Hay barns and straw storage
The simplest base on the farm, and the one building where the honest answer might be that you do not need a slab at all. Which makes the ground underneath it the thing worth getting right.
- Typical slab
- 150 mm
- Sub-base
- 150 mm
- Concrete, 12 × 18 m
- 35 m³
- Topsoil to shift
- 81 m³
The lightest on the list. Some barns get away without one.
Compacted Type 1, spread around 190 mm loose.
Around 10 m³ less than the same shed built for machinery.
More than twice the concrete volume, and the figure most people guess.
Bales are an easy load
A stack of bales is heavy, but it is heavy in the way a slab likes: spread out, sitting still, in the same place all winter. Nothing turns on it, nothing jacks up on it, nothing drops a loaded bucket on it. That is why 150mm with mesh does a hay barn when a machinery store wants 200mm.
Plenty of hay barns run on compacted stone with no slab at all, and there is nothing wrong with that if the building will only ever hold fodder. What the slab buys is a clean, dry, sweepable floor, the bottom row of bales kept off the ground, and the option of using the building for something else in ten years.
The sub-base is not the variable
This is the thing worth knowing before you decide. Whether or not you pour a slab, you still strip the topsoil, still level the platform, and still lay and compact the stone. The groundworks happen either way.
81 m³
Topsoil to strip off the working platform for a 12 × 18 m barn at 250 mm depth — more than twice the volume of the concrete. Whether you lay a slab or not, that muck still has to go somewhere.
So when you price the slab against no slab, you are pricing 35 cubic metres of concrete and the mesh, not the whole job. The earthworks and the stone are already spent. That usually makes the slab a smaller decision than people expect.
Spec it for what it will end up being
Farm buildings change use. A hay barn becomes somewhere to park a trailer, then somewhere to keep the sprayer out of the frost, then somewhere the telehandler lives. Nobody plans it and it happens on most farms.
The difference between a 150mm hay barn floor and a 200mm machinery floor on the same building is about ten cubic metres of concrete. Relaying a floor that turned out to be too light, with a building already up over the top of it, costs an order of magnitude more than that. If there is any real chance the building ends up holding machinery, spec it now.
Dry fodder and fire
More hay is spoiled by damp coming up from the ground or rain blowing in through an open side than by anything to do with slab thickness. Ground levels around the building, where the water goes when it runs off the roof, and which way the open side faces are worth more attention than the concrete specification.
Fire is the other one. Stored hay and straw carry a genuine fire risk, and it is a reason to think about where the building sits relative to everything else on the yard rather than putting it in the most convenient gap. Worth a conversation with your insurer and with whoever is designing the layout, at the point where moving it still costs nothing.
Planning
On a working holding a hay barn is usually agricultural development rather than a full planning application. Part 6 of the General Permitted Development Order gives rights on agricultural units of five hectares or more under Class A, and narrower rights on units between 0.4 and five hectares under Class B. The building has to be reasonably necessary for agriculture on that unit.
Rather than a planning application you apply to the council for a determination on whether prior approval is needed for the siting, design and external appearance, and they have 28 days to respond. There is a floor area limit that has moved twice — it was 465 square metres until 2018, rose to 1,000, and was amended again in May 2024, so check the current position rather than an older guide.
Rights are restricted on protected land, which covers a substantial part of Cornwall, and scheduled monuments were added to the exclusions in 2024.
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. Talk to Cornwall Council about your own site.
Work out the base for your hay barn
Set to a hay barn specification. To compare it against a machinery floor, change the slab thickness to 200mm and watch the concrete figure.
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 are sent with it. We are not a steel building manufacturer — we pass your details to a steel building supplier so they can price the job and contact you. How we handle your details.
Common questions
- Does a hay barn need a concrete floor at all?
- Not always. Bales are a distributed load and they are not driven over the way a machinery store floor is, so plenty of hay barns run on compacted stone rather than a slab. What a slab buys you is a clean, dry, sweepable floor that keeps the bottom row of bales off the ground and lets you use the building for something else later. If you are undecided, price both — the sub-base is needed either way, so the slab is the only variable.
- How thick should a hay barn slab be?
- Where a slab is laid, 150mm with mesh is the usual specification and it is the lightest of the farm building types. Stored bales sit still and spread their weight, so the floor is not doing the work a machinery store floor does. If there is any chance the building will end up storing machinery instead — and there usually is — spec it as a machinery store now rather than relaying it later.
- What matters more than the floor in a hay barn?
- Keeping the fodder dry and thinking about fire. Damp from below or wind-driven rain through an open side will spoil more hay than a thin slab ever will, so falls, ground levels around the building and which way the open side faces are worth more attention than concrete thickness. Fire is the other consideration — stored hay and straw carry a real fire risk, which is a reason to think carefully about how close the building sits to other buildings on the yard.
- Do I need planning permission for a hay barn?
- On a qualifying agricultural holding it is usually permitted development rather than a full application. Part 6 of the GPDO gives rights on units of five hectares or more under Class A and narrower rights on units of 0.4 to under five hectares under Class B, provided the building is reasonably necessary for agriculture on that unit. You apply to the council for a prior approval determination on siting, design and external appearance, which they have 28 days to decide. Rights are restricted on protected land, which covers a substantial part of Cornwall.