How do you damp-proof a shed floor?

We recommend using a concrete base with a damp proof membrane (DPM) to avoid water damage to the shed flooring. Placing a DPM inside the concrete stops moisture from rising and collecting under the shed floor.

gardenroom.guide - How to Waterproof a Shed & Damp Proof - Garden Room Guide

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A well-maintained shed can keep the items inside protected and secured for years. Waterproofing a shed may be required if you see watermarks on the interior as the building begins to get older. A garden shed is up against the elements constantly. Throughout the summer the frequent sun can dry out and crack the wood, and during the winter a constant barrage of wet weather can put even the best waterproofing to the test. Most sheds contain thousands of pounds worth of equipment and tools which we need to keep dry, and water damage can lead to rust and broken electrical equipment. One of the most common reasons for a shed needing to be replaced is due to water damage. The effects will start slowly and within a few years, the damage is visible on both the interior and the exterior of the building. Wood such as timber used in the construction of most sheds is not naturally waterproof and can be damaged by water and moisture over time. It’s important to take steps to reduce water damage and prolong the life of the shed.

Inspecting a leaking shed

Before beginning any work to waterproof your shed, it’s important to inspect exactly where the water is getting in. The best time to check is during or after heavy rainfall which will expose any water that has recently found itself inside. Areas of wood that have absorbed moisture will be darker in colour and easy to spot. Start at the roof and work downwards to trace where the water comes in and how far it goes. Water travelling further such as all of the way to the floor indicates a bigger leak. As well as the roof, another common location for leaks is the sides and corners where wooden panels join together. Timber naturally contracts in the summer and expands in the winter. This is due to adjusting to the moisture levels in the atmosphere i.e. in a damp winter the timber will take on more moisture and expand. The result of this is small movements in the wood that can lead to gaps appearing where panels are joined together. Natural movement is particularly troublesome on log cabins with interlocking logs but can still affect any garden building. If the shed has been leaking for a long time, damage to the wood may have already occurred that can be difficult to repair. Rotting wood will need to be replaced and if there is lots of damage it may be easier to replace the whole shed.

condensation vs leaks

If there’s moisture inside your shed but no obvious leaks, it may be due to condensation instead. Finding out which one is causing water in your shed is key as the treatment for each is slightly different. Condensation occurs when warm moist air in the atmosphere comes into contact with a cold surface. The cooling effect of the cold surface turns the moisture in the air back into water that will sit on the surface. Condensation is common in colder areas of a shed such as the windows but if really bad can be noticed throughout, including on the roof. The best way to resolve condensation is with better ventilation. The ventilation prevents warm humid air from getting trapped inside the shed and is instead constantly replaced with dry cooler air.

How to waterproof a shed

To get the best results, waterproofing needs to be considered as early as the construction stage of a shed. Several steps can be taken to ensure your shed stays dry and protected.

Start with the base

A shed with issues in the base is usually easy to spot as the floor will be damp and beginning to rot. The base is one of the most important elements of any garden building as it will provide the foundation of the structure for years to come. The base should not be positioned somewhere that is going to be exposed to standing water such as the bottom of a slope. A base slightly raised from its surroundings will avoid water puddling up around the bottom of the shed. We placed our shed in a location that gets direct sunlight so any rainwater will dry quickly instead of sitting around. The type of base is just as important as the location. Placing a shed on turf or grass and using a grid base can lead to damp issues. As the moisture from the ground rises it can build around the bottom of the shed and begin to damage the wood over time. We recommend using a concrete base with a damp proof membrane (DPM) to avoid water damage to the shed flooring. Placing a DPM inside the concrete stops moisture from rising and collecting under the shed floor.

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Use ventilated flooring

A common mistake when installing a new shed is not leaving a gap between the concrete base and the shed floor. Using wooden framing between the floor and base can raise the shed floor and avoid it sitting in water. Beams are positioned on the base and the shed floor is constructed on top. Leaving a gap also creates ventilation in the flooring. Ventilation allows air to constantly pass through, preventing the build-up of moisture underneath which can lead to dampness. This step needs to be taken during construction to ensure the shed floor is raised and away from the water below. For best results, the air should be able to flow from one side of the floor to the other without anything blocking its path.

Weatherproof the timber

Treating a timber shed with high-quality products as soon as possible is the best way to provide weatherproof protection. A new shed will arrive with untreated timber that has little protection from the elements and will take on water as it rains. Another issue for new timber is sunlight as exposure to the suns UV rays can lead to the shed beginning to turn grey as the natural colour of the wood fades. We recommend treating a shed as soon as possible once it has been installed. If the weather has been raining, you will need to wait a few days for the shed to dry out completely before painting to avoid trapping moisture in the wood. A base coat clear wood preserver can be applied to the wood first. The role of the wood preserver is to protect against rot and fungal growth from damp conditions. Next, high quality shed paint can be used to add colour and provide long-term protection. The paint will weatherproof and seal the wood to prevent moisture from penetrating through the surface of the wood. For best results, several coats will need to be applied and most paints will last up to five years before needing to be applied again. For a previously treated shed, the weatherproofing protection of the paint may have faded and a new coating may be required. We can test this by pouring water onto the wall of the shed. If the water beads on top then the paint is still providing a waterproof coating.

