For most garden buildings, a concrete base needs to be 100mm thick over a compacted 100mm sub-base. Heavier structures such as garden rooms, workshops with machinery or large log cabins usually justify 150mm. The right answer depends on what you are putting on it and what the ground underneath is like, so here is how to work it out properly.
A standard timber shed or summerhouse up to around 3m x 4m sits happily on 100mm of concrete. That is enough to spread the load, resist cracking and give fixings something solid to bite into. Going thinner to save money is a false economy, because a 75mm slab over soft ground will almost always crack within a couple of winters.
For anything heavier, step up to 150mm. That covers garden offices and garden rooms with proper stud walls and insulation, workshops housing lathes, mills or a car lift, and large log cabins with 44mm or 70mm walls. If you are planning a home gym with heavy free weights, or you might one day park a ride-on mower or quad inside, 150mm is the sensible choice from the start.
The slab is only as good as what it sits on. Strip off all topsoil and vegetation until you reach firm ground, then lay 100mm of MOT Type 1 hardcore, compacted in layers with a wacker plate. Skipping this step is the single most common cause of cracked bases we get called out to look at.
Around Chorley and much of central Lancashire the ground is often heavy clay, which holds water and moves as it wets and dries through the seasons. On clay you should treat 100mm of compacted sub-base as the minimum, and on particularly soft or recently disturbed ground it is worth going to 150mm. A damp proof membrane laid between the sub-base and the concrete stops moisture rising through the slab into your building, which matters a lot if the interior will be lined and insulated.
A C20 mix is fine for sheds, while C25 is the better choice for garden rooms and workshops. If you are mixing by hand, a 1:2:4 ratio of cement, sharp sand and coarse aggregate gets you close to C20, though for anything over a few square metres a ready mix delivery is easier and far more consistent. For slabs of 150mm carrying real weight, a sheet of A142 steel mesh set in the middle of the slab adds a lot of crack resistance for modest cost.
Size the base either flush with the building footprint or around 25 to 50mm inside it, so rain running off the walls drips past the concrete rather than pooling on it and soaking the timber. Make sure the finished slab sits at least 75mm above surrounding ground level, check it for level carefully, and give the concrete time to cure. You can usually build on it after about three days in summer, but wait a week before loading it heavily, and longer in cold weather.
Concrete is the most durable option, but it is not always necessary. A small tool shed on free draining ground can sit on paving slabs over compacted hardcore, and plastic grid systems work for lightweight buildings on level plots. Where concrete earns its keep is with heavier structures, sloping or soft ground, and anywhere you want a base that will still be flat and solid in twenty years.
If your garden slopes noticeably, the base may need to be cut into the bank or built up with shuttering on the low side, which changes the excavation and concrete quantities considerably. That is the point at which it is worth getting someone with a digger involved, because hand digging a stepped base into wet Lancashire clay is miserable work and rarely ends up level.
It can work for a very small, light shed on firm ground, but it leaves little margin and cracks easily if the sub-base settles. For the small extra cost, 100mm is the safer standard and what we would always recommend.
Usually not, as a base for a garden building normally falls under permitted development. Check with Chorley Council if you live in a conservation area, a listed property or plan a large garden room, as different rules can apply.
As a rough guide, expect somewhere in the region of 70 to 130 pounds per square metre supplied and laid, depending on access, ground conditions and slab thickness. Sloping sites, poor access and deep dig outs on clay all push the price up, so a site visit is the only way to get an accurate figure.