Render and lime
Multi-coat render: coat thicknesses and volumes (BS EN 13914-1)
- where
- total render thickness (mm)
- first undercoat (scratch coat) thickness (mm)
- second undercoat thickness, 0 if none (mm)
- final coat thickness (mm)
- undercoat wet volume (m³)
- area to render (m²)
- waste allowance (%)
- final coat wet volume (m³)
- total wet volume (m³)
Explain this equation
This builds up a multi-coat render, adds up the total thickness and gives the wet volume of the undercoats and the final coat separately, so each mix can be ordered on its own.
t1 is the scratch coat, t2 an optional second undercoat (used on severe exposure) and t3 the final coat. T is their sum, which is checked against the usual minimum total. Vu and Vf are the volumes of the undercoat and final coat mixes.
Each coat is a slab of area A and its own thickness, so volume is area times thickness, with the 1000 changing millimetres to metres and the waste term adding an allowance for droppings and uneven backgrounds.
Sense check: a 10 mm scratch coat and 8 mm final coat make 18 mm, above the 16 mm minimum for sheltered and moderate exposure. 50 m² of that is about a cubic metre of render including 10% waste.
Watch out: these are wet volumes. Put each one into the sand and cement calculator with its own mix and the dry-volume factor to get cement and sand. Never make the final coat thicker or stronger than the undercoat.
Builds up a two- or three-coat render, checks the total against the usual minimum thickness and gives the wet volume of each coat, so each mix can be ordered separately.
Enter your values to see the result.
Good to know
BS EN 13914-1 National Annex (Table NA.3) typical figures: first undercoat 8–12 mm, second undercoat 6–10 mm. Mortar industry guidance gives an undercoat of 10–12 mm and a finishing coat of about 8–10 mm, never thicker than the undercoat. Nominal total (excluding texture) not normally less than 16 mm for sheltered/moderate exposure and 20 mm for severe/very severe, where two undercoats are preferred. Feed each volume into the sand and cement calculator with its own mix ratio.
Common mistake
Making the final coat thicker or stronger than the undercoat. Each coat should be no stronger and no thicker than the one beneath, or it can crack and debond.
Limits
Wet volumes only; use the sand and cement render calculator for materials. Thicknesses are typical figures; follow the specification, background type and exposure zone. Not for thin-coat or proprietary through-coloured systems.
Sources
- BS EN 13914-1:2016 Design, preparation and application of external rendering and internal plastering. External rendering (National Annex Tables NA.3, NA.4), as summarised by Premier Guarantee, Site made renders
- Mortar Industry Association / MPA, Mortar Data Sheet 04: Rendering (undercoat 10–12 mm, finishing coat 8–10 mm)
- NHBC Standards 6.11.6 Render mixes (minimum total thickness 16 mm / 20 mm by exposure)
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the manufacturers’ system and product data, BS 8212, BS EN 13914 and Building Regulations. Coverage rates vary by product, background and thickness. Check prices, VAT and CIS rules with your own records and HMRC.