat with a scratched key, and a 5 mm finish coat. 1 Masonry wall clean, damp, joints raked 2 Base coat 12 mm · 0–4 mm sand 3 Finish coat 5 mm · 0–1 or 0–2 mm sand
Cross-section, drawn much larger than life. Each coat is thinner and no stronger than the one under it.

At a glance

Difficulty
3 of 4
Coats
2 coats, about 17 mm in all
Tools
Mixer, hawk, plastering trowel, straight edge, scratcher or comb, wooden or sponge float, sprayer
Parts
  1. 1  Masonry wall
  2. 2  Base coat
  3. 3  Finish coat

What you need per m², 12 mm base + 5 mm finish, 1 : 1 : 6

MaterialSizeTypeAmount
Sand (base coat) 0–4 mm Washed, sharp, well graded 19 kg
Sand (finish coat) 0–2 mm Washed, for a textured or floated finish 8 kg
or 0–1 mm For a smooth finish
Cement – CEM I or CEM II 4 kg
Hydrated lime – Building lime 1.1 kg

Why 0–4 under, 0–2 on top?

0–4 mm

For the 10–15 mm base coat. Coarser, sharp grains make a strong, open coat that takes the scratched key and evens out the wall.

0–1 or 0–2 mm

For the thin 3–8 mm finish coat. The grains must be much smaller than the coat is thick, or they drag under the float and tear the surface. The finer the sand, the smoother the face.

Step by step

  1. 1

    Clean the wall and wet it.

    Brush off dust, loose mortar and paint. On smooth walls rake the joints out to give the render a key. Dampen the wall so it does not suck the water out of the render, but leave no standing water.

    Wetting the wall
  2. 2

    Apply the base coat, 10–15 mm, and rule it flat.

    Mix 1 cement, 1 lime, 6 parts 0–4 mm sand. Press it on hard with the trowel, then pull a straight edge across to flatten it. Do not go thicker than about 15 mm in one coat.

    Ruling off the base coat
  3. 3

    Scratch the base coat.

    When it has firmed up, comb shallow horizontal grooves across it. They give the finish coat something to grip.

    Scratched key
  4. 4

    Let it firm up and finish shrinking.

    Mist it with water so it does not dry out too fast. The next coat goes on only when this one is firm and has shrunk. With lime-rich mixes in damp weather this can take weeks.

    Misting the base coat
  5. 5

    Apply the finish coat, 3–8 mm, and float it.

    Same or weaker mix, with the finer sand. Lay it on evenly, then rub it with a float in small circles once it starts to stiffen.

    Floating the finish
  6. 6

    Keep it damp and shaded while it hardens.

    Mist it and keep it out of direct sun and wind for the first days. Render that dries too fast cracks.

    Finished render kept damp
Specifications
PartSieve sizeThicknessHow
1 Masonry wall––Clean, sound, dampened. Smooth surfaces keyed.
2 Base coat0–4 mm10–15 mm1 : 1 : 6. Ruled flat, scratched, left to firm up and shrink.
3 Finish coat0–1 or 0–2 mm3–8 mm1 : 1 : 6 or weaker. Floated. Kept from drying fast.

Mix

Cement : lime : sand by volume. 1 : 1 : 6 for the base coat on ordinary brick, block or concrete. On soft or old masonry use a weaker 1 : 2 : 9. The finish coat is the same mix as the base coat or weaker, never stronger.

Thickness

Base coat about 9–16 mm, finish coat about 3–10 mm. Two coats at those thicknesses come to roughly 12–26 mm in all. No single coat should be more than about 15 mm.

Sand

Sharp, washed and well graded, without clay or dirt. Dirty sand makes the surface craze. Use finer sand in the thin finish coat (see the chapter page).

Sources

  1. Mortars, plasters and renders (Spelthorne Borough Council, advisory leaflet 7): base coat 9–16 mm, finish 3–10 mm, each coat thinner and the same strength or weaker; 1 : 2 : 9 for weaker backgrounds; sharp, well-graded sand; dirty sand causes crazing
  2. US National Park Service, Preservation Brief 22: stucco: two or, more commonly, three coats; the first two together generally no more than about 16 mm (5/8 in), finish coat about 6 mm (1/4 in); stucco must not dry too fast or it cracks
  3. University of Babylon, Methods of construction and estimation, lecture 14: cement-sand rendering 1–2 cm; dry materials shrink by about a quarter when mixed; bulk weight of cement 1,400 kg/m³
  4. Public-sector works specification for screeds and plaster (UK Contracts Finder): scratch coat at most 15 mm and must dry and shrink before the next coat, which can take several weeks; avoid rapid drying
  5. NA to BS EN 1996-1-1 UK National Annex to Eurocode 6 (BSI 2007), Table NA.2: cement : lime : sand mixes 1 : 1 : 5–6 (M4) and 1 : 2 : 8–9 (M2) by volume
  6. NPTEL (Indian Institutes of Technology), soil mechanics worked examples: dry sand about 1,600 kg/m³
  7. US Office of Surface Mining (OSMRE), lime products, citing the National Lime Association: hydrated lime about 400 kg/m³
  8. EN 998-1 Specification for mortar for masonry: rendering and plastering mortar
  9. EN 13914-1 Design, preparation and application of external rendering
  10. EN 13139 Aggregates for mortar; ASTM C926 Application of portland cement-based plaster; ASTM C897 Aggregate for plaster

Amounts: 12 mm base plus 5 mm finish is 0.017 m³ of render per m². Dry materials take about a third more volume than the wet mix, so about 0.023 m³, split 1 : 1 : 6, with loose bulk densities of 1,400 kg/m³ for cement, 400 kg/m³ for lime and 1,600 kg/m³ for sand. They include no waste. Order about 10 % extra, more on uneven walls.

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