Any building is only as good as its foundation system, and garden offices are no different.
A few years ago the only option when building a garden office was to create a concrete slab foundation, and whilst this remains a popular option with many suppliers, there are now a number of new garden office foundation options.
The site should dictate the style of foundation
Whilst 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.
- Concrete slab foundation
- Plinth foundations
- Concrete piles
- Ground screws with plastic beam
- Jack foundations
Concrete slab foundations have been used in garden office design since the early days and they are still a popular and reliable choice today.
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 and the concrete poured and leveled. Once the slab has set – this can take several days – the timber frame / shuttering is knocked away from the slab and the DPM trimmed.
Concrete slab foundations are generally made to be about 50mm bigger in each direction than the footprint of the garden office, this offers a degree of tolerance for ‘square’ and measurements.
It takes skill to install a concrete slab, it is important that the finished foundation is ‘square’, level and there are no undulations in the surface. This is why they have gone out of fashion with some suppliers, in addition they are labour intensive, use a lot of materials and take time to set so they are more expensive than other types of foundation.
Its worth checking if the garden office supplier organises the installation of the slab, some suppliers specify them but expect the customer to find a local builder to install the slab before they arrive with the building.
Plinth foundations have become very popular with garden office designers this is for a number of reasons – they are simple to install so can be fitted by the general construction team. They don’t involve wet materials so there is no setting time, meaning the garden office can be built as soon as the foundations are installed. They have strong eco credentials in that they can be removed cleanly from the site should the garden office ever be moved and they can be reused – all these reasons make them cost effective too!
The premise with plinth foundations is that the plinths made up of base pads and a top pad which has adjustable fixings are spaced evenly under the garden office.
There are three key brands of plinth foundation used by the garden office industry – Swift Plinths, Jack Pads and the EasyPad. These three systems work in much the same way, the materials used in their construction differ, some are made from concrete the others from plastic.
Plinth foundations can easily overcome problems with an uneven site, by adding additional base pads to raise the foundation where the ground drops away.
In most cases plinth foundations are included in the cost of the garden office project.
A cross between a concrete slab and a plinth foundation that many garden office suppliers use are concrete pile foundations, evenly spaced under the garden office.
The size and depth of the pile depends on the site conditions and the size of the building and the method of anchoring the building 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 form tube into which the concrete is poured, this keeps a uniform shape, other suppliers just pour the concrete into the hole.
Depending on the choice of fixing, some suppliers will insert metal work into the wet concrete this has adjustable fixings which will attach to the floor frame, other suppliers will level the pile ready for concrete beams or steel frames to sit on the piles.
Another eco friendly foundation system are ground screw systems, as the name suggests a large screw is driven into the ground.
With this system a large screw is driven by machine into the ground, this gets right down into the solid ground. A adjustable head is attached to the top of the screw pile and a modular beam system is used to connect the screws.
Once the beams have been positioned an insulated floor slab is slotted in-between the beams. The walls of the garden office are built off this structure.
Because there is no concrete used in this system it can be removed cleanly from the site should the building ever be removed.
A leading name in this style of foundation are Quick Base, their website has lots of information which is worth a read.
Some suppliers use adjustable fixings that are controlled by tightening a nut, other suppliers use small jacks to adjust the height of the floor frame.
These small jacks work on the same principle as a car jack, they have small hydraulics fitted which allows them to be finely adjusted to create a level floor.
These jacks normally sit on concrete pile foundations.