Mortar holds bricks together and plaster protects and finishes the walls. Both are made from cement, sand and water, but the proportions differ, and getting them wrong shows up as crumbly joints, cracked plaster or walls that sound hollow when tapped. This guide sets out the mixes the Concrete Society of Southern Africa publishes for South African builders, with the material quantities you need to order.
Mortar classes
SANS 2001-CM1 groups mortar into classes by strength. On houses you will meet two:
- Class II is the normal mix for load-bearing walls. Corobrik describes a 1:6 cement and sand mix as Class II for normal load-bearing applications.
- Class I is stronger, for work where the plans or an engineer ask for it.
Mortar quantities per 1,000 bricks
| Mortar | Cement | Building sand | Batch (bags : barrows) |
|---|---|---|---|
| Class II, common cement | 150 kg | 0.6 m³ | 2 : 6 |
| Class II, masonry cement (MC 22,5X or MC 12,5) | 200 kg | 0.55 m³ | 2 : 4 |
| Class I, common cement | 200 kg | 0.55 m³ | 2 : 4 |
| Class I, masonry cement (MC 22,5X) | 350 kg | 0.5 m³ | 2 : 2 |
These figures are for standard bricks. Bigger units such as maxi bricks and blocks use more mortar each but need fewer units per square metre; the mortar calculator scales the quantities by joint volume.
Common cement or masonry cement?
Common cement is the general-purpose cement used for concrete, mortar and plaster. Masonry cement is made for mortar and plaster. In the Concrete Society’s figures you need more bags of masonry cement than of common cement for the same class, but less sand. Use one type consistently on a wall, and follow the bag’s instructions.
Plaster quantities per 100 m² at 15 mm
| Plaster | Cement | Plaster sand | Batch (bags : barrows) |
|---|---|---|---|
| Internal, common cement | 600 kg | 2.3 m³ | 2 : 6 |
| Internal, masonry cement (MC 22,5X or MC 12,5) | 700 kg | 2.25 m³ | |
| External, common cement | 700 kg | 2.25 m³ | 2 : 5 |
| External, masonry cement | 850 kg | 2.25 m³ |
External plaster is richer in cement because it must resist the weather. For other thicknesses, scale the quantities: plaster 20 mm thick uses about a third more than at 15 mm. The plaster calculator does this for you.
Joints and cement per square metre
The brick counts in BouWise assume 10 mm bed and head joints, which is what the standard brick’s coordinating size of 232 × 116 × 83 mm allows for. Working the Class II figure back to the wall: a single-brick wall uses about 52 bricks per m², so about 7.8 kg of common cement and 0.031 m³ of building sand per m²; a double-brick wall needs twice that. Thicker joints use more mortar than this, so keep them even.
Sand
Use building sand for mortar and plaster sand for plaster, and keep them apart from the concrete sand on site. Clayey or dirty sand makes weak, crack-prone mortar and plaster. If in doubt, ask your supplier for sand that suits the job.
Batching on site
- Measure sand in a builder’s wheelbarrow filled level (about 65 litres), and cement in whole 50 kg bags.
- Mix the dry materials until the colour is even, then add water a little at a time.
- Mix only what can be used before it stiffens. Don’t add water to stiffened mortar to bring it back.
- Mix on a board or steel plate, never on bare ground.
Plastering well
- Wet the wall before plastering in hot weather, so the bricks don’t pull the water out of the plaster.
- Don’t plaster over the damp-proof course. Stop above it, or cut a 10 to 15 mm V-joint at DPC level, as SANS 10021 asks. See damp-proofing.
- Keep new plaster damp for a few days so it cures rather than dries.
- Seal new plaster with a plaster primer before painting.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check SANS 10400 (the National Building Regulations) and your local municipality’s building control officer for your project.