Farm workshop buildings
The smallest building on most farms and the one with the most going on in the floor. What breaks a workshop slab is rarely the tractor that drove in. It is what happens once the tractor is up on stands.
- Typical slab
- 150–200 mm
- Sub-base
- 150 mm
- Concrete, 10 × 15 m
- 33 m³
- Watch for
- Occupation
Sources differ. Point loads and vehicle access decide it.
Compacted Type 1, spread around 190 mm loose.
At 200 mm. Around 25 m³ if you spec 150 mm.
A workshop people work in may not sit where a barn does on planning and Building Regs.
Point loads, not weight
A slab does not really care what a machine weighs. It cares how that weight arrives. Four tyres spread a tractor across a reasonable area of floor. Two axle stands put the same tractor through a few square centimetres, and that is an entirely different calculation.
Everything permanent does the same thing. A press, a pillar drill, a bench bolted down, a jack that always lives in the same corner. Those are the loads that crack a workshop floor, and they are the ones nobody mentions when they ask how thick the concrete should be.
Where the thickness range comes from
Ask two sources and you will get two answers, and both are right about different buildings. UK concrete guidance commonly puts a workshop floor at 150mm with a heavier mix and A252 mesh. Agricultural and commercial floors generally sit at 200mm with mesh.
A farm workshop is both. It has the point loads of a workshop and the vehicle access of a farm building, and which one dominates depends entirely on your building.
33 m³ or 25 m³
The same 10 × 15 m workshop, at 200 mm or 150 mm. That is roughly a third more concrete for the heavier spec — worth deciding deliberately rather than by default.
The question to answer is simple enough: does a tractor or a telehandler regularly drive in and turn round? If yes, spec it as an agricultural floor. If it is somewhere you walk into to work on things, the lighter spec is defensible. The calculator will do either — change the slab thickness and watch the concrete figure move.
Oil, and what it does to concrete
A workshop floor collects diesel, hydraulic oil and whatever drains out of a gearbox. Untreated concrete absorbs all of it. The floor stains, the surface softens over the years, and it becomes progressively harder to keep clean and safe to walk on.
A sealed or oil-resistant finish is a decision to make while you are ordering the mix, not two years later with a floor full of machinery. It costs comparatively little at that point and it is close to impossible afterwards.
The planning question is what it is for
Part 6 of the General Permitted Development Order covers buildings reasonably necessary for the purposes of agriculture on the agricultural unit. A workshop for maintaining your own machinery is a far stronger case than a workshop running a repair or contracting business for other people, however identical the two buildings look from the road.
Occupation is the other thing to think about. A building people work in regularly is not in quite the same position as a barn that stores things, both for planning and for whether Building Regulations apply. That is a question worth putting to the planning authority before you commit rather than after, because the answer decides which route the whole project takes.
Where permitted development does apply, the usual tests hold: Class A on agricultural units of five hectares or more, Class B on units of 0.4 to under five hectares, and a prior approval determination on siting, design and external appearance that the council has 28 days to decide.
A summary, not planning advice. The legislation is Part 6 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, as amended. Whether Building Regulations apply to a particular workshop is a question for Cornwall Council's building control team.
Work out the base for your workshop
Set to a workshop specification at 200mm. Drop it to 150mm in the slab thickness box to see the lighter option.
What this does not work out: the pad foundations
The concrete pads the stanchions sit on are sized by a structural engineer, not by a formula. They change with the span, the eaves height, the wind loading on your particular site and what the ground will bear. Anyone quoting you a pad size without a design has guessed it.
Want this priced properly?
Send us the job and we'll get you engineered drawings and a quote for the frame — with the pad sizes this tool can't work out. The figures above go with your enquiry, so you don't have to type them again.
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Common questions
- How thick should a farm workshop floor be?
- There is a genuine range here rather than one answer. UK concrete guidance commonly puts workshop floors at 150mm with a heavier mix and A252 mesh, while agricultural and commercial floors generally sit at 200mm. A farm workshop straddles the two — it has a workshop’s point loads and a farm building’s vehicle access. If tractors and telehandlers drive in and turn, spec it as an agricultural floor. If it is a bench-and-tools building people walk into, the lighter spec is defensible. Decide it against what will actually drive in, and confirm with your groundworker.
- Why do point loads matter more than the weight of the vehicle?
- Because a load spread across four tyres and the same load concentrated under a jack, an axle stand or a machine foot are very different things for a slab. A tractor sitting on its wheels is a distributed load. The same tractor on stands, or a press bolted to the floor, puts the load through a few square centimetres. That is what cracks a floor that was thick enough on paper.
- Does the floor need to resist oil?
- It helps. A workshop floor collects diesel, hydraulic fluid and engine oil, and untreated concrete absorbs them — which stains, softens the surface over time and makes the floor harder to keep clean and safe underfoot. A sealed or oil-resistant finish is worth considering at the specification stage, when it is a mix and a coating rather than a remedial job on a floor already covered in machinery.
- Is a farm workshop covered by agricultural permitted development?
- It depends what it is for. Part 6 of the GPDO covers buildings reasonably necessary for the purposes of agriculture on that agricultural unit — so a workshop for maintaining your own farm machinery is a much stronger case than one used for a contracting or repair business serving other people. Occupation matters too: a building people work in regularly may not sit where a straightforward barn does. Establish the position with the planning authority early, because it decides which route you are on.