Mold On Exterior Walls: How To Clean And Prevent

Mold is certainly something you don’t want on the inside of your house, but it’s equally as unpleasant to look at growing all over the exterior walls of your house or property.

So, what can you do to stop mold growing on your walls in the first place? How can you remove what’s already grown, is it definitely even mold, and if it is, can it be damaging to both your health and the material it’s growing on?

In this article, we answer all these questions and more, so for all you need to know about mold growth on exterior walls, keep reading.

exterior wall mold

Why does mold grow on exterior walls?

Mold, mildew, and many forms of fungi will grow wherever the circumstances are suitable for it. They may be very complex organisms, but their needs are no different from our own, and as long as they have moisture, nutrients, and oxygen, they will thrive.

Let’s look a little deeper into some of the mold’s requirements and the types of environments it prefers to settle in.


Mold and mildew require a humidity level greater than 55% in order to be able to survive. In the case of mold on external walls, this moisture typically comes from outside sources such as frequent rainfall and high-humidity climates.

Tropical climates where a combination of both heavy, frequent rainfall and warm temperatures are most likely to suffer from mold growth on external walls.


The nutrients fungi feed off are most frequently sourced from rotting vegetation and other organic materials.

When mold grows on interior walls, it is far easier to keep them clean to remove all sources of nutrition, however, for exterior walls, it is not possible to keep the walls completely clean so other measures must be taken, (more on this later).

Lack of natural sunlight

If there’s one thing mold really doesn’t care for, it’s natural sunlight. This is because the UV radiation given off by the sun is powerful enough to break down its very DNA, sterilizing it.

This is why you will most often find mold growing on north-facing walls and surfaces, as they receive the least amount of sunlight over the year.


Whilst there are some strains of fungi that can grow at relatively low temperatures (with some even begin able to grow in refrigerators), most thrive in temperatures between 60-80 degrees Fahrenheit (15-26 Celsius).

In temperature climates where warm temperatures are reached year-round, fungi and mildew growth are more prominent.

What type of mold is it?

The most common strains of mold you are likely to see growing on an exterior wall are either Alternaria or Cladosporium. Both of these strains have a green to dark-green hue, and present with a fluffy to powdery texture.

They grow in temperate climates (preferably between 64-82 degrees Fahrenheit), where there has also been significant rainfall.

Is it definitely mold?

Much as seeing patches of green growing on the surface of exterior walls may make you immediately think of mold, this may not be the case.

There are several other explanations for the growth, including lichen, algae, or moss, all of which are commonly found growing in similar environments to mold and mildew.

Take a look at what you see growing and see if it matches any of these descriptions, to see if it is definitely mold you are dealing with.


Lichen is found growing very closely to the surface it’s attached to, and does not have stems or leaves. They can come in many different colors, including shades of green and orange.

Lichen is actually a combination of both fungi and algae living together in symbiosis. So they are not in fact a fungi or algae, but are a combination of both.

The texture is often described as being “leafy”, “crust-like”, or “scurfy”, which is quite dissimilar to the powdery and velvet-like textures of mold species.


Algae are plant-like organisms that mostly grow in water, but can grow on surfaces above water when there is very high humidity or frequent rainfall.

They are often green but can come in many shades, and have a stringy texture to them, again, very far away from the texture of mold.


Mosses produce spores just like mold does in order to reproduce and spread. They often form in clumps or mats together, covering large patches of walls or rotting vegetation.

They are technically a plant species, as they convert light to energy, and absorb minerals from the soil and surfaces they are attached to.

The texture of moss is best described as having a soft and spiky surface.

How to test if you have mold growing

Using the above methods to determine whether you are dealing with mold growth can be very useful, but if you are still uncertain, there is another method at your disposal.

Of course, you can purchase an at-home mold testing kit for more accurate testing that allows you to not only confirm that it is a type of fungi, but will tell you the specific strain.

Another option, however, is simple, cheap, and still effective.

Dip a cotton bud (q-tip) in bleach and apply it to the affected area. If it is dirt, algae, lichen, or moss, little will happen, however, if it is mold, the area will quickly lighten and you can be certain you are dealing with mold growth.

Is it dangerous?

Mold growth within a property is certainly more hazardous than external growth, as you are less likely to breathe in large doses of spores that can contain allergens.

However, for people with allergies, even small amounts of exposure can lead to health consequences such as:

  • Breathing difficulties
  • Skin irritation
  • Coughing
  • Nausea
  • Runny nose

Not all molds are dangerous, and many of them do not create the mycotoxins that cause the types of symptoms listed above, but if you are planning on removing any mold yourself, you should always use the correct protective equipment to prevent inhalation or coming into physical contact with any of the spores, just to be safe.

Can it damage your property?

Yes, if mold, mildew, or fungi are not removed from whichever surface they are attached to, it will slowly eat away at the material, weakening it over time.

The reason this occurs is that as the mold consumes organic matter for nutrition, it produces enzymes that break them down. These enzymes are powerful enough to break down brick, drywall, concrete, and other building materials, although this is a very slow process.

How to remove mold from exterior walls

Whether it is purely for ascetic reasons, you are worried about building damage, or concerned about the potential health implications, it is certainly best to remove any mold buildup on exterior walls.

Certain building materials can be porous, and so will require different methods to effectively kill and remove the fungi. With that, use the following methods to remove mold from these materials.

Before treatment

Before you begin remediation of mold on an exterior wall, you must first wear the appropriate safety equipment to ensure no spores are inhaled or come into contact with the skin, as this has the potential to cause allergic reactions.

