Garden rooms need to be built on solid foundations but there are several types to choose from:
A concrete slab is perhaps and is most common type of garden room foundation and is at its most suitable on level ground. The slab should be approximately 50mm bigger than the garden room footprint to allow for any discrepancy in size. If a 100mm concrete slab is being created the site will need to be excavated to a depth of about 275mm. 150mm of hardcore should then be compacted into the hole and levelled off, on top of this 25mm of soft sand is spread. Finally a damp proof membrane (DPM) is laid over the sand, the sand stops the hardcore splitting the DPM.
A timber frame is then levelled on top of the DPM; this is called shuttering and forms a mould for the concrete. Once everything is perfectly level the concrete is poured, this is a mix of cement, sand and ballast. Once poured the concrete is levelled to the height of the shuttering using a long wooden plank and a tapping action. Once the concrete has ‘gone off’ the timber shuttering is removed and the edges of the excavation back filled with soil.
A concrete slab can also be used on a sloping site but an expanse of concrete will be visible at the lower end of the slope, this can be disguised with paint or timber cladding but other forms of foundation may offer a more aesthetically pleasing finish.
Stilted foundations can be made of steel, concrete or a durable wood such as oak. Posts are spaced at a maximum of 1000mm and are set into the ground to a depth of a least 1000mm depending on soil conditions. Steel and oak posts should be set into a concrete mix, whilst concrete piles are created by excavating a hole and inserting a former – normally made from strong cardboard and filling it with concrete. Once the posts have set, a grid work frame is built connecting all the posts and forming a base onto which the garden room is built.
This type of foundation is particularly useful on a sloping site and allows for free airflow under the garden room.
The most modern type of foundation is a concrete plinth, these are made up from sections of concrete which interlock and have a steel fixing which attaches to the garden room floor joist and is easily adjustable. These plinths are spaced across the foundation like a stilted foundation and are environmentally friendly as they use much less concrete and can be recycled at the end of life. For more information have a look at the Swift Foundation website.