A house I looked at had three different render systems on it, visible as three different zones of slightly different colour and texture. Not because of deliberate design, but because it had been repaired and patched over 40 years by three different people using three different products. The base was lime, the patches were sand:cement, and someone had applied an acrylic coat over one elevation. None of them were compatible with each other and the whole facade was cracked, stained and bubbling in places.
Render compatibility is real and it matters. Apply a hard sand:cement patch to an old lime render and you create a stress differential that causes cracking around the patch. Apply silicone render over sand:cement without proper priming and it will delaminate within a few years.
Understanding the render types before you specify or buy saves a lot of problems down the line.
The main external render types
Sand:cement render
The traditional workhorse of UK external rendering. Mixed on site from Portland cement and sharp sand, applied in two coats (scratch coat and finish coat), painted after curing.
Strengths: Robust, widely understood, easy to repair, cheap materials, compatible with most masonry substrates.
Weaknesses: Needs painting, long curing time before decoration, hard and relatively inflexible - prone to cracking if the substrate moves, or if the mix is too strong for the brick.
Best for: Standard brick and block construction, DIY application (within skill level), surfaces that will be repainted regularly.
Mix ratio: 1:4 or 1:5 (cement:sharp sand) for most applications. Avoid 1:3 on soft brick.
Monocouche render
A factory-mixed, pre-coloured single-coat render applied at 12-15mm. The colour goes through the full depth of the product, so minor chips or scratches do not show as white marks. No painting required.
Strengths: No painting, factory-controlled mix consistency, professional appearance, fast application, manufacturer warranties of 20-25 years common.
Weaknesses: More expensive per bag than sand:cement, colour options are fixed (though wide ranges are available), requires more skill to achieve a good finish, cannot be matched exactly if a later repair is needed.
Best for: New build and major refurbishment where a long-term low-maintenance finish is the goal.
Acrylic render
A polymer-modified system applied in thin coats (3-5mm) over a mesh-reinforced base coat. Often used as the finish coat of an external wall insulation (EWI) system, or as a decorative finish over a scratch coat.
Strengths: Flexible (accommodates some substrate movement without cracking), wide colour range, thin application, can be applied over EWI boards.
Weaknesses: More prone to biological staining than silicone, shorter lifespan than monocouche or silicone in exposed conditions, some products can look plastic.
Best for: Thin decorative finish coat, EWI systems, rendered insulated panels.
Silicone render
Similar to acrylic but with silicone-modified binders that give it excellent hydrophobic (water-repelling) properties and resistance to algae and moss. Often specified as the finish coat of a premium EWI system.
Strengths: Excellent weather resistance, self-cleaning properties in rain, low algae growth, flexible, factory-controlled colour.
Weaknesses: Higher cost than acrylic, requires experience to apply well, must go over a compatible base coat.
Best for: High-quality finishes, exposed locations, EWI systems where long-term appearance matters.
Lime render
The traditional pre-Portland cement render, experiencing a significant revival for period property restoration. Uses hydraulic lime (NHL) or putty lime as the binder rather than Portland cement.
Strengths: Breathable (essential for solid-wall period buildings where moisture must be able to escape), flexible and self-healing for minor cracks, compatible with soft historic brick, sympathetic appearance.
Weaknesses: Slower to set, more skill required, fewer contractors familiar with it, cannot be applied in cold weather (below 5C), more expensive.
Best for: Pre-1920 solid brick buildings, listed buildings, any project where breathability is a requirement. Using Portland cement render on a solid-wall property is a common cause of damp problems.
Comparison table
| Render type | Needs painting | Life expectancy | DIY friendly | Cost (relative) |
|---|---|---|---|---|
| Sand:cement | Yes | 20-30 years | Moderate | Low |
| Monocouche | No | 25-30+ years | Less so | Medium-high |
| Acrylic | No | 15-20 years | Moderate | Medium |
| Silicone | No | 25+ years | Less so | High |
| Lime | Sometimes | 30+ years | Skilled | Medium-high |
My tips on choosing external render
Match the substrate history. If your house has lime render already, patch it with lime or strip and re-render. Using Portland cement on a lime base, or lime on a Portland cement base, creates incompatibility. If you are unsure what is there, get a small sample analysed.
Consider the exposure. A sheltered garden wall behind a fence can have a lighter mix and lower specification than a fully exposed gable end facing the prevailing weather. UK coastal locations are particularly demanding.
Do not specify monocouche if you want to change colour easily. The strength of through-colour is also its limitation. If you think you might want to change the colour in 10 years, a painted sand:cement system gives you that flexibility.
Plan the movement joints. Any render system on a building will crack without planned movement joints. Joints at door and window reveals, at changes of substrate, and at regular intervals across large elevations are essential. This applies to all render types.
Get three quotes and ask to see finished work. The skill of the applicator matters as much as the product choice. A well-applied sand:cement render will outlast a poorly applied monocouche by years.
Use the Rendering Calculator to work out how many bags you need for your project.