Garden rooms, hot tubs and garden offices have become one of the most common reasons people ring us. For most of them, it is the first time they have ever ordered concrete.
That is completely normal, and the questions are always the same three. What mix do I ask for? How thick should the slab be? How much do I actually need?
This guide answers all three, for each type of base, in plain numbers you can use. It also explains why a hot tub base needs more thought than a shed base, and how concrete delivery in Cambridgeshire works when your job is far too small for a full lorry load.
Start with the weight, not the shed
Every decision about a base comes back to one question. How much weight will sit on it, and how is that weight spread out?
A garden shed with a lawnmower and some tools in it is light. A garden office with a person, a desk and a couple of bookcases is heavier. A hot tub full of water and people is in a different league altogether, and we will come to why in a moment.
Get the weight right in your head first. The grade, the thickness and the reinforcement all follow from it.
One quick note before you dig. Most garden rooms and hot tub enclosures fall under permitted development, but there are limits on height and on how much of your garden you can build on. The government’s Planning Portal sets out the rules for outbuildings clearly, and it is worth five minutes of reading before you order anything.
The concrete grade to ask for
Concrete grades are just a measure of strength. The higher the number, the more load the concrete can take before it cracks.
You do not need to understand the chemistry. You just need to ask for the right one.
GEN 1 is fine for a standard garden shed base. Light timber shed, garden tools, a bike or two. Nothing heavy, nothing permanent.
GEN 3 is the sensible choice for most garden rooms and garden offices. It is rated C20/25, so it is noticeably stronger than GEN 1, and it stands up much better to frost and to the freeze-thaw cycle we get every winter. That matters because a garden base sits outside in all weather with nothing on top of it but a timber building. Our guide on Gen 3 concrete for garden projects goes into where it does and does not make sense.
C25 is the step up, and it is where we would push you for anything heavy or anything you cannot easily put right later. Heavier structures, bases that will take a vehicle, or anything holding a large volume of water.
People often worry about paying for a stronger mix than they need. Be honest about the difference. On a base of two or three cubic metres, going up a grade costs very little. Digging out a cracked slab and starting again costs a great deal.
Slab thickness for each type of base
Thickness does two jobs. It gives the concrete enough depth to resist cracking, and it spreads the load out over a wider patch of ground underneath.
Here is what to aim for.
| Base type | Grade to ask for | Slab thickness | Compacted sub-base | Mesh reinforcement |
|---|---|---|---|---|
| Small timber shed | GEN 1 | 100mm | 100mm | Not usually needed |
| Large shed or workshop | GEN 3 | 100mm | 100mm to 150mm | Worth adding |
| Garden room or garden office | GEN 3 | 100mm to 150mm | 150mm | Yes, A142 mesh |
| Hot tub base | C25 | 150mm | 150mm | Yes, A142 or A193 mesh |
| Base taking a vehicle | C25 | 150mm | 150mm | Yes, A193 mesh |
A few things worth pulling out of that table.
The sub base is not optional. A sub base is the layer of compacted crushed stone, usually MOT Type 1, that sits under the concrete. It spreads the load into the ground and stops the slab from settling unevenly. A 150mm slab on soft, uncompacted soil is weaker in practice than a 100mm slab on a properly compacted sub-base. Dig out the topsoil, lay the stone, and compact it properly with a plate compactor in layers of no more than 75mm at a time.
Mesh does not make concrete stronger. That surprises people. What mesh does is hold the slab together if it does crack, so a hairline crack stays a hairline crack instead of opening up and letting one half of the slab drop. On a garden room or hot tub base, that is exactly the insurance you want. Sit the mesh on plastic spacers about 40mm to 50mm off the bottom of the slab, roughly in the middle, not lying on the membrane.
Put a damp proof membrane under everything. A damp proof membrane, usually called a DPM, is a heavy polythene sheet that stops ground moisture rising up through the concrete. Use 1200 gauge polythene, lay it over a thin blinding layer of sand so the stone does not puncture it, tape any joins, and lap it up the sides of your formwork. Without it, the inside of your garden office will feel damp, and your flooring will suffer. It is one of the cheapest parts of the job and one of the most important.
Why a hot tub base needs more thought
This is the part people underestimate, so it is worth doing the sums properly.
Water weighs one tonne per cubic metre. A typical six-person hot tub holds somewhere around 1,500 litres, which is 1.5 tonnes of water on its own. Add the tub itself at roughly 300kg, then add six people at an average of 75kg each, and you are at about 2.25 tonnes.
Now look at the footprint. A six-person tub is often around 2.2 metres square, so about 4.8 square metres of ground.
That works out at roughly 470kg pressing down on every square metre of your base. For comparison, a garden office of the same size might put around 150kg on each square metre. The hot tub is doing three times the work.
Two more things make a hot tub base different from a shed base.
The load never goes away. A hot tub stays full. That weight sits on the same patch of concrete, day and night, for years. Concrete under constant load behaves differently from concrete that gets loaded now and then.
