If a summerhouse has been left unused and locked up throughout the winter, it can come as a surprise to many owners to find mould has started growing inside. When mould is caught early it is easy to remove and prevent from coming back in the future.
For many summerhouse owners, the building is only used throughout the warmer months of the year. From October to March the summerhouse is often sealed up and apart from the occasional visit for storage, remains unused. If you last left your summerhouse and everything was fine, it can come as a shock to find mould growth inside.
Once mould takes hold it can quickly spread across surfaces in the interior including walls, flooring and even furniture. If left untreated, the mould and fungi can cause damage to the wooden timbers that make up the structure of the summerhouse. When combating mould it’s important to do so early and take preventative measures to avoid it coming back.
The challenge for summerhouse owners in the UK is that we experience a lot of rainfall and mixed temperatures. This creates the perfect opportunity for mould to grow and a summerhouse should be designed from the construction stage with steps in place to prevent mould growth.
What is the black mould in my summerhouse?
If your summerhouse does not have any obvious leaks, you’re probably asking yourself how mould has started growing. Even in a summerhouse without any leaks, the conditions for mould growth can still be created, especially when the building is not used for an extended period.
Black mould refers to several types of mould that feature a dark green or black appearance. Mould spores float in the air around us at low concentrations and are invisible to the naked eye. As they move around they land on surfaces, and just like seeds, if the ideal conditions are met they can form into visible mould growth. The mould is usually visible inside a summerhouse as a black coating on items where growth is present.
Black mould can be troublesome to both a persons health and the structure of the building where it is growing. While the mould is unlikely to lead to significant issues, exposure could lead to coughing, sneezing and watery eyes among other symptoms. For the summerhouse building, mould gradually causes damage over time as it feeds on organic matter (such as wood). Long term damp conditions can lead to wood-decaying fungus which that can cause significant rot.
Why does my summerhouse have mould?
For black mould to grow inside a summerhouse, the correct conditions need to be present including high levels of humidity (moisture), dampness and a lack of ventilation.
The design of a single-skin summerhouse does not usually include any form of ventilation. Therefore, when the building is locked up for an extended period, moist air becomes trapped inside with no way to get out. This can be exacerbated further by storing damp items ( such as garden furniture from out in the rain) or a leak that allows water to get inside. With no nowhere for the moisture to go, humidity levels rise and can stay high for a long time.
Rising damp from below can also lead to mould. This is commonly noticed by wet patches on a summerhouse floor or at the bottom of the walls. If a summerhouse does not have adequate ventilation beneath the floor, the trapped moisture from rainfall will eventually begin to cause rising damp.
When the weather outside turns colder, the trapped air inside the summerhouse begins to cool and the moisture in the air forms condensation when it touches cold materials. This can commonly be seen on windows and other items stored inside a summerhouse where sitting water can be noticed, even without any leaks.
A surface humidity of more than 70% creates the perfect conditions for mould to begin to grow. When a summerhouse is locked up for several months without fresh air or ventilation, the high levels of humidity will see black mould begin to form.
In short, humidity, rising damp and lack of ventilation can lead to mould in a summerhouse.
How can I remove mould in my summerhouse?
If a summerhouse already has mould growth, it’s important to remove the mould before taking preventative measures to avoid it returning in the future. Thankfully, when black mould is caught early it is mostly superficial and can be easily removed.
Before beginning to apply a treatment, open all of the windows and doors to let the summerhouse ventilate. Any items such as furniture should be removed as mould removal treatments can often lead to staining. We recommend wearing gloves when treating mould to avoid touching the mould and also protecting yourself from the treatment.
Lots of common household cleaning solutions can be used for mould removal. One-part bleach to four-parts water can be mixed together and sprayed on the affected area. A scrubbing brush can then be used to remove any stubborn mould and the area rinsed and dried after. Be careful with bleach to avoid staining the wood, we recommend testing on a small patch first.
White distilled vinegar can also be used. Simply spray on and leave for at least 30 minutes to break up the mould. The area can be scrubbed and then rinsed with a damp cloth. We find the dedicated mould and mildew products give the best results. Most home-ware shops will have products available and even the low-cost options work great. Many only require a quick wipe with a cloth to take the mould off the walls.
Once the summerhouse has been treated, we recommend leaving the building for 24 hours with the windows & doors open. This allows time for the wood to dry but also for the strong smell of the mould treatment to ventilate out. The result may see the interior timber stained from the mould or treatment, but this can be covered up with garden building paint.
Guide: Preventing condensation in a summerhouse
How to prevent mould in a summerhouse?
