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By Amy Hanlon on 24 Feb 2024

How to get rid of mould in your shed.


Not since the invention of penicillin has anyone been pleased to see unexpected mould growing. A quick Google will reveal that there are tons of suggestions on how to get rid of mould, but there’s so much conflicting information that a blog seems like a good idea.

So if you’ve spotted black spots or green fur growing in your shed, read on to find out what your options are.

What causes mould in my shed?

Mould is caused by an overgrowth of fungus. Scientists believe that there may be as many as six million different species of fungus on Earth, even though only around 150,000 of them have been identified so far. 

Like bacteria, fungi are everywhere. They are microscopically small and don’t usually cause us problems. But when the conditions are right, fungi will grow and grow and grow. When this overgrowth happens, we start to see visible signs of the fungus. We call these visible signs mould.

Mould needs moisture and darkness to thrive. Sunlight and fresh air are its natural enemies. If you start to notice visible signs of mould, this is a clear sign that there’s water in your shed. 

This can be caused by a leak, and finding the source of the leak will remove the water.

But if the problem is caused by water ingress, things are a little bit more complicated. 

What is water ingress?

A shed wall and floor, showing signs of water ingress where the wall and frame meet.
Signs of water ingress on shed framing.

Water ingress happens when the air outside your building is full of moisture. Because timber is porous it eventually soaks up this moisture, and when there’s enough of it, water shows up on the inside of your building. 

If you have a single-skinned building in the Scottish weather, water ingress will always happen to some extent. However, it shouldn’t be an ongoing, consistent problem that causes your shed to regularly be soaking wet. It may simply be a small patch or stain left behind by water. 

We need to talk about water ingress because often customers are disappointed to find water in their shed when they’ve spent time and money on a really good quality building. But single-skinned buildings are always susceptible to water ingress to some extent. 

You can minimise water ingress by ensuring that your shed is painted with a good quality water-based paint. Painting your building every 3-5 years will help keep water out. 

But you should also watch out for spells of very wet weather.  This is when water ingress is likely to happen. Ventilating your shed well after heavy rain will minimise the impact that water ingress has. 

This is why we don’t build single-skinned summerhouses any more. We want to make sure that our products meet our customers’ expectations. If you need a building that’s never ever going to suffer from water ingress or mould, a single-skinned shed isn’t enough.

If the mould in your shed is caused by water ingress, it can be more difficult to remove the source of the moisture that’s so vital for the fungus to grow. 

Will bleach get rid of mould in my shed?

A wooden shed wall showing black spots of mould.

Ohhh, the internet! Go on – have a quick look. I’ll wait. 

There are SO MANY places that will tell you that bleach gets rid of mould. However, the situation isn’t that simple. Bleach will often remove the visible signs of mould. This is particularly useful if you’re dealing with unsightly black mould – the bleach will reduce or sometimes remove the spots and stains. 

But if you treat your mould with bleach, expect to have to repeat this treatment fairly regularly. 

Why doesn’t bleach kill mould?

Time for the science bit!  

Bleach is the common name for sodium hypochlorite. This is a chemical that is always trying to “steal” hydrogen ions from whatever else it comes in contact with. Hydrogen ions live a free and easy life, bonding with literally anything that will have them. These bonds are easily formed and easily broken. 

However, hydrogen can also bond to other hydrogen atoms, and these bonds are very sturdy indeed. Unfortunately, these are the bonds that hold together the cell walls of fungi. 

To dissolve these bonds, you need very strong bleach, and lots of time. If you have a non-porous surface, you can pour neat bleach onto this surface and leave it for a few hours. This will break down the hydrogen bonds in the cell walls of the fungus that’s causing the mould, killing the organism.

But…when I say non-porous, I’m talking 100% non-porous. Most materials are porous at a microscopic level. And if there are microscopic pores, there’s room for mycelia.

A mycelium is like a root for a fungal cell. It grows out of the cell and branches off into many smaller roots. When a fungus takes root, it grows mycelia into all the microscopic spaces in the material it’s growing in. Getting rid of your mould involves getting rid of all the mycelium strands. 

So even if the cell walls of the fungus are broken down by extended exposure to neat bleach, mycelia will still be rooted into your shed wall. As long as the mycelia are there, the cells will regrow. 

