External Wall Render Systems: Types, Finishes & Performance

Exterior wall rendering systems serve a number of purposes. Most importantly, they help protect buildings against rain penetration while allowing the wall(s) to remain vapour permeable, and they produce an aesthetically pleasing surface. Depending on the chosen product, the render might produce an attractive finish in its own right, or it might provide the base for a separate decorative finish.

In addition, some suitably designed render systems can perform other functions, including improving a building’s airtightness (which can help to improve its energy efficiency), helping to limit thermal gain by reflecting a portion of the sun’s energy, and protecting concrete substrates against the damaging effects of carbonation or chloride ion penetration.

Render and Weather Protection

In most parts of the world, buildings are exposed to the risks of water ingress, which can be harmful to many different types of structure, and for different reasons. It can lead to:

• Frost damage in masonry and some brickwork
• Damage to timber frames (warping and rotting)
• Damage to carrier boards and sheathing
• Corrosion of metal elements such as steel frames, the steel rebars in reinforced concrete, and the fixings that hold certain building components together
• The degradation of insulation materials

Moreover, if it penetrates far enough, moisture can cause considerable damage to electrical systems and to porous materials such as timber and plasterwork. It can also increase the risks of condensation, damp and mould, which can present a risk to occupants’ health.

Preventing water penetration is therefore essential, and it’s a fundamental role of external render systems to protect walls made from materials that are not, in themselves, completely waterproof. This can include concrete, sheathing boards and many types of brick, blockwork and masonry.

There are numerous render systems available to specifiers. The choice will ultimately depend on factors such as the project’s design requirements, aesthetic concerns, and the type, age and condition of the substrate.

Render Materials

Some render systems have been in use for many decades. For example, aggregate-based ‘pebble dash’ systems have been used in Britain for well over a century, having first been used as means of waterproofing porous brickwork walls. Over time, more sophisticated products have been introduced. Today, architects and other specifiers can choose from options including:

• Polymer-enriched dash renders
• Polymer-modified cement-based renders
• Brick-effect renders and brick slips
• Advanced silicone-based renders
• Other systems for specialist applications

Performance and Design Considerations

On retrofit projects, a key concern will often be the ability to produce an appearance that is sympathetic to the general architectural style of the surrounding area. For example, if the majority of local buildings are of traditional red brick, then the specifier may well seek to use a render finish that maintains the same appearance – e.g. using brick slips or brick-effect render. In some cases, replicating local styles may even be a requirement for securing planning consent.

In other cases, there may be fewer planning constraints and the choice of render might then focus more on making a striking impression. Here, a self-coloured render will often be a preferred choice. Architects can choose from silicone, acrylic and other polymer-based systems, many of which will be available in hundreds of different colours.

Silicone is a particularly cost-effective option because it is naturally water-repellent, and flexible enough to tolerate minor cracks and substrate movement without losing its integrity or adhesion. Modern silicone renders also tend to exhibit superior colour-fastness and to shed dirt through the natural actions of weathering. This helps to preserve a bright, clean appearance without the need for regular and expensive cleaning.

Minimising the need for maintenance is important on any building but it can be especially important on buildings where access to external walls is limited or difficult. An obvious example is high-rise buildings, where regular cleaning is more costly and technically challenging.

It is also important to consider the environmental conditions to which an external wall rendering system is likely to be exposed. For example, tall buildings and those on elevated ground may be exposed to higher-than-average wind loads, so it’s important to ensure that the façade system is designed to resist expected wind load.

Similarly, buildings in damp and regularly shadowed settings may be more than usually at risk of organic growth developing on external walls. Here, it can be useful to specify a silicone-based render system with special additives that helps to inhibit the growth of mould and algae. These, together with the render’s natural dirt-shedding properties, should help to preserve the walls’ appearance for longer than traditional renders.

Fire safety is another key concern. High-rise buildings require the use of suitably certificated systems, as do certain other buildings, including some schools and many public buildings. Besides the obvious considerations such as inherent combustibility and resistance to the surface spread of flame, specifiers should also take note of whether the system will produce burning droplets and how securely a render finish will remain in place in the event of a fire. Choosing the right system will help to eliminate the possibility of falling debris and associated risks to fire & rescue teams or to occupants who may be evacuating the building.

The EN 13501-1 classification system shows the combustibility of the material, the extent of smoke emissions and the propensity to produce burning droplets. So, for example, a Permarock mineral fibre external wall insulation system featuring Permarock Silicone^Ultra K Finish achieves an A2-s1,d0 classification, which indicates limited combustibility, very limited smoke production and no flaming droplets.

Exterior Render Finishes

Different external render systems allow designers considerable freedom in in terms of colour, texture and the opportunity to replicate certain traditional architectural styles. Individual product details vary by manufacturer but listed below are examples from within the Permarock range:

Permarend: a range of thin-coat, polymer-modified renders. Suitable for use on substrates including brickwork, blockwork, and concrete, Permarend systems exhibit good flexibility and resistance to both cracking and impact damage. They provide a secure base for other protective and decorative renders, or specialist paints such as Permarock Silicone^Ultra Façade Coating.

Permarock Silicone^Ultra K Finish: a textured, self-coloured render that is light, vapour permeable and resistant to discoloration and organic growth. It provides excellent weather protection and is available in over 1250 colour shades.

Permarock Dry Dash: a combination of mineral-cement Dashing Mortar and colour-graded aggregates that, together, produce a classically ‘traditional’ appearance. The resulting finish is hardwearing, durable and almost entirely maintenance-free. It is available in 20 standard shades, textures and grain sizes.

Permarock Brick Slips: a lightweight synthetic resin brick that can replicate the appearance of many different styles of brickwork and stone walls. Available in 12 standard colours and 3 textures, it permits the creation of attractive design features such as quoins and soldier courses.

Permarock Brick-Effect Render: a self-coloured render based on polymer-modified cement. Applied in two differently coloured coats, it convincingly simulates the appearance of many types of brickwork. It can be used to recreate metric or imperial brick sizes, and it permits the creation of visual effects such as soldier courses and brick quoins.

On many projects, more than one render system may be installed in order to suit specific design and aesthetic requirements.

For more details, please visit the individual product pages, or see an overview on our Renders, Finishes & Effects page.

Render or Insulated Render?

Another consideration that will narrow down the options for specifiers is whether the render is intended to work as a standalone finish – which may be the case on buildings that are already well-insulated inside – or whether it will be used as part of an external wall insulation (EWI) system.

In the first instance (standalone finish), the key considerations will be:

• Colour, texture and any optional decorative effects
• Ability to satisfy local planning consent criteria
• Suitability for the building / substrate
• Suitability for the expected conditions (e.g. wind load)
• Compliance with relevant regulations and standards, including fire safety

In the second instance (when used as part of an EWI system), those same factors will apply but, in addition, specifiers must consider:

• The suitability of the substrate to receive an EWI system
• Compatibility with the insulation system and the impact on any warranties
• The performance and certification of the whole system
• Whether the insulation component imposes additional requirements (e.g. that the render system must have good impact resistance)

Insulation Render Finishes

Shown below are examples of the finishes achievable with different insulation render systems. However, please note that they only show one colour and texture for each option. With Permarock systems, the choice of finish, colour, texture and surface effects is almost unlimited.