How to clean rendered walls, stone and brick

Before cleaning renders, stone masonry or brickwork, it’s essential to consider the cleaning method to avoid surface damage, damage to surrounding materials and the environment. Ensuring proper care and maintenance of these surfaces not only enhances the aesthetic appeal of your property but also extends its lifespan. It is crucial to understand the nuances of each of these material types, as they have distinctive characteristics that require different cleaning techniques.

Whether you are dealing with porous brickwork, delicate stone masonry, or a traditional lime render, this guide will provide you with expert advice to help you clean effectively, protect your surfaces, and keep the environment in mind. From identifying the right render cleaning agents to the application of different cleaning techniques, we’re here to make the cleaning process a bit easier to tackle.

This guide will cover what to consider, what to avoid, plus a four-step render cleaning process to get the best results whether you are cleaning natural stone, brick, coloured render or painted renders.

Understanding lime tender, stone masonry and brickwork

In traditional construction, often called composite masonry, multiple units, such as brick and stone, are set with mortar to create a wall. Lime mortar has long been used in this type of building, and in the UK, it’s estimated that 1 in 5 buildings is classified as historic construction. Similarly, lime renders have long been essential for external walls, functioning as a protective layer for conserving stone masonry, brick, or cob beneath.

Lime render can be made from different types of lime, including natural hydraulic lime, lime putty, or even quicklime. Despite their potential variations, lime mortars and renders are distinctively more breathable and softer than their cement counterparts. They allow the solid walls of traditional buildings to ‘breathe’ and release moisture while accommodating the natural subtle settlement movements.

View our range of lime mortars and renders

For stone masonry, natural stones such as granite, limestone and sandstone are used to create unique building facades. These surfaces often feature intricate detailing and have a beautiful texture. Care must be taken when cleaning these materials since some types of stone can be sensitive to certain products or techniques, which can cause damage.

Finally, there is brickwork, a type of masonry construction that has long been a popular choice for creating beautiful buildings with character. While the average lifespan of brick can vary, brick buildings can undoubtedly last a century when applied with sympathetic materials, such as breathable and flexible lime mortars and maintained appropriately.

While bricks are often considered durable and low maintenance, their porous nature and overall stability, especially with age, are always worth considering. Older bricks tend to be softer and if the brickwork has been subjected to hard cement pointing, there is a chance that the bricks have been weakened.

Learn more about this historic use of lime around the world

Four steps for cleaning rendered walls, stone masonry & brickwork

1. Consider the building fabric and time of year

It is also worth considering the building fabric before undertaking a cleaning project. For example, excessive use of water to clean a porous surface could lead to saturation, and in the winter, this could lead to freeze-thaw damage.

Therefore, before undertaking any large cleaning project, consider the building’s structure, the cleaning method, and the time of year. In addition, be mindful if you have areas where moisture might remain present, for example, in shaded areas with minimal sunlight. Want to learn more? Gain practical insights and helpful tips on breathability, paints and maintenance by joining our next free Breathable Paints and Maintenance webinar.

2. Remove algae, fungi, mould, and lichens

Fungi, mould, lichens, moss, green algae and in coastal locations, stubborn red algae, are considered an unsightly nuisance by many. These biological materials can be found on various surfaces and often in damp spaces where moisture and oxygen help them thrive.

Removing this organic growth not only keeps render surfaces and exterior walls looking fresh, it can also prevent damage and decay over time.

Moulds, lichens, and other biological materials can cause mechanical weathering or the breaking down of materials as the lower parts of the organisms, referred to as hyphae, penetrate the surface.

Think of these hyphae as the biological materials’ root structure, which can penetrate the surface of natural stone, brick or render. Over time, the expansion and contraction of the hyphae can cause the surface to break down and loosen mortar and brickwork. These hyphae are also why many surface treatments do not entirely remove biological materials or prevent them from reoccurring. Instead, these treatments often remove the materials or staining from the surface rather than penetrating further and breaking down these underlying hyphae. As such, they can often make a swift return.

Fungi, mould, algae and lichens can also produce acids that can lead to chemical damage. These acids, over time, can begin to dissolve lime in renders, pointing, limestone and other natural stones such as slate and sandstone. Further, self-coloured renders and premixed mortars and renders can contain lime additions. So keeping them at bay is a wise choice.

While at first glance, this may appear only relevant for older buildings or historic properties with lime renders, pointing or exposed natural stone. However, it also presents an issue for modern rendered buildings, or those featuring lime-pointed brickwork or clad with natural stone. Both new and old surfaces can have staining, discolouration and damage.

