Steel building groundworks calculator
A frame quote prices the frame. The base underneath it is a separate job — and on a sloping field it is rarely the smaller number. Put your building size in and this works out what you need to order: concrete, Type 1, mesh, membrane, and how much muck is coming off.
- Typical slab
- 150–250 mm
- Set by what drives on it, not by the size of the building.
- Sub-base
- 150 mm
- Compacted Type 1. Spread around 188 mm loose to finish there.
- Muck-away
- 12 m³ a load
- Eight-wheel tipper, loose volume. Dug ground takes up more room than it did.
- Not in here
- Pad sizes
- They come off a structural design. Anyone quoting one without it has guessed.
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.
Every enquiry gets a reply the same day, and your figures go with it. The building itself is designed and priced by South West Steel Buildings, our building partner, so your details go to them too, and they will be in touch. How we handle your details.
What a steel building quote does not cover
This is the gap the whole site exists to close. A steel frame quote is a quote for steel — stanchions, rafters, purlins, bracing, sheeting, fixings and, usually, a team to stand it up. It is a complete and honest price for that, and it is not a price for a building you can walk into.
Everything between the field and the frame is somebody else's job. Stripping the topsoil. Cutting and filling to a level platform. Carting the spoil away, or finding somewhere on the holding to put it. Digging and pouring the pad foundations. The sub-base, the membrane, the slab, the drainage, the apron outside the doors. On a flat site with shallow topsoil that is a manageable second bill. On a slope it can rival the frame.
The one place the two trades meet is the pads. The frame supplier's structural design says how big they are and where the holding-down bolts go; your groundworker digs them, pours them and sets the bolts to that drawing. Get those two talking to each other early, because a pad in the wrong place is a very expensive thing to find out about on erection day.
The slab
Slab thickness is decided by what drives on the floor, not by how big the building is. A thirty metre hay barn that never sees anything heavier than a telehandler at the door needs less floor than a fifteen metre workshop with a tractor on axle stands in it.
The thing that catches people out is how the load arrives rather than what it weighs. A tractor on its tyres is spread across a reasonable area of concrete. The same tractor on two stands puts the whole of that weight through a few square centimetres, and so does a press bolted down in the corner or a jack that always lives in the same spot. Those are the loads that crack floors that were thick enough on paper.
The calculator sets a thickness from the building type and then lets you override it, because the preset is a sensible starting point and your groundworker's opinion about your ground beats a preset every time.
Ordering the concrete
Net volume is length by width by thickness, and nobody orders net volume. The tool adds 5% for spillage, over-dig in the formation and the simple fact that a slab is never exactly as deep as it says it is, then rounds up to the 0.5 m³ increment the batching plant actually delivers in.
Order short and you have a cold joint across your floor where the second lorry met the first. That is the one mistake on this page you cannot fix afterwards.
Mesh, fibre or neither
Mesh comes in 2.4 × 4.8 metre sheets and has to be lapped, so the number of sheets is always more than the floor area divided by the sheet area. The tool allows 300 mm on the laps plus a cutting allowance, which is why the sheet count comes out higher than the arithmetic you would do in your head.
Fibre goes in at the plant instead — 30 kg per cubic metre for steel fibre, around 5 kg for macro polypropylene. It saves fixing mesh on site and it works through the whole depth of the pour rather than at one plane. It is a specification decision, not a swap you make because the mesh has not turned up.
The membrane is sized at slab area plus 12% for laps and the upstand at the edges. It is a cheap item that is miserable to retrofit.
Sub-base, and why the dig is bigger than the building
Type 1 has to be spread loose and compacted down, so the loose depth is always more than the finished depth. At the compaction factor this tool uses, a 188 mm loose spread finishes at 150 mm compacted. Order by the finished depth and you will be a lorry short.
It is bought by weight and laid by volume, at around 2.1 tonnes to the cubic metre compacted, which is the conversion that turns a depth into something a quarry can put on a ticket.
The working platform is wider than the slab — the tool allows 1.5 m beyond the slab edge on each side. That is not generosity, it is where the machines stand and where the formwork goes, and forgetting it is how a dig ends up short on the last corner.
Muck-away: the number that surprises people
Ground that sat tight in a field breaks up when you dig it and takes up more room loose than it did in the ground. Topsoil bulks by roughly a quarter. Clay bulks more. Broken rock can come out at half as much again as the hole it came from.
×1.5
Broken rock bulks to around 1.5 times its in-situ volume — clay around 1.3, topsoil around 1.25. The loose figure is the one that fills lorries, so it is the one that costs money.
At 12 cubic metres of loose material to an eight-wheel tipper, the arithmetic runs away quickly on a sloping site. This is routinely the largest single line in a groundworks bill, and it is almost never the line anyone asks about first.
