In our last article, we looked at the concrete base often used as the foundation for a garden room. Today, we are going to look at a variation of the traditional concrete slab - the insulated concrete slabs.
While not seen as often as the traditional concrete slab. Insulated concrete slabs are popular with several garden room designers. The main reason being that they negate the need for a timber floor frame. The final floor covering can be laid directly over the concrete base.
Removing a timber floor frame from the build-up not only reduces the quantities of materials needed, but it also offers extra height within the room. A timber floor frame is typically 150mm thick, remove this from the equation, and you have 150mm extra height to play with within the room. This is particularly important when building a garden room that is no taller than 2.5m to comply with Permitted Development. It is also a popular option when building a garden gym, where extra headroom is very beneficial.
Quick guide to insulated concrete slabs
Usually installed as part of a turnkey project
With traditional concrete bases, you are usually asked to engage a local company to install it before the garden room installation team arrive on-site. This is an additional expense that you need to budget for.
However, with an insulated concrete slab, this is typically installed as part of a turnkey project. In these cases, you work with the one company from groundworks to final decoration and are quoted one price for all the works.
Installed a couple of weeks before the garden room
You will find that the installation site arrives on site a couple of weeks prior to the main installation date to undertake the groundworks and install the concrete foundation. They then leave the site while the concrete hardens. Returning to start work on the core structure.
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 diagonal from corner to corner. 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.
Rigid insulation is laid over the damp proof membrane. The thickness of the sheets will be decided upon based on the size of the garden room. Rigid insulation will also be fitted around the perimeter of the slab.
A second damp proof membrane is laid over the insulation; the sides mitred to stand up at the sides of the frame.
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.
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 wall frames to sit on top.
