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By Amy Hanlon on 21 Feb 2024

How to stop mould in your shed


It’s that time of year again… The time when you’re all frantically Googling how to stop mould in your shed.

As soon as you’ve cut the grass for the final time of the year, it’s easy to forget that your shed even exists. When things are quiet in the garden you might be nowhere near your shed for months at a time.

But if you want to open up your shed in the Spring to find everything inside safe and dry, you can’t ignore it in the winter months. Because that’s when mould takes hold!

What is mould?

Mould is a structure formed by some species of fungi. Like bacteria, you can find fungus almost anywhere if you get your microscope and white coat out. Most fungi are harmless or only exist in microscopic amounts that don’t cause any problems. 

But if the conditions are right, the fungus will grow rapidly and build up into mould. Depending on the species of fungus, mould may form as black spots, grey dust, or white, green or brown fluff. 

Why do sheds get mouldy?

Mould forms in places that are moist, dark and warm. The inside of a shed is a perfect place for mould to grow, as many sheds tick all of these boxes.

Why do plastic sheds get mouldy?

Plastic sheds are a popular choice for people who don’t want to spend time maintaining their sheds. They don’t need to be painted and rarely need repairing. However, they are extremely prone to mould. This is because water can’t pass through plastic walls.

Surely this is a good thing? Not when it comes to mould. Unless you make sure that everything inside your shed is absolutely dry, any moisture inside the building will be trapped in there. It has nowhere to go, meaning that it condenses on the walls, and pools and gathers in corners and crevices inside the building. This is a perfect environment for mould to grow. 

We often use sheds to store garden tools, lawnmowers, wellies – all the things that get wet out in the garden that we don’t want to trail into our home. In a plastic shed, the moisture carried by these things isn’t going anywhere, and any fungus in the air will lap it up and get growing.

Why do concrete sheds get mouldy?

Like plastic sheds, concrete sheds are also prone to mould. While water can pass through concrete walls, this happens at a very slow rate, and only in the presence of extreme amounts of moisture. 

Concrete walls also get cold quickly, causing any moisture inside the building to condense against the cold walls. Just like plastic sheds, concrete sheds are great places for fungus to grow.

Why do timber sheds get mouldy?

Timber sheds are both a boon and a bother when it comes to staying mould-free.

Because timber is porous, any water inside the building is absorbed by the walls and can make its way out of the shed. However, because timber is porous, any water outside the building is absorbed by the walls and can make its way into the shed. Wooden sheds deal with condensation much better than plastic or concrete sheds. 

This is why it’s essential to protect the exterior of your shed with a good coat of paint. Water-based paints allow some moisture in and out of the building, but also minimise the amount of water that can travel into the building from outside. This helps prevent any damage from water ingress

But timber sheds also suffer from mould because fungi can feed on the sugars in the timber. Not only can a wooden shed create the perfect environment for mould, it even puts on a buffet!

That’s why it’s particularly important to maintain your timber shed.

How can I prevent mould in my shed?

A Gillies & Mackay apex wooden shed with steel box profile roofing and double doors. It is stained brown and sitting on a concrete base.

Prevention is EVERYTHING when it comes to mould. Once you have mould in your shed, it’s very difficult to remove it completely. If you’re looking for advice on how to remove mould – we have a whole article about just that right here. But it’s so much easier to prevent mould from forming in the first place. 

Preventing mould before you buy.

If you’re shopping for a timber shed, check to see what steps the manufacturer has taken to prevent mould in your building. The best prevention is an anti-fungal wash. This means that your building has been treated with chemicals that prevent mould growth.

Bear in mind that pressure treatment is often used to prevent mould. This treatment uses pressure to force an anti-fungal solution deep into the pores of the wood. Pressure-treated wood is sometimes described as Tanalised timber, after the Tanalith E solution that’s used.

But while this seems like a great way to stop mould in your shed, pressure treatment also increases the likelihood of water ingress in your building. If the entire building is pressure-treated (particularly the walls) you may find that mould still causes problems because it’s easier for water to get into your shed. 

Five top tips for preventing mould in your shed.

A Gillies & Mackay apex wooden shed painted pale green with its double doors open to ventilate the shed to stop mould

Once your shed is installed, there are five simple things you can do to prevent mould from forming.

  1. Ventilate your shed
  2. Make sure everything is dry
  3. Add moisture absorbers or a dehumidifier
  4. Keep perishables off your shed floor
  5. Add insulation

How do I ventilate my shed?

