The correct cement plaster mix ratio in South Africa is set by SANS 2001-EM1. With ordinary common cement it is 1 part cement to 5 parts plaster sand for internal walls, and 1 to 4.5 for external walls.
Those numbers come from the Cement & Concrete SA mix leaflet, not from site habit. Getting them wrong is the single most common cause of cracked plaster in Cape Town homes.
The cement plaster mix ratio by wall type
The ratio changes with the cement you buy. Common cement (SANS 50197) and masonry cement (SANS 50413) are not interchangeable.
- Internal walls, common cement: 1:5 by volume
- Internal walls, masonry cement MC 22,5X or MC 12,5: 1:4.5
- Internal walls, masonry cement MC 12,5X: 1:2.5
- External walls, common cement: 1:4.5
- External walls, masonry cement MC 22,5X or MC 12,5: 1:3.5
Read the bag before you mix. A merchant will happily sell you masonry cement when you asked for cement, and the ratio you were told on the last job no longer applies.
Why 1:4, 1:5 and 1:6 all get quoted
You will hear all three on site. They are not all wrong, but they are not all right for the same job either.
- 1:3 — a rich repair mix for filling cracks and small patches only
- 1:4 — strong, used externally and where exposure is severe
- 1:5 — the internal standard with common cement
- 1:6 — weak for a base coat; more common as a thin finish coat
Richer is not better. A mix that is too strong shrinks harder than the brickwork behind it, and that shrinkage is what pulls a wall into a map of fine cracks.
Quantities per 100 square metres
For 15mm thick plaster over 100m2, the published quantities are straightforward.
- Internal, common cement: 600kg cement plus 2.3m3 plaster sand
- External, common cement: 700kg cement plus 2.25m3 plaster sand
- External, masonry cement MC 22,5X: 850kg cement plus 2.25m3 sand
That is 12 to 17 fifty-kilogram bags per 100m2, plus roughly one and a half cubic metres to two and a half cubic metres of sand. Order 10% over. Nobody has ever finished a wall with the exact bag count.
Measuring on site
Volume batching only works if the containers are the same. A builder’s wheelbarrow holds 65 litres, levelled flat with a plank.
- Batch cement by the whole bag wherever possible — it is more accurate.
- Use one container for every material in the batch.
- Level the sand off, do not heap it.
- Mix dry first, then add water.
Two 50kg bags of common cement take five level wheelbarrows of plastering sand for an external mix, or six for internal. That is the same ratio expressed in barrows rather than buckets.
Water, workability and the mistake everyone makes
There is no fixed water figure. You add enough to make the mix plastic and workable, and no more.
Extra water makes plaster easy to spread and weak once it dries. It increases shrinkage and it is why a wall that went on beautifully crazes within a month. If the mix is stiff, the answer is usually more mixing time, not more water. Mix ratios and workmanship standards are published by the NHBRC.
- Wet the brickwork before the first coat, but do not soak it
- Use the batch within about two hours of mixing
- Never re-temper a batch that has started to stiffen
Strength versus cracking
Cracks in new plaster almost always come from three things, and none of them are the bricklayer.
- The mix is too rich. Excess cement means excess shrinkage.
- It dried too fast. Cape Town’s south-easter will pull the water out of a wall in hours.
- It was not cured. Plaster needs to stay damp for three to seven days.
Curing is free and it is skipped constantly. Mist the wall lightly twice a day for the first week, or shade it during the hottest part of the afternoon. It costs nothing and prevents the defect that costs the most.
Before you paint
New sand-cement plaster needs a minimum of 28 days before paint goes on. Painting early traps moisture and the coating bubbles and peels within months. If the finish you want is glass-smooth, a skimming plaster coat goes on top of cured plaster, not instead of it.
When to mix it yourself and when to hire
A single patch or a garden wall is a reasonable weekend job. A whole house is not, and the reason is not the mixing.
- Keeping a batch consistent across a full day is harder than it looks
- Getting a wall flat and plumb over three metres takes practice
- Rework costs more than the original job, because it must be hacked off first
For anything structural or full-house, our brickwork and plastering team mixes to the SANS ratios above. Compare the finished rates in our plastering prices guide, or read the process in the wall plastering overview.
We offer a free on-site assessment and there is no call-out fee. Send us your wall measurements and we will quote the mix, the materials and the labour separately so you can see exactly what you are paying for.
Frequently asked questions
How do you calculate a plastering ratio?
Work by volume, not weight. Add the parts together, then divide. A 1:5 mix is one part cement to five parts plaster sand, so cement is one sixth of the total volume. Use the same container for every material in the batch, level the sand off flat, and batch cement by the whole 50kg bag wherever you can because it is more accurate than measuring loose.
What is the best ratio of cement and sand for plastering a house?
With ordinary common cement, use 1:5 for internal walls and 1:4.5 for external walls. If you are using masonry cement, the ratios change to 1:4.5 internal and 1:3.5 external for MC 22,5X. A 1:3 mix is only for crack repairs and small patches, because a rich mix shrinks hard and crazes a full wall.
What is the cement-mortar ratio for wall and ceiling plastering?
Wall and ceiling plaster use the same internal specification, which is 1:5 with common cement. Ceilings are harder to apply rather than differently mixed, since the plaster must hold overhead without slumping. Many Cape Town ceilings are skimmed with a gypsum-based product instead of sand-cement plastered, which is a different material entirely.
How many wheelbarrows of sand to one bag of cement for plastering?
Two 50kg bags of common cement take five level builder’s wheelbarrows of plaster sand for an external mix, or six for internal. That works out to roughly two and a half to three barrows per bag. The barrow must be a 65-litre builder’s barrow, levelled flat with a plank rather than heaped.
How much cement and sand do I need to plaster 100 square metres?
At 15mm thick you need about 600kg of common cement plus 2.3 cubic metres of plaster sand internally, or 700kg plus 2.25 cubic metres externally. With masonry cement the external figure rises to 850kg for the same sand volume. Order roughly 10% extra to cover wastage and the inevitable drop off the trowel.