How to stop mould on outside walls — plus when it's a sign of something more serious

With many of us still reporting mould as an issue in the home, it comes as no surprise that mould removal matters. But if you think you've spotted it on the outside walls of your home, you might be wanting to confirm it is mould as well as how to get rid of it.

"The good news is, black, green, blue and even red patches on the outside of your house are often algae rather than mould," says Dave Bendon, director of D Bendon Construction Ltd.

Here, in our step-by-step guide, we look at how to stop mould – or algae – on outside walls as well as looking at when it might be a sign of something more serious in your home in our FAQs at the bottom.  

Dave Bendon

Dave Bendon has over 30 years experience in construction, working on a variety of commercial and residential projects, including listed buildings and older properties that have been subject to damp and water ingress.

How to stop mould on outside walls – common causes

Even if you feel you know everything about removing mould from walls and you've become an expert on the types of mould that exist, how you tackle the issue on the outside of your home will be different to the methods you use inside. 

Most importantly, whereas the cause of mould in a house is often due to a build up of condensation, this is not a factor when it comes to external walls. Instead, there are a number of different reasons for the build up of mould-like damage and getting to the root of the problem is essential.

"Mould build up on external walls could be from a number of factors, such as a leak inside the property or poor condition of the exterior walls allowing moisture to permeate," says Checkatrade approved member, Edward Jonkler from Remora Restorations. 

"It’s also often down to a lack of maintenance with guttering. Once bricks or exterior materials are saturated they become a potential environment for mould growth to occur. Rising damp is another source of moisture in external walls."

Edward Jonkler

Edward Jonkler is a Property Care Association Qualified Technician, and a British Damage Management Association Technician, focusing on damp and mould issues in homes. 

Step-by-step advice for getting rid of mould on outside walls

1. Find the cause of the mould

When it comes to knowing how to stop mould on outside walls, all the experts agree that if you can't find an obvious cause to fix the problem yourself, the most important first step is to call in the professionals.

“My advice for mould and damp issues is to seek a specialist to work out what the root cause of the issue is," says Andy Simms, construction expert at Mybuilder.com.

"Once you've discovered the cause, get it fixed as soon as possible. There's zero point treating the effects of mould without fixing the problem causing it. 

“General builders are great for fixing guttering, repairing brickwork or roofing should a leak be your obvious issue. However, you might need a damp specialist to help you fully understand the root of your problem if it is not easily noticeable. They can tell you whether it’s rising damp, condensation, or a bigger issue such as penetrative damp." 

Contact a qualified damp expert using a service such as Mybuilder.com who have a handy guide for hiring a damp proof specialist or seek advice from a professional through the Checkatrade website.

Andy Simms

With almost a decade of experience on the front line in construction as a multi-trader, Andy Simms is well versed in many of the damp issues faced by residential home-owners.

2. Clear the area of any overgrown greenery

Buildings need to breathe which is why home ventilation is so important. And while we are often aware of the importance of letting air flow and move around the home, it's also essential to expose your external walls to air and sunlight. The biggest culprit for preventing this can be overgrown trees, bushes and plants.

Sunlight prohibits mould growth and constantly wet greenery pressing against your home will pave the way for penetrating damp to become an issue. Knowing how to stop mould on outside walls therefore means keeping on top of your tree growth and if maintenance is an issue, consider low maintenance garden ideas instead.

3. Clean the affected areas regularly

Once any problems have been fixed, next on the agenda is removing any build up of salts, algae, mould and dirt in order to allow the walls to breathe more effectively.

"Using a fungicidal chemical designed for exterior walls such as Sika Mould Buster would work best," says Edward Jonkler. 

Sika Mould Buster from Amazon is currently priced at £22.99 for 5 litres. It's also suitable for patios and driveways and has the added bonus of being biodegradeable.

Don't forget to clean in shaded areas such as underneath window sills and eaves, and for extra elbow grease consider using a stiff brush and then rinse with a hose or sponge off with water. And while it may be tempting to reach for chlorine-based bleach, remember this can run-off the walls and end up killing your plants and grass. 

If the area you need to tackle is much larger, then other options include using one of the best pressure washers or steam cleaning.

"Pressure washing will achieve a great clean," says Dave Bendon, "but it's important to check it's suitable for your external wall finish before you start. While it may work well on bricks, it can for example cause damage to stone or render.

"You'll also need to be prepared to clean all the walls with it once you start," adds Dave, "as otherwise you'll end up with a patchy finish."

Steam cleaning, which has a much gentler approach, may be more suitable if you are dealing with a listed stone building.

If the main issue with your external walls is algae and dirt, the reality also is that even once your external walls are clean and you've repaired any problems, regardless of their construction methods, you will likely need to repeat the procedure every few years.

4. Waterproof coatings and facade creams

Once your walls are clean, you may want to consider adding a waterproof coating or facade cream depending on the external finish. Although they may not entirely inhibit mould or algae, they will provide a protective layer to prevent any future water ingress from permeating the walls.

Dave Bendon notes that while these can certainly be helpful, it's important to ensure these are also breathable so that moisture doesn't get trapped inside the walls.

Having once used a waterproof coating on an exterior wall that was exposed to driving wind and rain, my experience is that it had a limited success rate. However, it was a number of years ago and progression in product development makes it likely that the efficacy rate is now much better.

Re-painting with a masonry paint offering extra waterproof protection may also be an option, but ensure you check on the label or with a reputable tradesperson that it is suitable for your wall finish before applying any new coating, cream or paint product.

Check out these protection products

FAQs

When is mould on outside walls a sign of something more serious?

Penetrating damp is caused by water ingress into the external walls of your house that eventually tracks its way inside when left unresolved.

It's the most common cause of the mould-like arrival of algae on your home and can be caused by a number of different issues:

• Cracks and gaps in your external wall finishes
• Problems with guttering and downpipes
• Leaks in the roof, eaves, fascias or soffits
• Overgrown trees, bushes and plants in consistent contact with walls

In general, all of the above can be remedied making it less likely for mould to appear inside your home in the long run.

But if the mould or damp problem only appears on the ground floor walls of your property, and is also close to the ground, you could have a rising damp issue that needs more than just a quick power wash.

Generally caused by a failure, or breach of your damp proof course (DPC), the most common signs on outside walls are "visible saturation of bricks, such as a darker area, or there could be visible salt deposits on the walls," says Edward Jonkler.

It's also likely you'll see this rising tide mark on both the inside and outside walls of your home – particularly those internal walls that are outside facing. 

If you think this could be a problem, it's essential to seek professional advice to repair your DPC course, or to assess if the issue is caused by external hard landscaping such as patios or driveways being laid above the level of your DPC.

Can rainfall cause mould on outside walls?

Rainfall causing algae or mould growth on outside walls is common and easily fixable.

"Algae thrives in damp conditions," says Dave Bendon. "The combination of the UK's relatively high rainfall and the fact that most external wall finishes – even renders which are classed as smooth – still have a textured surface that airborne algae will attach to.

"Or the walls are made from materials such as stone which lichen loves," he adds, "meaning you are likely to see a build up of discolouration every few years."

And while leaving dirt and algae won't necessarily lead to any bigger problems, it's always recommended that you deal with it to ensure it doesn't hide anything such as new cracks or external wall damage that can cause longer term problems such as permeating damp.

If you've tackled the problem outdoors and now need to repair the damage inside, follow our advice on how to remove mould from walls without damaging paint and try these budget-friendly products to get rid of mould.