Mould has two deadly enemies: sunlight and fresh air. 

If your shed is single-skinned (built of a single layer of timber cladding) the most important thing you can do to prevent mould in the building is to ventilate it regularly.

This is as simple as opening the doors and windows and leaving them open for a couple of hours on a dry day. It’s particularly important to do this after heavy and prolonged rainfall because your building is more susceptible to water ingress during rainy weather. 

Many garden sheds are built with insulation vents that improve airflow in the building. While these help, they’re no substitute for opening up the doors and windows regularly. Don’t forget to close them after you’re done – your ventilation tactics are no good if they let a load of rain into the building!

How do I make sure that everything in my shed is dry?

Wet garden furniture. Soggy wellies. Damp grass cuttings all over the lawnmower. It feels like there are tons of ways to accidentally bring moisture into your shed.

I’m not saying that everything in your shed should be bone-dry before it crosses the door. Let’s be realistic here. But keep an eye on ways to reduce the amount of moisture in your building.

For example, if you’re storing patio furniture in your shed, give it a quick wipe down with a towel before putting it in there. Leave your wellies out to dry for a bit. Clean the lawnmower thoroughly.

In Scotland, there will definitely be days where it’s not possible to do any of these things. You know, those days when it feels like the rain has rain on it? You can’t leave your wellies out to dry if it’s tipping it down. 

But do what you can. If you know that you’ve put a load of wet things into your shed, make sure that you ventilate the building as soon as you’re able to.

What sort of moisture absorbers should I use in a shed?

For those times of year when it feels like it’s been raining forever, moisture absorbers are another great way to prevent mould in your shed. 

There are different types – some are single-use and some are reusable after being refilled with crystals or salts to absorb water. Moisture absorbers are great in the winter months and in the spring when it’s rainy and damp. 

You can sit these on shelves or windowsills, or in any spare corner of the shed. Make sure to empty them regularly – you might be surprised by how much water they collect!

Why should I keep perishables off my shed floor?

Sheds are built for storage. But if you’re storing perishables you need to take extra care. 

Just like fungus feeds on the sugars in timber, it also feeds on other sugary materials like paper, cardboard and textiles. All of these things retain moisture as well as being fungus food, so they’re more likely to be affected by mould. 

By keeping this sort of material off the floor of your shed you’re preventing mould from spreading to the timber itself. Storage boxes or shelving are perfect for keeping books, old clothes, or whatever other perishables you have. 

Should I insulate my shed?

This is one of the questions we hear most often. Adding insulation is a great way to prevent mould in your shed. You can pay someone to add this, or you can do it yourself if you’ve got the DIY skills. 

By adding insulation and an air gap, you add an extra layer to your single-skinned shed. Insulation not only helps prevent condensation, but it also means that any water that soaks through the walls doesn’t get inside the building. It’s a great way to stop mould in your shed.

Bear in mind that a shed floor should be insulated when the building is installed – otherwise, the entire building needs to be dismantled. If your shed is already in place, underlay and laminate flooring are a good alternative to insulating the floor. 

Insulating the walls and roof is a manageable job that will make a real difference to the building. 

How do I guarantee that my shed stays mould-free?

A Gillies & Mackay pent potting shed painted Stephanotis Cream. It has four windows and sits on a slabbed base.

Of course, the very best way to stop mould in your Shed is to buy a building that won’t let any water in, ever. 

Does such a thing exist? It does! 

Our 3-Tier Sheds are built with a layer of timber cladding, an air cavity and a layer of OSB for added insulation and structural integrity. We use this specification for sheds larger than 12’ x 10’ (4.3m x 3m).

3-Tier Sheds are built the same way as our Garages, minus the garage door. The only other difference between the two types of buildings is the floor. Garages don’t have a timber floor, but 3-Tier Sheds do. 

As well as making sure that moisture, and therefore mould, stays out of your building, 3-Tier Sheds are perfect for workshops, studios, or hobby rooms. If you add electricity to your building you can install heat and light and use the building all year round. 

How do I stop my shed from getting mouldy?

Whether you’re going for a swanky 3-Tier Shed, or looking for somewhere to store your garden equipment, you don’t have to endure mould in your shed. 

By taking a little time to care for your building, you can keep it and its contents safe, dry and mould-free. 

If you’re looking for more information on Shed maintenance, have a look at our Learning Centre. Or check out the blogs below:

How to look after your shed in winter

Where can I buy paint for my shed?

Shed Maintenance: How painting your shed will help it last longer

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