How to waterproof your shed

A shed in your garden is a helpful place for storing tool, equipment or bicycles - but water can quickly cause problems like rotting wood and mould that weaken the structure of a shed and damage what you keep inside.

Our guide has straightforward tips on how you can keep your shed safe and dry for years.

EASY AS HACK 

Covering your shed roof with waterproof material is essential. Felt roofing is popular as it’s good value for money and easy to install.  Watch this short video ^[1] to learn how to do it.

what can cause water to get into your shed?

Base
Where you build your shed can be a significant cause of water getting into your shed. For example, placing it directly on the ground or a poorly constructed foundation or base can allow water to seep through the floor.

Water can also enter your shed because of a shift in the structure. Movement in the base that makes it uneven will allow water to pool underneath.

Quality
Low-quality timber for a shed might be a short-term saving – but inconsistent grain, knots in the wood, and cladding that’s not tongue and groove boards will all make it easier for water to get in.

Once the shed is up, holes or gaps in the roof, floor and walls – or poorly sealed joints between them – can all cause water to get into your shed.  

Roof
Sagging or damaged roof structures can mean water gets into the shed rather than being channelled away.

Sealant
This can crack or shrink with age, especially around window frames and doors; and create gaps and holes that let in water.

Rotting wood^[2]
This is something else to look out for because the fungus that causes it feeds off the timber until it begins to break down, weakening its ability to resist water penetration.

how to build a waterproof shed

To build a shed that is waterproof, follow these steps.

1. Foundation or base – a solid and even foundation or base is crucial if you want your shed to be waterproof, providing stability and keeping the shed away from direct contact with soil, preventing moisture from spreading onto the floor.
   
   Standard foundation materials are concrete, paving stones and plastic. For extra protection with concrete or plastic, add a waterproof membrane beneath the foundation.
2. As a further precaution, elevate the shed – resting the floor on treated timber fencing posts or battens is one simple but effective way. Creating a gap between the shed and the foundation improves airflow, helping to keep it dry underneath.
3. Cladding - Fit cladding such as tongue and groove timber boards. There are different kinds of tongue and groove, such as shiplap, but in each case, the interlocking boards slot together with a tight fit to resist rain and moisture from the air.
4. Install insulation – this helps to control the temperature inside your shed, stopping it from falling below the dew point when water vapour in the air condenses into water.
   
   Types include fibreglass roll, cellulose, expanded polystyrene or foam, breathable membranes, plywood board and even bubble wrap. Find out more in our detailed guide to how to insulate a garden shed.
5. Use waterproof roofing material – mineral roofing felt is the most popular because it is cheap, forms a sound barrier against rain and is lightweight, making it ideal for sheds.

Alternatives roof coverings include:

• Roof shingles - individual, overlapping tiles made of felt, asphalt, wood or slate.
• Ethylene Propylene Diene Monomer (EPDM) - a synthetic rubber with a lifespan of decades^[3] but costs more.
• Seal windows and doors – gaps around windows and doors are an open invitation to unwanted water. Apply silicone sealant around window frames and use weatherstripping tape on the door frame to create a tight seal.
• Install ventilation – proper airflow prevents the build-up of moisture. A basic approach is installing small vents shaped like grilles. Place them high up on the gable walls at the ends of your shed. Alternatively, you could consider using a roof vent with a wind-powered turbine, known as a whirligig.^[4]
• Apply a waterproof treatment – once your shed is up, cover it with wood preservative or waterproof shed paint. A wood preservative protects against rot, decay and insect damage, while waterproof shed paint creates a surface coating that repels water and prevents moisture absorption.
• Install gutters – these keep a shed waterproof by channelling rainwater away from the shed’s roof and foundation. There’s lots of help online to put up gutters.^[5]

how to waterproof a shed floor

Start by building your shed on a solid and even foundation, using concrete, paving stones or plastic, then elevate the shed off the foundation by, for example, resting the floor on treated timber fencing posts or battens. Other measures include:

• Clear gutters to prevent water pooling around your shed’s foundation.
• Apply silicone sealant along the base/wall joints and between joints in the floorboards. 
• Use extra water-resistant pressure-treated plywood for the shed floor.
• Add gravel borders around your shed to improve ground-level water drainage.

how to waterproof shed walls

Take the following steps to keep your walls waterproof:

• Regularly check walls for holes or gaps between cladding boards and use a waterproof silicone sealant to fill them.
• Apply waterproof shed paint to damaged areas as needed. The whole shed should be repainted, on average, once every five years, but more regularly if you live in areas with high levels of rainfall.^[6]
• Replace rotten wood. See this video online for some tips on how to do this.^[7]

how to waterproof shed doors and windows

Windows and doors are common entry points for water in a shed because of gaps in the frames or seals.

Fill any cracks and holes with silicone-based sealant, which is designed to create a watertight seal but remains flexible even after curing, helping to prevent cracking. This benefit makes it preferable to an alternative caulk sealant.^[8]

Dig out and replace rotten patches or fill with wood filler, which is usually made from of a mixture of wood fibres, a binding agent and colour pigment that hardens after application.

You can keep door seals tight by using weatherstripping around the frame and a door sweep at the bottom of the door. Tightening hinges and ensuring proper door alignment can also keep out water.

how to waterproof a shed roof

A shed’s roof is its first line of defence against rain, so keeping it clear of debris like leaves, twigs and anything that could trap moisture, leading to decay in the roofing material, is essential.

It’s good to check for damage to the roofing material yearly ^[9]and after severe weather. Repair cracks or holes with sealant or replace roofing material if needed. Keeping gutters clear will make sure they can do their job of allowing water to run off the roof.

why should you waterpoof a shed?

Waterproofing your shed will keep the contents dry but also protect against the damage to the shed’s timber structure. It will:

• Prevent damp, rotting, warping and sagging – dampness encourages wood rot, which leads to mould and mildew, creating unpleasant smells and potentially irritating your skin, lungs, nose and throat. Warping weakens the shed, making it vulnerable to high winds, while sagging in the roof can create a concave shape where water can pool.
• Prevent the growth of moss, algae, and lichens, which are an eye sore, cause unpleasant smells, and weaken timber’s resistance to moisture.
• Prevent leaks in the roof and cracks in window and door frames – all allowing water to enter.
• Ensure items in your shed stay undamaged – and tools stay rust-free.

Typically, building insurance covers garden sheds, excluding wear and tear – but here’s our guide to what home insurance covers in your garden.

Find out more about Ageas Home Insurance.

Sources

[1] - YouTube

[2] - YouTube

[3] - Epdm rubber rooding for sheds

[4] - Guide to shed ventilation 

[5] - YouTube

[6] - How often should you repaint your shed or fence

[7] - YouTube

[8] - Silicone vs Caulk

[9] - Look after your roof