In our series of articles looking at the different foundation systems used in garden room design, many of the options such as ground screws have only really been adopted by designers in the last five years or so. The one foundation system that has been used from day one, and is still the prefered choice for many designers today, is the concrete slab.
Concrete slabs take skill to get right and need to be installed more than a week before work can start on the main construction of the garden room. This is why other options such as ground screws and plinth foundations have become popular with designers, as work on the building can start as soon as the foundation has been installed.
Quick guide to concrete slab foundations
Concrete base not always included in the price
While a garden room company may specify a concrete slab foundation, they don't always include the installation in the price they quote. They either ask you to organise its installation with another company; or if they will install it for you, it is a hidden extra, that is quoted for on top of the building.
When comparing garden rooms, if one seems significantly less expensive than another, you may find that the foundation is not included in the price.
Usually, 50mm bigger than the garden room
A concrete slab is usually made to be 50mm bigger than the garden room footprint. This offers a degree of tolerance when it comes to positioning the core framework of the building.
The slabs thickness depends on the building and site conditions
While 100-150mm concrete slabs, laid over compacted hardcore and sand are typical for a garden room, the depth of the slab should be decided following a site survey where the ground is assessed along with the size and intended use of the building.
If you are asked to organise the concrete base before the garden room installation team arrive on-site, you would expect them to give you a detailed specification for the slab composition and dimensions.
A concrete slab is constructed from a series of layers
The first task is to remove the grass and any other vegetation from the site. The ground excavated and the sides and base levelled.
A timber framework - shuttering - is created. This will form a mould for the concrete to be poured into. So, it is essential that it is 'square' and level. To check for 'square' you measure the diagonals. If the measurements are the same the frame is 'square'.
A thick layer of hardcore is laid in the bottom of the hole. This is then compacted and levelled. A vibrating plate is used to get a solid base.
Sand is then laid over the compacted hardcore and again levelled.
A damp proof membrane is then laid over the sand, within the timber framework. The sides of the membrane are mitred so that it lines the sides of the timber shuttering.
With bigger garden rooms, a steel rebar gridwork will be fitted before the concrete is poured. This strengthens the concrete slab considerably. Special feet are used to support the rebar so that it doesn't cut into the damp proof membrane.
The concrete is poured into the mould. On smaller slabs, this can be mixed on-site in a cement mixer, but with bigger concrete bases the ready mixed concrete will be delivered by lorry and wheelbarrowed from the road to the site. This can be a much quicker option.
The timber framework is used to level the concrete; the installers will use the edges of the shuttering to support a length of wood, which is tapped to remove any air bubbles.
The timber framework is used to level the concrete; the installers will use the edges of the shuttering to support a length of wood, which is tapped to remove any air bubbles.
Once the concrete has set, the timber framework is knocked away, and the ground backfilled with earth.
A damp proof course is then laid on top of the slab, ready for the joists to sit on top.
