Question 1

What is the mix ratio for sand and cement render?

Accepted Answer

The standard mix for external rendering in the UK is 1:5 (1 part cement to 5 parts sharp sand) for a general-purpose render. A stronger 1:4 mix is used for the scratch coat or in exposed locations. Always use sharp sand (not building sand) and add plasticiser to improve workability.

Question 2

How many coats of render do I need?

Accepted Answer

Traditional sand and cement rendering typically requires 2 coats — a scratch coat (8-10mm) and a top coat (6-8mm), giving a total thickness of around 15-20mm. Monocouche (one-coat) render is applied in a single 20-25mm coat. Silicone and acrylic renders are usually applied in 1-2 thin coats (1.5-2mm each) over a base coat system.

Question 3

How much sand do I need per m²?

Accepted Answer

It depends on the coat thickness. For a typical sand and cement render with a 10mm scratch coat + 7mm float coat (17mm total), you need approximately 27 kg of sharp sand per m² (1 m² × 0.017 m × 1600 kg/m³). So 1 tonne of sharp sand covers roughly 37 m² for a typical 2-coat job — or about 63 m² at 10mm scratch alone (1000 kg ÷ 16 kg/m²) and 89 m² at 7mm float alone (1000 kg ÷ 11.2 kg/m²). Set each coat's thickness in the calculator independently: scratch coats are usually 10-12mm and float coats 6-8mm.

Question 4

How thick should each render coat be?

Accepted Answer

For traditional sand and cement: scratch coat 8-12mm (10mm typical — slightly stronger 1:4 mix for keying), float (top) coat 6-8mm (7mm typical — 1:5 mix), giving 14-20mm total. Single thick coats are 12-15mm. Heavy renders for uneven substrates can go up to 25mm total over 2-3 coats. Monocouche is applied in one 18-22mm coat. Silicone and acrylic thin-coat renders are 1.5-3mm depending on the product — TC15 is 1.5mm, TC30 is 3mm. Heritage lime renders are 8-12mm per coat over 2-3 coats. The calculator now takes a separate thickness for each coat so you can mix scratch + float thicknesses accurately.

Question 5

Why doesn't the sand quantity change when I switch mix ratio?

Accepted Answer

That's correct and intentional — the sand quantity for a given mortar volume is independent of the cement:sand ratio. This is the standard yield method used across UK construction (per BRE Digest 362, NHBC Standards 6.1, BS EN 998-1 and Hanson Cement's published rates): when you mix cement and sand together with water, the cement particles fill the voids between the sand grains rather than adding extra volume to the finished mortar. So 1 m³ of finished mortar always needs roughly 1.6 tonnes of sharp sand (or 1.5 t of building sand) regardless of whether you mix at 1:4, 1:5 or 1:6 — what changes is the cement bag count. Stronger mix (1:4) needs more cement (~14 bags/m³); standard (1:5) needs less (~12 bags/m³); weaker sheltered (1:6) needs even less (~10 bags/m³).

Question 6

What is monocouche render?

Accepted Answer

Monocouche (meaning 'one coat' in French) is a factory-made render that combines cement, aggregates, and additives. It is applied in a single 20-25mm coat, saving time compared to traditional 2-coat rendering. Popular UK brands include Weber, K Rend, and Parex. It comes pre-coloured, so no painting is needed.

Question 7

How long does render take to dry?

Accepted Answer

Sand and cement render should be kept damp for the first 2-3 days to prevent cracking, then allowed to dry for at least 7 days before the next coat. Monocouche render typically needs 24-48 hours before it can be finished. Full curing takes 28 days for cement-based renders. Do not render in freezing conditions. For internal plaster drying, see our Plaster Drying Time Estimator.

Rendering Calculator — Sand, Cement & Render Materials

How We Calculate This

The formula

Coat thicknesses — typical values

Frequently Asked Questions

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