It has to stay dead level. A hot tub that sits on a slab with even a slight slope will put uneven pressure on its shell, and the water line will look wrong every time you use it. Most manufacturers ask for a level base and will say so in their instructions, so read them before you set your formwork. Check with a long spirit level across the base in several directions, not just one.
So for a hot tub, go with 150mm of C25 on a properly compacted 150mm sub-base, with mesh in the middle. Our page on C25 concrete mix sets out the strength figures if you want the technical detail. Also give some thought to where the tub’s water will drain when you empty it, and leave a route for the electrical supply before you pour rather than after.
Working out how much concrete you need
The maths is simple, and you only need one sum.
Length in metres × width in metres × thickness in metres = volume in cubic metres.
The only trap is the thickness. It has to be in metres too, so 100mm becomes 0.1 and 150mm becomes 0.15.
Here are four worked examples using real base sizes.
| Project | Slab size | Thickness | Volume | Order with 10% added |
|---|---|---|---|---|
| Shed base | 3m × 2m | 100mm (0.1) | 0.6 m³ | 0.7 m³ |
| Garden room | 4m × 3m | 125mm (0.125) | 1.5 m³ | 1.7 m³ |
| Hot tub base | 3m × 3m | 150mm (0.15) | 1.35 m³ | 1.5 m³ |
| Garden office | 5m × 4m | 150mm (0.15) | 3.0 m³ | 3.3 m³ |
Always add 5% to 10% on top of your figure. Excavations are never perfectly flat, subbases settle slightly in places, and a little gets lost during the pour. Running out with a corner still to fill is a genuinely bad afternoon, because the joint between yesterday’s concrete and today’s will always be a weak point.
If your base is an odd shape, split it into rectangles, work each one out separately and add them together. For circles, use π × radius² × thickness. There is a full walkthrough with more examples in our guide on how to calculate exactly how much concrete your project needs.
One last measuring tip. Measure the hole you have dug, not the building you are putting on it. Your base usually needs to be slightly larger than the garden room itself, often by 50mm to 100mm all round, so check the manufacturer’s instructions before you set your shuttering.
Small load delivery, and why it matters here
Look back at that table. Almost every garden base comes in under 3.5 cubic metres.
A standard ready-mixed lorry carries around 6 cubic metres. So if you order from a traditional supplier, you have two unappealing options. Pay for a full load and pour the rest away, or pay a part load charge on top of a mix that was batched at the plant an hour ago and has been going off ever since.
Volumetric delivery works differently. The lorry carries the sand, stone, cement and water separately and mixes them on your driveway as the concrete comes out of the chute. That means three things for a job like yours.
You pay for what you use. If your 1.5 cubic metre order turns out to need 1.7 because the sub-base settled, we mix another 0.2. If you only use 1.3, you are not charged for concrete you did not take.
The mix is genuinely fresh. Nothing is batched until it reaches you, so the clock only starts when the concrete does. On a small base where one person is barrowing round the side of a house, that extra working time is worth a lot.
You can change the mix on the day. If you decide halfway through that the last section wants to be a bit stiffer, we can adjust it. A pre-batched load is whatever it was when it left the plant.
Our page on the benefits of on-site concrete delivery covers the full picture, including staged deliveries if you are doing a base and a path at different times.
What to have ready before the mixer arrives
Delivery day goes well when the preparation is done. Work through this list a day or two before, not on the morning.
- Excavation finished and squared off. Topsoil out, sides straight, and the depth checked in several places rather than one.
- Sub base laid and compacted. MOT Type 1, put down in layers of no more than 75mm and compacted with a plate compactor. Walking on it should not leave a footprint.
- Formwork fixed and level. Timber shuttering, pegged securely, checked with a spirit level, and braced well enough to hold wet concrete without bowing outwards.
- Damp-proof membrane down. Sand blinding first, then 1200 gauge polythene, joins taped and edges lapped up the formwork.
- Mesh in place on spacers. Sitting 40mm to 50mm up, not resting on the membrane, with sheets overlapped by at least one full square.
- Access checked. Roughly 3.5 metres of width and 4.5 metres of headroom for the lorry, and a firm, well drained spot for it to stand.
- Tools out and ready. Rake, shovel, screed board, float, wheelbarrow and wellingtons. Have them by the base, not in the shed you are about to demolish.
- Enough people lined up. Even a 1.5 cubic metre pour is hard going on your own. Two or three pairs of hands makes it straightforward.
- A wash-out point agreed. A lined pit or an agreed corner, sorted in advance.
- The weather checked. Very hot weather speeds up setting and gives you less working time. Frost is a problem at the other end. Aim for a mild, dry day if you can.
Talk it through before you order
If you are not sure whether your base needs GEN 3 or C25, or whether 100mm or 150mm is right, just ring and describe the job. Tell us the size of the base, what is going on top of it, and what the ground is like. It takes two minutes, and it is the cheapest insurance there is.
We mix on site with volumetric mixers, so you get exactly the mix you need and exactly the amount you use, with no waste and no part load penalty on a small garden job.
Call Cardinalis Concrete on 01954 288027 or email enquiries@cardinalis.co.uk. We deliver across Cambridgeshire, Suffolk, Hertfordshire, Essex and Norfolk.