The best way to avoid mould coming back is to take preventative measures to inhibit its growth. For a new summerhouse, these steps can be completed during the construction stage. If the building is already several years old, any existing mould should be removed first.
Wood treatment
One of the easiest ways to prevent mould growth is to apply a high-quality wood treatment to both the interior and exterior of a summerhouse. A new summerhouse will be constructed from untreated timber that is porous and able to absorb moisture. By applying a treatment, moisture absorption is prevented and the organic material is separated & protected from mould by an additional layer.
The exterior of a summerhouse should be painted to prevent rainwater from getting into the walls. Painting inside a summerhouse is just as important as painting the outside. A clear paint can always be used if the natural look is preferred.
When treating a garden building, we first apply a base layer of preserver such as Cuprinol Clear Wood Preserver. The preserver will penetrate deep into the wood to provide protection against rot, decay and staining mould. After the base coat has dried, a garden building paint can be applied to provide both colour and protection to the wood. A waterproof coat is formed that will see water bead from the surface instead of puddling on top.
Ventilation
A practical way to prevent mould is to keep a summerhouse ventilated. When a building has adequate ventilation, the air inside is regularly replaced by fresh dry air and this avoids the build-up of moisture and humidity. Most summerhouses will not have any ventilation, so when the building is locked up for months not being used, it’s not possible for the air to ventilate.
A quick & effective way to ventilate a summerhouse is to leave the door or windows open for a period during the day regularly. when opened, the air is able to move around and ventilate easily. If the summerhouse has a window catch, it can be left in the permanently open position. Be careful to take steps to keep the summerhouse secure while the windows and doors are open.
An additional step to improve air circulation is to install a passive ventilation system. This simply involves air vents fixed to the wall. A small air vent high on one of the summerhouse walls will allow warm moist air to naturally ventilate out before building up. Improved results can be achieved by using two air vents. The first is placed low down on the wall and the second is placed on the opposite wall at a higher position. The air is effectively tunnelled through the building and constantly replaced.
Seal for watertightness
As we know, mould is caused by high moisture levels in a summerhouse, so it’s important to prevent water from getting in. A common cause of high humidity is a leak that is letting water into a garden building.
Start by inspecting the base as this can be a common location for rising damp through the floor. When a summerhouse does not have a gap between the floor and concrete base underneath, it’s not possible for the air below to ventilate properly. This will be visible by patches of water on the summerhouse floor and rising up the walls from the bottom. The best resolution is to construct the summerhouse raised above the base with wooden framing so air can pass underneath the floor.
As it’s not practical to redesign the base of a summerhouse already in position, we can still take steps to reduce the water from rain. Wood treatment should reduce the amount of water soaking through. Guttering can also be installed to prevent rainwater from collecting next to the summerhouse floor. Instead, the water can be redirected away from the building.
In an older summerhouse, natural movement in the wood can see small gaps appears in areas where different sections join such as near the ceiling and corners. The result can be small leaks that over time can let in a lot of water. A wood filler or sealant can be used to prevent leaks and stop water from getting in.
A summerhouse roof can also be a weak spot for leaks. Roof shingles are nailed into position but as the wood naturally moves slightly in different seasons, the holes can become bigger. This can lead to small areas where water can get in. Sealant can also be used to prevent water from leaking through.
Further steps
As well as the steps discussed, further measures can be used to prevent mould. If a summerhouse has electricity, a dehumidifier can be used to reduce moisture levels in the air. A few hours a day is plenty for a summerhouse and will only be needed to dry the building out from a bad leak. An electric dehumidifier should be avoided long term as drying the timber out too much can lead to cracking.
Non-electric dehumidifiers that contain hydrophilic crystals can be purchased online and placed in a summerhouse. While the effect of these is small, they make a good choice to leave inside throughout the winter.
Insulating and heating a summerhouse allows for use throughout the year and also protects against mould. The heating and regular use keeps the air inside dry and reduces moisture build-up. An insulated summerhouse also contains a vapour barrier inside of the wall to prevent moisture from getting trapped inside and leading to dampness.
Guide: Waterproof a garden building
Summary
Finding black mould inside your summerhouse is never a nice surprise, especially if you’re looking forward to using the building when the weather begins to warm up. Fortunately, mould is easy to remove and when caught early is unlikely going to lead to any long term damage.
The best way to protect against mould is to take preventative measures that will inhibit growth. Wood preserver and garden building paint protect the timber and stops mould from forming on the surface. Keeping the building ventilated is the most effective way to stop moisture building up inside and simple measures such as opening a window or installing an air vent provide an effective solution.
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