Which is why the finished level question matters more than it looks. Cutting into the slope gives you a level platform and a mountain of spoil to pay to remove. Building the level up needs imported fill, which is a different bill. A balanced cut and fill moves the least material and keeps most of it on the plot, and on a farm there is usually somewhere a heap of subsoil can usefully go. Tick the spoil-off-site box in the calculator and untick it, and watch what happens to the number.
Ground conditions in South Wales
This is where a national calculator and a national contractor both start guessing, and it is the reason this site exists rather than a spreadsheet.
South Wales is not flat. Valley-side and hill farms have fields that look level from the gate and carry several hundred millimetres of fall across the footprint of a building, and fall is the single biggest driver of earthworks cost — it decides how much you cut, how much you cart, and whether you need a retaining detail at the high side. Measure it before you price anything. Corner to corner with a laser level takes ten minutes and changes the whole budget.
Underneath, the first answer is often clay, much of it left by the glaciers, which holds water, moves with the seasons and bulks badly when you dig it. Below that the rock changes across the region: sandstones and mudstones through the coalfield, limestone around the Gower and the Vale of Glamorgan, and older sandstones, shales and mudstones further west and north. Broken rock of any kind comes out bulky, so the muck-away figure climbs even when the dig itself was straightforward.
Then there is the coal. Much of South Wales sits on the coalfield, and old shallow workings and forgotten mine entries are a foundation question before they are a planning one. The Mining Remediation Authority (formerly the Coal Authority) splits the coalfield into a Development High Risk Area and a Development Low Risk Area. In the High Risk Area most planning applications need a Coal Mining Risk Assessment, and the pads may need a ground investigation first. Check your site on the authority's map before anyone designs anything — its guidance explains how.
And the water. Welsh rainfall makes drainage a design item rather than an afterthought, and in Wales it is a legal one too: construction of 100 m² or more needs its surface water drainage approved by the SuDS Approval Body before work starts. Where the roof water goes, whether the yard falls away from the building or into it, and what happens at the door threshold in February all belong in the design.
The honest summary is that the only way to know what is under your field is to dig a trial hole in it, and the only way to know the fall is to measure it. The calculator will take both once you have them.
The order it all happens in
Strip the topsoil
Off the whole working area, not just the building footprint. It compresses unevenly under load and it will keep doing so for years. Measure the depth in a trial hole first.
Cut and fill to a platform
Level, and to the finished level you have chosen rather than the one the slope suggests. This is where the muck-away is decided.
Pads, to the structural drawing
Dug, poured, and with the holding-down bolts cast in to the erector's setting-out. Not before you have the design, and not to a size somebody worked out over the phone.
Frame up
Standing on those pads. Erection plant working over a finished slab is how a new floor gets damaged before it has been used once.
Sub-base, compacted in layers
In layers, not in one lift. A single deep spread does not compact all the way down whatever the roller looks like it is doing.
Membrane, edge detail, then pour
Slab last, inside a building that is already up and out of the weather. Drainage and the apron outside the doors get designed at the same time, not after the first wet winter.
What this tool will not tell you
Pad foundation sizes. They come off a structural design, from the span, the eaves height, the wind loading on your site and what the ground will bear. This tool has none of those inputs and is not going to pretend otherwise.
Prices. Deliberately. Ready-mix, aggregate and haulage are all priced by distance and all move, and a calculator quoting you a rate from last year would be confidently wrong. Take the quantities to a local supplier and a local groundworker and you will get a real number.
Drainage, services and the yard outside. Real costs on a real job, and all site-specific enough that a formula would mislead you.
Whether your ground will take it. Only a trial hole, and somebody standing in it, will tell you that.
Every assumption behind the figures — the waste allowances, the bulking factors, the compaction, the lorry capacity, and the maths behind the cut and fill — is published in full with its range.
Common questions
- Is the groundworks included in a steel building quote?
- Almost never, and this is the single most expensive misunderstanding in the whole process. A steel frame quote covers the frame: the stanchions, rafters, purlins, sheeting, bracing, fixings and usually the erection. The base it stands on is a separate trade and a separate contract. The one point where the two meet is the pad foundations — the frame supplier gives you the pad sizes from the structural design, and somebody else digs and pours them. So a quote that looks complete is genuinely complete for what it covers, and you still have a bill to come for levelling the site, carting the muck away, the sub-base, the slab and the drainage.
- How much does a concrete base for a steel building cost?