This is why it’s so hard to get rid of mould, even with bleach! Mould will regrow as long as it’s warm enough, moist enough, and there are still mycelia roots around. 

Can I use bleach on mould in my shed?

The problem with using bleach isn’t just that it only tackles the surface of the mould. It also (eventually) breaks down the chemical bonds of the wood in your shed. 

If you leave bleach on your shed for long enough to break down the fungus, you also risk damaging the timber that your shed is made of. 

Are there other chemicals that I can use to kill mould?

Unfortunately, these problems extend to most other chemicals that are powerful enough to kill mould. Either they kill off the cell walls of the fungus but not the mycelia, or they’re so strong that they damage the timber of your shed. As long as there are still mycelia present, mould will regrow. Because of this, the most effective use of chemicals is to prevent mould. 

Prevention really is the name of the game here.

What can I do about mould in my shed?

A wooden shed interior showing mould on the walls and framing.

If the prevention ship has sailed, and you’ve found black, white, grey or green mould inside your shed, don’t worry. Your building isn’t ruined. However, you will have some work to do.

The first and most important thing to do is to remove the source of the moisture inside the building. Whether it’s coming from outside or inside, removing the moisture will limit the growth of mould. 

Remember that sunlight and fresh air are the enemies of mould. Ventilate your building regularly, and consider adding moisture absorbers to the building.

Washing the affected area with bleach will improve the appearance in most cases. Dab the mouldy area carefully with neat bleach (don’t forget your gloves and mask!) Remember that you may have to repeat this treatment fairly regularly if the mould regrows.

Is mould bad for my health?

It’s worth pointing out that if you’re prone to allergies, disturbing mould may irritate these. 

When mould is attacked it responds by releasing spores into the air, which cover you with the potential to spread the fungus wherever you go. If you’re sensitive, breathing in mould spores can cause allergy-type reactions and headaches.

The vast majority of healthy humans won’t be bothered by most types of mould, but mould can make you sick if you’re sensitive. 

If you’re worried about this, dig out a clean face mask, wear eye protection, put your clothes right in the washing machine and take a really hot shower once you’re done. If you know that you’re sensitive to mould, get someone else to clean the affected area.

Will cleaning with bleach remove mould from my shed?

Remember – bleach will improve the appearance of the affected area, but it won’t kill the mould. You’ll have to keep an eye on this area of your shed to check that the area is no longer wet and to ensure that the mould hasn’t regrown. If it does reappear, clean it again with the bleach solution. 

Whether you use bleach or an anti-fungal chemical, mould will regrow unless the moisture is removed.

What can I do if the items in my shed have gone mouldy?

The same rules apply if you have perishables in your shed that are affected by mould. First, make sure that they’re not in a moist environment, then clean them as best you can. 

Be aware that mould may reappear on these items if they get damp again. However, if they stay dry and in a well-ventilated area they won’t cause any problems unless you’re sensitive to mould.

How do I remove mould completely from a shed?

The only surefire way to completely remove mould is to replace any materials that contain mycelia. In a timber building, this might mean replacing a few boards of the shed. However, since mycelia are not visible without a microscope it’s very difficult to know for sure whether you’ve removed all of the affected materials. 

For the most part, we can coexist quite happily with fungi. As with bacteria, we know it’s there, and we don’t want it to grow out of control. But trying to remove it entirely is pretty much impossible. If you find mould in your shed, the best way to prevent it from regrowing is to address the water problem that’s causing the mould in the first place.

How do I keep a shed completely dry inside?

A large Gillies & Mackay 3-Tier pent shed painted pale blue. It has double doors which are open showing the wooden interior
A Gillies & Mackay 3-Tier Shed

Preventing mould is much easier than treating it, especially in a timber building. This means ensuring that your building never suffers from leaks or water ingress. 

In practice, this means you need a shed that isn’t single-skinned, like our 3-Tier Sheds.

These buildings have triple-layered walls and use a clever air cavity to prevent moisture from getting into the building, even in the wettest weather. 

The 3-Tier Shed specification is based on our Garage specification, which was created to address the problem of water ingress. So if you’re looking for a bone-dry building that won’t suffer from mould, a 3-Tier Shed is just what you need. 
If you’d like to find out more about 3-Tier Sheds, come and see us at our Show Area. Or if you’d like some dedicated time with our Team to ask questions, you can book a Consultation.

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