While most suppliers have a preferred foundation system for their garden offices, the final choice should always be based on the site conditions. This is where a site visit comes into its own as it allows the supplier to assess the site and test the soil conditions to decide which foundation type is right for your garden.
Some companies don’t conduct a site visit prior to starting work. They rely on you to describe the site. Typically, these firms will either ask you to have a concrete slab foundation installed before they arrive on-site, or rely on systems they have developed in-house which they believe can overcome any site issues once they arrive on-site. If you choose to work with a company who does not conduct a site survey, ask them about how they handle unforeseen site issues, and who will be liable, if work is delayed.
While a relatively new technique in the garden office industry. Ground screw piles have been used in domestic housebuilding and commercial building for many decades. Often overcoming very tricky site conditions.
With this system a large screw is driven by machine into the ground, the length chosen so that it gets down into the solid ground. The beauty of this system is that it is so easy to level an uneven site. Even a large site sloping in multiple directions can be levelled within a couple of hours. The head of each screw is then fitted with a bracket, which supports the floor frame.
With concrete slab foundations, the vegetation is removed from the site, and the ground excavated – the depth will be dictated by the site conditions and the size of the building. A layer of hardcore is laid and then compacted to create a solid level base.
A timber framework is built for the concrete to be poured into. A damp proof membrane (DPM) is laid within this frame. On bigger projects, steel reinforcement bars will be fitted over the DPM, before the concrete is poured and levelled. Once the slab has set – this can take several days – the timber frame / shuttering is knocked away from the slab, and the DPM trimmed.
A cross between a concrete slab and a plinth foundation that some garden office suppliers use are concrete pile foundations, evenly spaced under the garden office.
The size and depth of the pile depending on the site conditions and the size of the building and the method of anchoring the structure to the piles.
The ground is excavated down to solid ground, this is either dug by hand or by using a power auger which drills out the earth. Some suppliers then insert a former tube or timber shuttering into which, hardcore is compacted, and then the concrete poured in. These forms ensure a uniform shape, other suppliers just pour the concrete into the hole with a more rustic shape.
