Why Render?
External render provides a weather-resistant, decorative finish to blockwork or masonry. For extensions in London, it is popular because:
- It gives a clean, contemporary appearance
- It allows colour to be specified precisely
- It can match an existing rendered house
- It is appropriate for the many types of blockwork construction used in modern extensions
The main decision is between traditional sand-and-cement render and the modern polymer-modified or silicone-based systems.
Types of Render
Sand and Cement (Traditional)
The original render system — a mix of sand, cement, and water applied in two or three coats and finished with a float or scratch texture.
Advantages:
- Low material cost
- Widely understood by plasterers and builders
- Can be applied to most substrates
Disadvantages:
- Requires regular painting (typically every 5–10 years) to maintain appearance and weather resistance
- More susceptible to cracking as the substrate moves
- Retains moisture if cracked or if paint fails — can lead to damp problems
Monocouche (Through-Coloured Render)
A factory-mixed render applied in a single coat (sometimes two), with the colour mixed into the product. The finished surface does not require painting.
Advantages:
- Through colour — no painting required
- Clean, uniform finish
- Good coverage in a single visit
- Many colours available from leading manufacturers (Weber, Parex, K Rend)
Disadvantages:
- Higher material cost than sand and cement
- Colour cannot be changed without re-rendering
- Scratched or damaged areas are harder to patch invisibly
Finish options: Scraped (fine texture from scratching the surface), tyrolean (machine-applied granular texture), smooth (rare — requires specific products).
Silicone Render
A polymer-modified render system with a silicone-based finish coat. Available in hundreds of colours, smooth or fine-textured finish.
Advantages:
- Hydrophobic (water-repellent) — water runs off rather than being absorbed
- Flexible — resists cracking better than sand and cement
- Self-cleaning properties in many products
- Through colour — no painting required
- Longest low-maintenance life of commonly available systems
Disadvantages:
- Highest material cost
- Requires experienced applicator — poor application can lead to patchy finish
- May not be repairable invisibly if damaged
Lime Render
Traditional lime-based render (either hydraulic lime or hot lime) used for older or historic buildings. Essential for listed buildings and often preferred for Conservation Area properties and Victorian/Edwardian houses.
Advantages:
- Breathable — allows moisture to pass through the wall rather than trapping it
- Appropriate for older buildings that use lime mortar in the masonry
- Softer and more forgiving than cement — less likely to crack
- Can be repaired and re-pointed with compatible materials
Disadvantages:
- Slower to apply and cure
- Higher labour cost
- Requires application by experienced lime plasterer
- Usually requires painting (unless using natural hydraulic lime with a natural stone texture)
Critical point: Never use cement render on a listed building or Victorian/Edwardian solid wall property. Cement traps moisture, causes deterioration of the masonry, and may require Listed Building Consent to apply.
Choosing a Colour
Most monocouche and silicone render systems offer extensive colour ranges. Common choices for London extensions:
- White/off-white: Classic contemporary look; shows dirt more than mid-tones
- Light grey: Currently popular for modern extensions
- Warm stone tones: Work well with brick elevations on the existing house
- Dark grey/anthracite: High-impact but more expensive and harder to maintain looking clean
Colour samples should be viewed on a large area (minimum A3) in the actual daylight conditions of the site — small paint chips in a showroom are not reliable.
Application Notes
Render should be applied to a clean, sound substrate. New blockwork should be allowed to cure fully before rendering. Expansion joints should be incorporated in large, uninterrupted render panels to manage movement.
Most render systems require a primer or base coat on challenging substrates (smooth engineering brick, blue engineering brick, certain blockwork types).
RCB Design & Build
RCB specifies render systems as part of the external finishes package on extension and refurbishment projects, advising clients on the appropriate product for their substrate, budget, and aesthetic preference.