Mortar is the cement used outside the structure — in walls and plaster — and together it takes a large share of a house's cement. Choosing the right ratio for each job saves cement where strength is not needed and avoids cracks where too much is used.
Which Ratio for Which Job?
| Ratio (cement : sand) | Typical use |
|---|---|
| 1 : 3 | Pointing, ceiling plaster, waterproofing coats, repairs |
| 1 : 4 | External plaster, 4.5 inch brick walls, plaster in wet areas |
| 1 : 5 | Masonry in exposed conditions |
| 1 : 6 | 9 inch brick and block masonry, internal plaster |
| 1 : 8 | Non-load-bearing, non-exposed masonry |
Cement and Sand per Cubic Metre
Mortar quantities use a dry volume factor of about 1.27 (some estimators use up to 1.33 to include wastage).
| Mix | Cement | Cement (bags) | Sand |
|---|---|---|---|
| 1 : 3 | 1.27 ÷ 4 = 0.318 m³ ≈ 457 kg | 9.1 | 0.95 m³ |
| 1 : 4 | 1.27 ÷ 5 = 0.254 m³ ≈ 366 kg | 7.3 | 1.02 m³ |
| 1 : 5 | 1.27 ÷ 6 = 0.212 m³ ≈ 305 kg | 6.1 | 1.06 m³ |
| 1 : 6 | 1.27 ÷ 7 = 0.181 m³ ≈ 261 kg | 5.2 | 1.09 m³ |
Estimators who use a factor of 1.33 get figures about 5 percent higher. Either is reasonable; use one convention throughout your estimate.
Why Not Make Everything 1:3?
- Cracking. Richer mortar shrinks more as it dries, causing cracks in plaster and joints.
- Stiffness mismatch. Mortar much stronger than the bricks or blocks concentrates movement and cracking at the joints.
- Cost. A 1:3 mix uses nearly twice the cement of 1:6.
The aim is mortar suited to the masonry and exposure, not maximum strength.
Mixing Good Mortar
- Use clean, well-graded sand, sieved for plaster.
- Measure by box, not shovel.
- Mix dry, then add water to a workable consistency.
- Use mortar within about 30 minutes of adding water; never re-temper stiffened mortar with more water.
- Cure masonry and plaster for at least 7 days.
The Short Version
1:6 for thick walls and internal plaster, 1:4 for thin walls and external plaster, 1:3 for pointing and specialist coats. Leaner is not weaker when it matches the masonry, and it cracks less.