3. Look for black pollution staining & white mineral salt staining

Black staining is caused by carbon and pollution. Often found on external render, stone walls, bricks, monuments and statues in urban areas or near roads due to years of emissions caused by traffic or nearby industrial activity.

While most agree these stains are less than attractive, it’s less known that issues can arise due to sulphur carbonating or reacting to produce damaging salts.

There are various sources of salts, including groundwater, salt added to roads during icy periods and sea spray. Acid rain, which contains nitric and sulfuric acids, is also a common cause and can damage many structures, but particularly those made from limestone suffer.

In some cases, the salts may be present in the materials during production, such as bricks. These can cause damage as moisture cycles through a surface; the water dissolves the salts and carries them to a drying front. As the water evaporates, the salts recrystallise and often have an extraordinarily high expansive force, pushing mortars and renders apart. Salt damage can appear as a powdering of the surface or as thin layers of delamination where the salt has crystallised behind the face of the layer.

4. Avoid pressure washing and scrubbing

When considering abrasive methods for any render cleaning process it’s essential to consider not only the surface to be cleaned but the structure itself and, where relevant, the building fabric. As such, high-power jet washing might produce clean render, but it may also produce damaged render.

High-powered pressure cleaning and excessive scrubbing with a stiff bristle brush can lead to physical damage to the render surface, stone, lime pointing and paint. If the paint removal during your project is under consideration, see our helpful guide to painting stripping.

A low-pressure washer is a soft washing option to loosen heavy biological materials if needed. For polished stone, it’s worth noting that a roughening of the surface can also result in a glossy finish being dulled and damaged on limestone, marble, and travertine.

If the render, stone masonry, or pointing is already friable (weak or crumbly), avoid pressure washing to prevent further structural issues. When using a brush, we recommend a soft nylon or natural bristle brush, avoiding abrasive metal brushes wherever possible.

Browse our range of tools and PPE

Commonly used masonry and render cleaners and what to consider

Before buying a commonly recommended render or masonry cleaner for your project, it’s worth taking a closer look at each and how it might impact your building in the short and long term.

Cleaning and maintenance can keep surfaces looking fresh, help keep long-term maintenance costs down and even assist your building in maintaining its value. However, while a clean surface is satisfying, it should not come at the materials’ expense. Different cleaning products can negatively affect surfaces, even where the surface is new, stable and robust.

It’s also worth considering how cleaning methods can impact those doing the cleaning, animals, plants, the watercourse and the environment.

Diluted household beach

While generic household cleaners like household bleach may seem convenient and affordable, it will often struggle to remove heavier soiled areas and prevent biological materials from returning.

Bleach also contains soluble salts that can react with lime in render or pointing to form gypsum/ettringite crystals that can lead to salt damage. Cleaning render with bleach may also leave a film behind on walls where it’s applied.

For those looking to clean planted areas or within gardens, bleach can cause harm to plants and animals.

Peroxide bleaches

These are often reasonable cleaning solutions for most surfaces as they typically leave no residual salts or films behind. However, they are often also a potent colour bleaching agent.

As a result, this can change the colour of the surface after treatment, which is particularly important when looking to clean coloured renders, wood or paint.
Like household bleach, peroxide bleaches can cause damage to plants and animals. Whenever using any bleach, ensure the appropriate care is taken and the individual applying the bleach wears protection.

Diluted Muriatic Acid (Hydrochloric Acid)

While this can be used as a cleaning solution diluted with water, this acid can also be found in other cleaning products. Known for its distinctive pungent smell, muriatic acid is typically considered for those looking to clean heavily soiled brick, concrete or stone.

Muriatic acid can damage certain stone types, including marble, travertine and some slates and sandstones. Further, it can cause damage to limestone and, most crucially, will damage lime render and lime pointing for stone or brickwork by dissolving the lime.

Remember that muriatic acid is corrosive and can cause health issues if misused without appropriate care and PPE. If considering this method, take precautions for those applying the product and the environment or animals around the area.

Summing It Up

We love a spring clean here at Cornish Lime, but before cleaning any renders, stone masonry or brickwork, it’s essential to consider the cleaning method to avoid damage and protect any surrounding materials. While many commonly used cleaning products may have pros and cons, our years of experience have led us to recommend D/2 Biological Solution as our go-to cleaning product. Designed by conservators, D/2 can be an effective solution for achieving a fresh, clean surface on various masonry surfaces. As always, every project is unique, so speak to a professional before getting started.