Concrete Bases for Garden Rooms, Garages and Outbuildings

Each building asks something different of its base. A garage slab carries a car and needs to be tough and well reinforced; a garden room or office needs a dead level base so the frame and glazing go up square; a shed or summer house needs a solid, free draining pad; and a hot tub, full of water and people, is a heavy load that a thin slab or an ordinary patio is not built for.

Tell us what is going on the base and, if you have it, the supplier's base specification. Where there is one we build to it exactly, including the size, the thickness and where any service ducts need to come up through the slab for power or water.

We dig out the topsoil and any soft ground to reach a stable formation, then lay MOT Type 1 crushed stone in layers and compact each one with a plate or roller. A well compacted sub base is what stops a slab settling and cracking, and it takes as much care as the concrete on top of it.

Timber formwork is set to line and level around the edge and braced so it cannot move under the weight of wet concrete. Steel mesh goes in where the load needs it, raised on spacers so it ends up inside the slab rather than lying on the ground beneath it, and a damp proof membrane goes down first where the building needs a dry floor.

Some bases need to be dead level, others need a slight fall. A garage apron or an open hardstanding is laid with a fall so rainwater runs off rather than pooling, while a base for a timber garden room is usually laid level and kept just inside the building's footprint, so rain running down the walls drips clear instead of soaking in underneath.

We set the level against the house, the garden and the drainage before we dig. On a sloping garden that can mean cutting into the bank on one side and building up on the other, so the base finishes at a sensible step height and no water is directed back towards the house.

We source and book ready mixed concrete for each pour, in the mix and quantity the base needs, so the whole slab goes down in one continuous pour rather than being mixed by the barrow load. Where the wagon cannot reach the back garden, the concrete is barrowed or dumpered through, or a concrete pump is booked where that is the better route, and we plan the route on the site visit.

The concrete is spread, compacted and struck off level against the top of the formwork, then finished to suit the use: a tamped finish for grip on an outdoor hardstanding, or a smoother float finish where a building will sit on it.

Concrete goes on gaining strength for weeks, but only while it keeps its moisture. Curing means stopping the new slab drying out too soon, usually by covering it with polythene, and keeping frost off it: industry guidance says the surface should not fall below freezing until the concrete reaches a strength of at least 5 newtons per square millimetre.

In practice a base poured in cold weather is covered and protected overnight, and one poured in hot or windy weather is covered so the surface does not dry and crack. We tell you when the base is ready for the building to go on, and when it can take heavier loads such as a car or a filled hot tub.

According to the Planning Portal, building regulations do not normally apply to a detached outbuilding with a floor area under 15 square metres and no sleeping accommodation. Between 15 and 30 square metres approval is usually not needed either, provided there is no sleeping accommodation and the building is at least 1 metre from any boundary or built mainly of non combustible materials.

Planning permission is a separate question, decided by the rules on the size, height and position of outbuildings, so it is worth checking both before the base goes in. Where the building sits, and how close it comes to the boundary, is fixed when we set out the base, so have that settled before the day of the dig.