Guide: Best shed paint

Improve the roof

The roof of a shed should be inspected regularly as it can be prone to wear over time. If there’s one area guaranteed to get wet and cause leaks if it is not sealed correctly, it is the roof. The type of roof design can affect how water runs off the shed and also how much storage space you will have inside. A gable roof design with 2 sloped sections is common. This design allows rainwater to fall off easily and it will collect near the sides of each slope. To combat this we can ensure the water at each side can flow away from the shed once on the ground instead of collecting around the bottom. If the roof has several leaks it may be easier to replace the roofing material and upgrade to one which is better quality. Roof tiles and shingles are popular for sheds due to their durability and appearance. However, a challenge with roof shingles is that they are nailed in place and so when the wood naturally contracts & expands over time it can cause these holes to get bigger and leaks to occur. Damaged shingles can be replaced and sealant used to fill any gaps in the shed roof. For flat roofs, we recommend an EPDM rubber roof. This type of roofing is installed in one large piece on top of the sheds wooden roof. Using one sheet provides the benefit of no obvious weak point for the roof to fail and water to get in through. EPDM rubber is great value and easy to install as a DIY task. The lifespan is also extremely long and will often outlast the rest of a wooden shed.

Check the windows & doors

Depending on the quality and age of your shed, the windows and doors may have small gaps around the edges. As well as cold, they can also allow water to get through and into the shed. Repairing these gaps can be tricky as we don’t want to affect the functioning of the windows or doors. For large gaps, a wood sealant can be used to fill them and prevent water from leaking through. A rubber weather strip can also be placed around the windows and doors to ensure a tight seal when they close.

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Consider guttering

If your shed has a sloped roof, water may puddle during heavy rainfall on the ground at the bottom of the slope. Standing water at the bottom of a shed is going to lead to damp issues as it penetrates into the wood. One way to prevent this is to install guttering. This will prevent water from collecting next to the shed and channel it off elsewhere. You could even connect the gutting to a water butt for collecting rainwater. If your shed already has guttering installed, be sure to check it regularly to prevent any blockages. Leaves and other debris can block the gutting and prevent water from moving freely, resulting in the water having nowhere to go.

Insulation & heating

Depending on how your shed is being used, insulation is a great option to keep water and dampness out. As well as keeping the cold out during the winter, insulation also provides a barrier to keep moisture out as well by using a damp proof membrane. Insulating a shed can be cost-effective and there are several different options available. Our favourite method is to use PIR insulation boards that can be cut to shape and placed between the interior framing of the shed. These foil-backed boards prevent moisture and the edges can be covered with aluminium tape to create an airtight seal. Once the insulation has been installed, the shed can be boarded on the interior with OSB or plywood to hide the insulating material inside of the wall. With insulation installed, a small heater can be used inside the shed to keep the damp away in the winter. Raising the temperature will dry out any moisture from the surface and into the air. If the shed is well ventilated the humid air will be replaced by fresh dry air in no time. Heating and insulation make a great idea if the shed is used as a workshop or to keep electrical equipment that can be sensitive to cold conditions.

Guide: How to insulate a shed

Ventilation

Installing additional ventilation inside a shed is one of the quickest ways to fix damp issues caused by condensation. Most sheds do not have any form of ventilation as standard. This is due to the building not being completely air-tight and some air still being able to circulate. If you notice lots of condensation it may be time to upgrade the ventilation and allow better airflow inside the shed. Passive ventilation comes in the form of small vents installed on the walls of a shed. These work to keep fresh air moving and create a fresher environment inside. One vent will be enough, but for particularly bad condensation a two vent system can be used. A vent will be placed on each side of the shed, allowing air to enter through one and leave through the other. Mesh can be used to stop bugs from entering the shed through vents. Improved ventilation can also be achieved by leaving windows and doors open regularly. If your shed doesn’t get opened much, the trapped air inside is going to lead to dampness and moisture issues. Leaving the shed door or windows for a short period every few days is enough for the air to circulate and prevent moisture from building up.

Guide: Stop condensation with ventilation

Summary

Keeping a shed waterproof involves many steps which all add up to keep the interior dry. The base and foundations of a shed are crucial elements to get right during the construction stage. A solid shed base should be in a location where water can easily run off and the foundations of the shed should allow the floor to be raised above any puddling water. A high-quality wood treatment will weatherproof the shed walls and stop moisture from penetrating through when it rains.

gardenroom.guide - How to Waterproof a Shed & Damp Proof - Garden Room Guide

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