5 Mortar, render and screed
Fine sand jobs. Washed sand with little clay, mixed with cement and often lime. The thinner the layer, the finer the sand.
- 5.1Laying bricks and blocksMortar for walls and piers. 0–2 or 0–4 mm sand in 10 mm joints, usually 1 cement : 1 lime : 6 sand.
- 5.2Rendering a wallA two-coat render on masonry. 0–4 mm sand in the base coat, 0–1 or 0–2 mm in the thin finish coat.
- 5.3Laying a floor screedA level sand-cement layer on a slab. 0–4 mm sharp sand, 1 cement : 4 sand, laid semi-dry.
The mixes in this chapter cement : lime : sand
| Mix | Used for | Measured by | Class |
|---|---|---|---|
| 1 : 1 : 6 | Walls above ground, render base coat (5.1, 5.2) | Volume | M4 |
| 1 : ½ : 4–4½ | Walls near or below ground, where they stay wet (5.1) | Volume | M6 |
| 1 : 2 : 9 | Weak finish coat on soft masonry (5.2) | Volume | M2 |
| 1 : 4 | Floor screed, cement : sand (5.3), about 1 : 3½ by bucket | Weight | – |
Mortar mixes and classes from the UK National Annex to Eurocode 6 (Table NA.2). Screed mix as used in 5.3. Loose cement weighs about 1.4 t/m³ and dry sand about 1.6 t/m³, so a 1 : 4 screed by weight is about 1 : 3½ by bucket.
Why the sand size matters
Every grain has to fit inside the layer with paste around it. A 10 mm joint or a 12 mm base coat takes sand up to 4 mm. A thin finish coat needs finer sand, and a smooth finish finer still. Fine and coarse aggregate are split at 4 mm (EN 12620), so all sand in this chapter is 0–4 mm or finer.
Chapter 5
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