A shed needs a solid foundation to keep the building secure and prevent issues such as movement in the future. When installing a timber shed we also need to consider how to prevent damp rising from the base.
Installing a shed is not as simple as assembling the building on a spare section of soil in the garden. The base needs to be prepared in advance, and in the case of a concrete foundation will need time to dry completely before use. While there are plenty of options for a shed base, concrete is the most popular and provides plenty of durability.
If there is one piece of advice we could give to every new garden building owner, it would be to invest in the base to ensure it’s both solid and level.
A solid base is required to ensure no movement over time. Sheds can weigh hundreds of kilograms, and that’s before garden tools and furniture are stored inside. The base needs to be level to ensure all of the shed’s panels slot together correctly. An uneven base can result in gaps and extra stress on the timber.
One of the most common questions we get asked is whether a shed can be placed directly onto concrete or if some form of additional base is required. Getting the foundations correct from the beginner will maximise the lifespan of the shed and avoid issues in the future.
Do I need a shed base on concrete?
In short, yes, pressure-treated bearers or similar should be used in addition to the base when a shed is placed directly onto concrete.
A concrete slab base is our recommendation when it comes to shed foundations. In brief, the ground is dug out and a concrete base is poured and left to set for several weeks in advance. The concrete slab sits on top of the ground underneath and is raised slightly above the surrounding area.
The challenge is that over time moisture from the ground underneath can work its way up through the concrete base and make it to the surface. If the shed floor is directly touching the concrete, this can result in damp issues as there is nowhere for the moisture to escape.
To overcome this, we can use pressure-treated bearers or a similar additional base to raise the shed floor and create a gap between the shed and concrete base. For a small shed, plastic bases make a good solution that can sit beneath the shed to prevent it from directly touching the concrete.
An additional step is to install a damp-proof membrane inside of the concrete when creating the base. This stops damp rising through the concrete from surrounding soil and drainage. The membrane is placed several centimetres above the surrounding ground level.
Can you put a shed straight onto concrete?
As we have discussed, it’s not recommended to put a shed directly onto concrete. The reason for this is to prevent damp issues when moisture rises through the concrete from the surrounding ground.
Most shed manufacturers will offer pressure-treated bearers that sit between the shed and the concrete base. They work by raising the floor of the shed away from the concrete. This creates a gap to allow airflow underneath, preventing the build-up of rising damp. During heavy rainfall, the slightly elevated shed is also protected from standing water that could soak through the wooden shed floor.
Wooden bearers come pressure treated. This process forces chemicals deep into the timber to protect against damp conditions. As an extra step, you could also treat the bearers with an additional product such as a wood treatment or preserver.
For a small shed, another option to consider is a plastic shed base. This is suitable for use on concrete or pavement and provides a waterproof layer between the shed and the ground below. The plastic base is usually filled with gravel for increased strength and allows enough airflow to prevent dampness on the shed floor.
Plastic bases (Amazon) are interlocking, ensuring they stay secure and can hold hundreds of tonnes once filled. The solution is easy to install and can be completed at the same time as assembling the shed. Unlike wooden bearers, no treatment is required for the plastic due to its already waterproof properties.
Should a shed base be bigger than the shed?
Getting the size of the shed base correct is essential to making sure the assembly runs smoothly. A shed base that’s way too small will stop the installation completely if there is no way of supporting the exterior walls.
If the base is just slightly too small you may be tempted to assemble the shed anyway. But this can lead to issues in the future, as the shed won’t have enough support on each of the sides. Over time the stress on the wood could lead to warping, making windows and doors difficult to open or gaps appearing.
A base that’s too big is less of an issue but still not ideal. The challenge with an oversized concrete base is that it creates a place for water to puddle around the sides of the shed and can also cause splashing on the shed walls when raining. This could lead to damp issues from water penetrating the bottom of the walls.
Ideally, the shed base should be just slightly larger than the shed. This allows all of the shed walls to be supported but prevents water from puddling around the exterior.
Waterproofing a concrete shed base
Waterproofing a concrete slab can be difficult. As we have mentioned, the best solution for a concrete slab base is to use a damp-proof membrane (Amazon) inside of the base. This prevents dampness from rising from the ground whilst leaving the top of the concrete slab untouched.
A concrete sealant (Amazon) could also be applied to the base before the shed and bearers are assembled on top. This type of product penetrates deep into the concrete and creates a waterproof barrier to prevent moisture from getting through. When dry, the sealant will be clear, leaving the natural look of the concrete on show.
The wooden bearers can also be treated with a preserver or treatment product to keep them protected. Applying several coatings will provide the best results as they will be inaccessible once installed. Pay particular attention to the ends of each bearer to ensure they are covered completely.
Can I put a shed on paving slabs?
While a concrete slab base is common, it’s not the only option for a shed base. Existing paving slabs are a common choice, with no additional cost if already installed correctly.
Paving slabs are a good option as each slab can be levelled out to create a perfectly flat base. If using existing paving, be sure to check its completely level to avoid issues with the shed panels joining together correctly. Sometimes patios can be slightly uneven to allow rainwater to run off but this won’t be suitable for a shed.
When using paving slabs, pressure-treated bearers are still required underneath to allow airflow and prevent damp issues. If the patio is much larger than the shed, guttering could be used to control some of the water runoff.
In summary, it’s best not to place a shed floor directly onto concrete. instead, pressure-treated wood bearers or a plastic grid base should be used in between. This is due to damp issues occurring when the flooring is placed directly onto concrete.
With a concrete base, water can rise from the groundwork below. Using one of the above methods creates an air gap to prevent moisture build-up over time. Using a damp-proof membrane inside of a concrete slab base is also a great way to reduce moisture issues further.