How much plaster sand do I need per square metre in South Africa?

For a two-coat plaster system (12mm scratch + 6mm finish): total plaster volume = 18mm × wall area. The total dry sand volume needed is approximately 0.025m³ per m² of wall (25 litres per m²), accounting for both coats and the cement-sand ratio. For a 100m² wall, you need approximately 2.5m³ of plaster sand — about 3–4 bulk bags (750L each). Always order slightly extra to account for variations in surface flatness.

How long should I wait between scratch coat and finish coat?

Wait at least 24 hours before applying the finish coat over the scratch coat — longer in cold or humid conditions. The scratch coat should be firm but not fully dry. Some plasterers wait 3–7 days for better bonding. Wet the scratch coat lightly with water before applying the finish coat if it has dried out completely — this prevents the dry scratch coat from sucking moisture from the finish coat and causing premature cracking.

Do I need to add a bonding agent or PVA to plaster in South Africa?

On smooth or dense surfaces (in-situ concrete, smooth blockwork, previously painted walls), a bonding agent (Renderoc, Masterbond, or diluted PVA at 1:3) is recommended before the scratch coat. On textured brick or rough blockwork, bonding is usually not required — the texture provides sufficient mechanical key. For interior plaster going over an existing plaster layer, always apply a bonding liquid. Avoid applying PVA directly to external plaster — it can reduce breathability and trap moisture.

Plastering Calculator South Africa

🖌️ Last Updated: May 2026 · SA Standard Mix Ratios

🖌️Plastering Calculator — South Africa

Wall Area to Plaster (m²) Net area after deducting large door and window openings

Plaster System

New brickwork always needs both coats

Scratch Coat Thickness (mm)

Finish Coat Thickness (mm)

Scratch Coat Mix Ratio

Finish Coat Mix Ratio

Cement Bags (50kg)

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Plaster Sand

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Wall Area

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Total Plaster Volume

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Bulk Bags (750L)

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Scratch Coat Cement

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Finish Coat Cement

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Approx. Water

Calculation Details

⚠️ For planning purposes only. Mix ratios should be confirmed with a qualified plasterer for your specific surface conditions.

SA Plastering Mix Ratios and Material Calculations

Getting your plaster material quantities right before starting a job prevents the two most common problems on SA building sites: running out of sand or cement mid-coat (which causes visible join lines in the finished surface), or over-ordering and wasting materials. The calculation is straightforward once you understand the mix ratios and how coat thickness translates to volume.

Plastering Formula

Volume per coat (m³) = Wall area (m²) × Coat thickness (m) Sand volume (m³) = Volume × [Ratio ÷ (Ratio + 1)] × 1.3 (bulking factor) Cement volume (m³) = Volume × [1 ÷ (Ratio + 1)] Cement bags (50kg) = Cement volume ÷ 0.035 m³/bag Add 10% to all quantities for waste

Standard SA Plaster Mix Ratios

Coat	Mix Ratio	Thickness	Cement/m²	Use Case
Scratch coat	1:4 cement:sand	10–15mm	~0.3 bags/m²	Standard exterior and interior new build
Scratch coat (wet)	1:3 cement:sand	12mm	~0.4 bags/m²	Bathrooms, kitchens, exposed walls
Finish coat	1:6 cement:sand	5–8mm	~0.1 bags/m²	Standard interior smooth finish
Finish coat (hard)	1:4 cement:sand	6mm	~0.15 bags/m²	High-traffic areas, exterior finish

Why the finish coat must be weaker than the scratch coat

This is the most commonly misunderstood rule in plastering. The scratch coat must always be stronger (lower ratio = more cement = stronger) than the finish coat. If the finish coat is stronger, it shrinks less during curing than the scratch coat beneath it — causing map cracking across the entire surface. The weaker finish coat accommodates slight movement in the backing without cracking.

Plaster sand vs building sand in South Africa

Plaster sand and building sand are different products in South Africa. Plaster sand is finer, cleaner, and more uniformly graded — it produces a smooth finish coat. Building (or pit) sand is coarser and suitable for mortar and the scratch coat, but not for finish coats. Always specify plaster sand when ordering for finish coat work. Using building sand for a finish coat results in a rough, difficult-to-smooth surface.

Frequently Asked Questions

What is the correct plaster mix ratio in South Africa?

Scratch coat: 1 part cement to 4 parts plaster sand at 12mm thick. Finish coat: 1 part cement to 6 parts plaster sand at 6mm thick. The finish coat must always be weaker than the scratch coat — a stronger finish coat causes map cracking as the backing shrinks more during curing. For wet areas, use 1:3 for the scratch coat.

How many 50kg bags of cement do I need for plastering?

For a standard two-coat system (1:4 scratch at 12mm + 1:6 finish at 6mm): budget approximately 1 bag of cement per 2.5–3m² of wall. This calculator gives the exact figure based on your wall area and selected mix ratios, including 10% waste allowance. Always round up to the next full bag.

How much plaster sand do I need per m²?

For a two-coat system (12mm scratch + 6mm finish): approximately 25 litres (0.025m³) of sand per m² of wall, including both coats and accounting for the mix ratios. For a 100m² wall, order approximately 2.5m³ of plaster sand — 3–4 standard bulk bags (750L each). Always order slightly more than calculated — sand volumes vary with moisture content and compaction.

What is the difference between a scratch coat and a finish coat?

The scratch coat (base/backing coat) is the first layer applied to bare brickwork — typically 10–15mm thick at 1:4, combed while wet to key the finish coat. The finish coat is applied over the cured scratch coat at 5–8mm thick at 1:6, and is finished smooth with a steel float or given texture with a sponge. Never apply a stronger finish over a weaker scratch coat.

How long do I wait between scratch and finish coat?

Wait at least 24 hours before applying the finish coat — longer in cold or humid conditions. The scratch coat should be firm and not powdery. If it has dried out fully (several days), lightly wet it before applying the finish coat to prevent the dry substrate from sucking moisture from the new coat, which causes premature cracking and poor bonding.

Do I need a bonding agent before plastering in South Africa?

On smooth or dense surfaces (concrete, smooth blockwork, painted walls), a bonding agent (Renderoc, Masterbond, or diluted PVA at 1:3) is essential before the scratch coat — without it the plaster will delaminate. On rough brickwork or textured blockwork, bonding is not usually required as the surface provides mechanical key. For re-plastering over existing plaster, always apply a bonding liquid first.

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