How thick should render be on external walls?

External render should be 18-25mm thick in total, typically applied in two coats. The base (scratch) coat should be 10-15mm and the top (finishing) coat 5-10mm. Applying render too thickly increases the risk of cracking. On blockwork, a total of 20mm is standard; on brick, 15-18mm is typical.

What mix ratio should I use for rendering?

For external rendering, a 1:5 (cement:sand) mix with plasticiser is the most common UK ratio. For the scratch coat, use 1:4 for extra strength. For the top coat, use 1:5 or 1:6 for flexibility. Always use sharp sand (not building sand) for rendering. Add a waterproofer to the final coat for weather protection.

How much does rendering cost in the UK?

DIY sand and cement rendering costs approximately £5-10 per m² for materials. Pre-mixed render (K Rend, Weber, Parex) costs £15-30 per m² for materials. Professional installation adds £20-40 per m² for labour, making the total £25-70 per m² installed depending on the product and finish.

Rendering Calculator — How Much Render Do I Need? (UK)

Q: How much render do I need per square metre?

For a standard two-coat sand and cement render at 20mm total thickness, you need approximately 1.8kg of cement and 9kg of sand per square metre per coat. For a complete wall, budget approximately 3.6kg cement and 18kg sand per m². Pre-mixed render (such as K Rend or Weber) typically covers 1.0-1.5m² per 25kg bag depending on thickness.

Complete Guide to External Rendering (UK)

External rendering is the application of a protective and decorative coating to the outside walls of a building. It improves weatherproofing, thermal performance, and appearance. In the UK, rendering is one of the most popular external wall finishes, found on millions of homes ranging from Victorian terraces to modern new-builds.

Types of Render

Render Type	Cost per m²	Lifespan	DIY Suitable?	Best For
Sand & cement	£5-£10	20-40 years	Yes (with practice)	Budget, DIY, traditional finish
K Rend / Weber	£15-£30	25-40 years	Moderate	Modern finish, through-colour
Lime render	£10-£20	50+ years	Specialist skill	Listed buildings, old stone
Acrylic render	£12-£25	20-30 years	Moderate	Smooth modern finish
Monocouche	£15-£25	25-40 years	Moderate	Single-coat, coloured finish

Sand and Cement Render

Traditional sand and cement render is the most affordable option and is widely used across the UK. The standard mix is 1:5 (1 part cement to 5 parts sharp sand) with a plasticiser to improve workability and reduce cracking. Key points:

Scratch coat: 10-12mm thick, 1:4 mix. Score (scratch) the surface with a comb scratcher to provide a key for the top coat. Allow to cure for at least 24 hours
Top coat: 6-8mm thick, 1:5 or 1:6 mix. Apply and finish with a sponge float for a textured finish or a steel float for smooth
Total thickness: 18-20mm on blockwork, 15-18mm on brickwork
Sand type: Use sharp (plastering) sand, NOT building (soft) sand. Sharp sand gives a stronger, less crack-prone render
Additives: Add waterproofer (e.g., SikaWP) to the top coat for weather resistance. Add plasticiser (e.g., Feb-Mix) for workability

Pre-Mixed Render (K Rend, Weber, Parex)

Pre-mixed renders come in bags with consistent colour and quality. They are more expensive but offer advantages:

Through-body colour: No painting needed. Available in 20+ colours
Consistent finish: Factory-mixed for uniform colour and texture
Coverage: K Rend Silicone TC30 covers approximately 1.0-1.5 m² per 25kg bag at 15mm thickness
Warranty: Many manufacturers offer 10-25 year colour and weather guarantees

Preparation

Proper preparation is essential for a long-lasting render finish:

Remove all loose material, old paint, and flaking render
Rake out mortar joints to a depth of 10-15mm to provide a mechanical key
Install stainless steel render beads on all external corners and stop beads at edges
Apply a render primer (PVA, SBR, or proprietary primer) to control suction
Fix stainless steel reinforcing mesh over any cracks, movement joints, or mixed substrates
Dampen the wall before applying render (but not soaking wet)

Common Problems and How to Avoid Them

Cracking: Usually caused by render applied too thickly, wrong mix ratio, or rendering in direct sun. Apply in coats no thicker than 12mm and protect from sun/wind during curing
Blow-outs (delamination): Caused by poor suction control or applying to a dusty/painted surface. Always prime the wall
Algae/green staining: Common in the UK climate. Use a render with fungicide or apply an anti-fungal treatment
Efflorescence (white salt stains): Caused by moisture drawing salts through the render. Ensure the wall is dry before rendering and use a waterproofer

Frequently Asked Questions

How much render do I need per square metre?

For sand and cement at 20mm total: approximately 3.6kg cement and 18kg sand per m² (two coats). Pre-mixed render covers 1.0-1.5 m² per 25kg bag at 15mm.

How thick should external render be?

18-25mm total in two coats: scratch coat 10-15mm, top coat 5-10mm. Do not exceed 25mm total or apply more than 12mm in a single coat.

What mix ratio for rendering?

Scratch coat: 1:4 (cement:sharp sand). Top coat: 1:5 or 1:6. Always add plasticiser and waterproofer to the top coat.

How much does rendering cost?

DIY sand and cement: £5-10/m². Pre-mixed: £15-30/m². Professional installation: £25-70/m² total including materials and labour.

Rendering Calculator — Sand, Cement & Pre-Mixed Render