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A Guide to Cleaning Rendered Walls, Stone Masonry & Brickwork

Need to clean stone masonry, render or brick? Find out more about cleaning methods to consider, things to avoid & tips to get successful results.

Black staining on a stone wall

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. This guide will cover what to consider, things to avoid and tips to get the best results whether you are cleaning natural stone, brick, coloured render or painted renders.

Removing algae, fungi, mould, and lichens

Fungi, mould, lichens, moss, 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 them not only keeps surfaces 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.

Mould algae and lichens on a stone wall

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 or 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 stone 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 occur.

Black pollution staining and white staining (mineral salts)

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.

Black staining on an external wall

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.

Paint peeling off a wall with salt visible

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.

Commonly recommended cleaners and what to consider

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.

If your project includes stone, but the type of stone is unknown and can’t be identified by visual inspection, a stone analysis can assist before undertaking a significant cleaning project.

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

Diluted Household Bleach

While 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. 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. However, muriatic acid can damage certain stone types, including marble, travertine and some slates and sandstones. Further, it can cause damage to limestone and, most crucial, 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.

Our Recommendation

For cleaning most masonry and architectural surfaces, including render, coloured render, brick and natural stones, we recommend D2 Biological Solution. Non-hazardous and biodegradable, this ready-to-use product helps to prevent the pitfalls of creating a solution that is too strong, dangerous to mix, damaging or just plain ineffective.

Developed by conservators, D/2 contains no harmful acids, salts or bleach and leaves no residual film behind once washed off. Further there is no concern about acidic or alkaline damage or the formation of potentially damaging salts in render and pointing. Find out more about D/2 Biological Solution and its application methods.

Pressure washing and scrubbing

When considering abrasive methods for cleaning, it’s essential to consider not only the surface to be cleaned but the structure itself and, where relevant, the building fabric.

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

A low-pressure washer is an 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.

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.

The information provided by Cornish Lime is for informational purposes only and does not amount to a specification. Every project is unique, so please consult a professional before undertaking a project. Use of this site and reliance on any information on the site is solely at your own risk.

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