In order to remove the fungi safely, ensure you wear the following:

  • Breathing mask
  • Gloves
  • Protective eyewear (goggles)


Brick is a porous material, and therefore mold can become deeply rooted, making it difficult to remove.

To clear a brick wall of mold effectively, pour undiluted white vinegar into a spray bottle and liberally spray over the affected area. Leave for a minimum of 30 minutes, then using a soft-bristled brush (so as to not damage the material), brush the wall using circular motions. If required you can use warm water and detergent to help remove the dead mold. If you struggle to work the vinegar into the brick using a soft-bristled brush, you can use a wire brush, but be careful not to damage to brickwork.

Once the wall has been cleaned, spray the previously affected area with the vinegar once more and leave it to soak into the wall and kill off any spores that you may have missed. You do not need to wash away this second coating.


Stone surfaces are slightly less porous than brick, but the same white vinegar method as described above can be used once again here.

Fill a spray bottle with undiluted vinegar and liberally spray over the affected stone. Leave it to sit for at least half an hour before scrubbing the area with a soft bristled brush and warm water. To be certain all the mold is dead, spray the area again and leave it on the stone, there is no need to wash the remaining vinegar off.

You can also use diluted bleach to clean stone surfaces, with oxygen-based bleaches being preferable over chlorine-based ones, as they are better for the environment.

If working on an exterior stone wall of your house that holds aesthetic value, check to see which type of stone it is, as the chemicals in bleach can damage some materials.

To clean an exterior stone surface of mold using bleach, make a 50/50 dilution of bleach to water, and apply it to the affected area using a soft-bristled brush. Leave the solution to work for at least twenty minutes, then rinse with clean water.

You may need to repeat these steps for well-established mold and mildew.


Both the previously mentioned vinegar and bleach methods work well to remove mold on concrete exterior walls, but as concrete usually has less aesthetic appeal, easier methods can be employed.

To remove mold from a concrete exterior wall, spray undiluted vinegar on the affected area and leave it to work for a minimum of twenty minutes. Then, using a pressure hose with a high-pressure setting, spray the surface using up and down strokes. This will remove all dead mold and any residue.

What can you use to clean outdoor mold that won’t ruin paint?

It’s frustrating to find a neatly decorated exterior wall covered with mold and mildew, and sometimes, rather than having to clean it with harsh chemicals that will require repainting after, it might be better to use other options that will not damage the paint but are still able to kill and remove the mold.

So, with this as your aim, what options do you have? Let’s take a look.


Vinegar is perfect for clearing mold from painted surfaces, as it has an acidity level high enough to break down the cell walls of mold to kill it, but is not powerful enough to corrode paint.

This allows you to liberally spray vinegar over large patches of painted walls without the fear of the paint blistering or peeling.

Hydrogen peroxide

Hydrogen peroxide can be bought from chemists, drug stores, and some department stores. It is used as a bleaching agent, disinfectant and can also be used to very effectively kill mold.

The solution is not corrosive enough to damage the paint, so can be applied and left to work for up to one hour before being wiped away.

Can you paint over exterior mold?

Whilst you certainly can paint over fungal growth on an exterior wall, this is not advised, as the wall will only remain clean and mold-free for a short period of time. When you paint over fungi, it is not killed, it is simply covered up, and will continue to grow and spread underneath the layer of paint.

Once the colony has grown large enough, it may appear through small cracks in the paint, spreading over the top layer once again.

How to prevent future growth

Preventing mold from growing on an exterior wall is certainly preferable to having to deal with the stains and potential damage it can cause once it has become well-established.

In order to stop mold growing, you need to keep the masonry as dry as possible so it does not have a source of hydration. Repairing cracks and breaks in brickwork, for example, is the first step to preventing the ingress of water from rainfall and high-humidity climates. The second step is to use a hydrophobic barrier, usually in the form of paint that adheres to the surface it is applied to and prevents water from reaching the masonry behind it.

This is by far the most effective manner to prevent mold on exterior walls, but there are several other quick steps you can take to make this less likely.

Check drainage and gutters

Faulty guttering can allow large amounts of water to fall directly from a roof to the ground, leading to pooling around the base of a wall. As brick and other building materials can be porous, allowing them to sit in water will give them a chance to soak up large amounts of liquids that get trapped within the brick.

If sufficient moisture stays within the brick, mold, and mildew will never be far away. So, ensuring as much rainwater as possible is directed to drainage systems will help keep the moisture levels of your external wall down, which in turn makes it less hospitable for fungi.

Repair cracks in grouting

Even small cracks in grout can allow water and moisture deep into a wall that can lead to damp and mildew formation, not to mention that during winter months, water will fill these gaps, freeze and damage the wall further due to the expansion of the ice.

Repair these cracks as soon as they are noticed to reduce the likelihood of water entering the brickwork or masonry.

When should an expert be called?

Should you notice large amounts of mold growth that continue to come back even after treatment and the correct preventative measures have been taken, it may be time to call in a professional.

Professional mold remediation technicians will be able to assess a situation to determine the potential cause of frequent mold growth. Once this cause is determined, it can be rectified and the problem dealt with before too much damage is incurred.


Mold on exterior walls can cause damage to brickwork and masonry, as well as being potentially harmful to both humans and animals. Testing to see if you are in fact, dealing with mold and not other forms of organic matter growing on an exterior wall is an important first step to determine the best removal methods. If dealt with promptly, mildew and fungal growth on exterior walls can be removed with little damage to the underlying materials.

Chris Walker

Chris Walker has struggled for several years with mold after buying his own property. After finding the solutions to several issues around his home, he decided to create this site in order to answer as many questions about mold and mildew as possible to help others dealing with the same problems.

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