How Thick Should Render Be?
Quick Answer
Two-coat sand and cement render is 14-20mm total; NHBC’s minimum is 15mm
Monocouche is 15-25mm in a single coat (20mm typical). A silicone or acrylic thin-coat topcoat is just 1.5-3mm, applied over a separate 4-5mm basecoat, not the whole render depth on its own
How We Calculated This
Render thickness is set coat by coat, not as one single figure: the total is the sum of however many coats the system uses. Take the same 3m × 2.4m wall (7.2m²) used across our render calculators, and compare the two ends of the sand and cement two-coat range:
- Wall area: 3m × 2.4m = 7.2m²
- At NHBC’s 15mm minimum (8mm scratch + 7mm float): mortar volume = 7.2 × 0.015 = 0.108m³
- At the thicker end of the guidance (12mm scratch + 8mm float = 20mm): mortar volume = 7.2 × 0.020 = 0.144m³
- Difference: 0.144 − 0.108 = 0.036m³, about 33% more mortar on the same wall
Converted to sand at the standard 1600kg/m³ bulk density for damp sharp sand (BS EN 13139):
- At 15mm: 0.108 × 1600 ÷ 1000 = 0.173 tonnes of sand
- At 20mm: 0.144 × 1600 ÷ 1000 = 0.230 tonnes of sand
That is the practical reason thickness matters for ordering, not just finish quality: going from the minimum to the top of the guidance range on the same wall adds an extra 0.058 tonnes, about 58kg, of sand per 7.2m².
Why Thickness Varies by Coat
A two-coat sand and cement render is not one 15mm layer. It is two different jobs stacked on top of each other:
- Scratch coat (1st, 8-12mm):keys mechanically into the substrate and needs to be the stronger of the two mixes, typically 4:1 sand to cement. It is scored, or “scratched”, once it has taken up, to give the top coat something to grip
- Float coat (2nd, 6-8mm): the finishing layer, ruled flat and floated to a smooth, even surface, usually a weaker 5:1 mix with plasticiser for workability
- Finish coat (3rd, 3-6mm, optional): only used on heritage or heavy three-coat specifications, rarely needed on modern domestic work
Two coats together run 14-20mm, and NHBC Standards Chapter 6.11 sets 15mm as the practical minimum for a durable, weather-resistant finish. Go thinner than that and the render loses the reserve of material it needs to bridge minor substrate irregularities without hairline cracking.
One-Coat and Thin-Coat Systems Are Different
Not every render system works in 14-20mm layers. Two other approaches are common on UK jobs, and mixing up which one you are pricing leads to badly wrong material orders:
- Monocouche (Weber Pral M, K Rend K Mono, Parex Monorex): a genuine single-coat, through-coloured render applied 15mm minimum, 20mm typical, up to 25mm maximum in one build. If the substrate is too uneven for one pass, installers build up in two passes, for example 12mm plus 8mm, that still total around 20mm. They do not stack a fresh 20mm on top of an existing 20mm
- Silicone or acrylic thin-coat (K Rend Silicone TC15/TC30, Weber Therm SR, Sto Stolit, Parex Sil RD): this is a topcoat only, applied over a separate 4-5mm basecoat (K Rend HP12 or equivalent) with reinforcing mesh. The topcoat itself is just 1.5mm for TC15, 2mm for a scraped texture, or 3mm for TC30. The basecoat is where the real depth and strength sit
Render Thickness by System
| Render system | Coats | Total thickness |
|---|---|---|
| Sand & cement (traditional) | 2 (scratch + float) | 14-20mm (15mm NHBC min) |
| Monocouche (one-coat, through-colour) | 1 (up to 2 passes on uneven substrates) | 15-25mm (20mm typical) |
| Silicone/acrylic thin-coat topcoat | 1 topcoat over a 4-5mm basecoat | 1.5-3mm topcoat |
Getting Thickness Right on Site
Go too thin and a sand and cement render loses its crack resistance and can let the substrate show through. Go too thick in one pass and fresh render is prone to slumping and cracking as it dries, and it uses far more material than the job needs. Keep to two proper coats rather than one thick one, dampen absorbent backgrounds before applying, and follow the specific product’s data sheet over generic rules whenever one is available. Every render system in this guide is built from published product data sheets or NHBC guidance, and they do not always agree with each other to the millimetre: even our own related pages quote slightly different typical figures for the same product, depending on which manufacturer datasheet or NHBC range they draw from.
Use our rendering calculator to work out cement, sand and coverage for your own wall and thickness, or the monocouche render calculator and K Rend calculator for one-coat and thin-coat systems.
Coat thicknesses match the defaults used across our render calculators, sourced from BS EN 13914-1, NHBC Standards Chapter 6.11 and current manufacturer product data sheets (Weber, K Rend, Parex, Sto). Last updated: July 2026