- It is the wrong question to ask first, because the honest answer is that nobody can tell you without the quantities. The same size building on a flat, free-draining paddock and on a sloping field with two feet of topsoil over clay are not remotely the same job, and the difference is mostly earthworks and haulage rather than concrete. Work out the quantities first — cubic metres of concrete, tonnes of Type 1, loads of muck off site — then price those against local rates, because ready-mix and aggregate are priced by distance from the plant and a quarry twenty miles further away changes the number. That is why this calculator gives quantities and not prices.
- How thick should the slab under a steel building be?
- There is a genuine range rather than one answer, and it is set by what drives on the floor rather than by the size of the frame. Storage buildings that only ever see a telehandler at the door sit at the lighter end. Anything tractors and trailers turn on regularly sits nearer 200mm with mesh. Workshops straddle the two, because they take point loads from jacks, stands and bolted-down machinery that a spread vehicle load never applies. Decide it against the heaviest thing that will actually be on the floor and how the load arrives, then have your groundworker confirm it against what the ground will bear.
- Why does the calculator not work out the pad foundations?
- Because pads cannot be worked out from the size of a building. The concrete pad under each stanchion is sized by a structural engineer from the span, the eaves height, the wind loading on your particular site and the bearing capacity of the ground it sits in. Two identical buildings a mile apart can need different pads if one is on rock and the other is on soft ground, or if one sits exposed on a hilltop. Any tool or supplier offering you a pad size without a design has guessed it, and guessing that particular number is not a corner worth cutting.
- How much muck comes off a building site?
- More than the hole looks, because dug ground bulks. Material that sat tight in the ground breaks up as it comes out and takes up noticeably more room loose than it did in situ — around a quarter more for topsoil, more again for clay, and half as much again for broken rock. That loose figure is what fills a lorry, so it is the figure that decides how many loads you pay for. It is also why the muck-away can quietly become the biggest single line in a groundworks bill on a sloping site, and why spreading spoil on the holding rather than carting it off is worth serious thought before the machines arrive.
- Do I need to take the topsoil off before building?
- Yes. Topsoil holds organic matter and water, it compresses unevenly under load and it will keep doing so for years after the building is up. It has to come off the whole working area before any sub-base goes down, which is one of the reasons the dig is always bigger than the footprint of the building. How much comes off is worth measuring rather than assuming — dig a trial hole and put a tape in it. On some fields it is a few inches and on others it is two feet, and that difference moves more material than a moderate slope does.
- What is the difference between mesh and fibre reinforcement?
- Mesh is steel fabric laid in sheets and lapped, positioned at a set depth in the slab so it carries tension where the slab needs it. Fibre is mixed through the concrete at the batching plant and works throughout the pour rather than at one plane. Mesh remains the conventional choice for agricultural and industrial floors and is what most structural specifications call for. Fibre can save a trade on site and suits some pours well, but it is a specification decision rather than a like-for-like swap, so it belongs with whoever is designing the floor rather than with whoever is ordering the concrete.
- What order does the groundworks happen in?
- Strip the topsoil off the working area, then cut and fill to get a level platform. Set out and dig the pad foundations from the structural drawing and pour them, leaving the holding-down bolts cast in to the erector’s setting-out. The frame usually goes up next, standing on those pads. Then the sub-base goes in and is compacted in layers, the damp proof membrane and any edge detail go down, and the slab is poured last, inside a building that is already standing. Pouring the slab before the frame goes up looks efficient and is how slabs get damaged by erection plant.
Other buildings we cover
Same approach on each: what the building is, how it gets sized, where you stand on planning, and what the base underneath it comes to.
Machinery stores
Clear span, no posts in the way, and a floor that works harder than most. What it costs to put up and what it costs underneath.
Grain stores
Walls that get pushed from the inside, a floor that takes a loader, and a roof that rains on the crop if you get it wrong.
Livestock buildings
Housing that has to ventilate, drain and be scraped out. Falls, channels and the slurry rules — plus the 400 metre planning trap.
Farm workshops
Room to get a machine in and work all round it, with a floor specced for the jack rather than the tractor.
Hay barns
Open sides, air through it, and the lightest slab on the farm — if it needs one at all.
General purpose buildings
The shed that ends up doing everything. How to spec a building for a use you have not decided yet.
Stables and arenas
Stable blocks, American barns and indoor arenas. The same steel frame as a farm building — and a completely different planning position.
Garages and workshops
Steel framed garages, home workshops and man caves at the house. The eaves limit that decides whether you need planning permission, and the floor underneath.
Industrial units
Workshops, warehousing, trade counters and light manufacturing. Full planning, full Building Regs, and a yard that often costs more than the shed.
Get the frame priced, with the pad sizes
Rough span and length is enough to start. You get engineered drawings and a price for the frame — including the pad foundation sizes this tool deliberately will not guess at. If you have run the numbers above, they go with your enquiry.