Technical Installation · 7 min read
How to Prepare a Shed Base: Options, Steps & Mistakes
Concrete, slabs, timber bearers or plastic grids? A shed maker's straight guide to a firm, level, well-drained base — and the mistakes that rot floors.
Groundwork first
How to Prepare a Shed Base
Every building we've made since 1995 has lived or died by what's underneath it. Here's how to get the base right — concrete, slabs, timber bearers or grids.
Ask our fitters what makes a shed last and they won't start with the timber — they'll start with the ground. A garden building needs a base that is firm, level and well-drained, sitting slightly proud of the surrounding ground so rainwater runs away rather than pooling underneath. Everything else in this guide is detail.
Get those three right and the doors swing true and the floor stays dry for decades. Get them wrong and no joinery will save it — an un-level base is the most common cause of doors catching our installation teams see. For the short version, see our quick shed base guide; this is the full one.
The four bases that work
Concrete slab
The most permanent option. 75–100mm of concrete over 75mm of compacted hardcore carries any building we make — the right call for large buildings and workshops.
Paving slabs
Quicker and better value than a pour. Slabs bedded on a sharp sand and cement mix suit most sheds — the one rule is dead flat, with not a single slab rocking.
Pressure-treated timber base kit
A bearer frame we make in our own workshop, tanalised like the buildings. Lifts the floor for airflow, quick to fit, and works over slabs or on firm, level ground.
Plastic grid system
Light, permeable and easy to handle — but not a shortcut. Grids need a levelled sub-base with a weed membrane beneath. Do that groundwork and they're a tidy solution.
Preparing a concrete slab base
The gold standard for big or heavy buildings and anything you'll use as a workshop. Set aside a weekend and rope in a second pair of hands.
- 1
Mark out and square up
Peg out the footprint with string lines, matching the base to the building rather than swamping it. Check square by measuring both diagonals — equal diagonals, square base.
Tip: A base much larger than the building collects rain-splash and throws it back at the cladding.
- 2
Dig out and lay hardcore
Strip the topsoil and any soft ground, then lay 75mm of hardcore and compact it hard — this layer is what stops the slab settling.
- 3
Build the formwork and mix
Set timber formwork to the finished height — slightly proud of the surrounding ground so water drains away. Level the formwork carefully: the concrete can only be as level as the frame holding it.
Careful: A slab that finishes below ground level becomes a pond. Proud of the ground, always.
- 4
Pour, tamp and level
Pour to 75–100mm, working the mix into the corners, then tamp flat with a straight board sawing across the formwork. Check with a spirit level on a long straight edge — in several directions, not just one.
Tip: Mix and pour in one session so the slab cures as a single piece.
- 5
Let it cure
Give the slab at least 48 hours before it takes any weight — longer in cold or wet weather. Concrete that looks dry on top is often still green underneath.
Careful: Don't book delivery for the day after the pour. A building on green concrete can shift as the slab hardens.
The timber base kit — the fast route, done properly
Not every garden needs a pour. Our pressure-treated timber base kit is a bearer frame made in the same West Midlands workshop as the buildings, from the same FSC-certified, tanalised (Use Class 3) timber — so the part nearest the ground is treated for exactly that job.
The kit lifts the floor clear of the earth, and that gap matters: air moves under the building, so the floor stays dry from below instead of wicking up damp. It's quick to fit with basic tools and works two ways — over existing slabs, or straight onto firm, reasonably level ground. For small and mid-sized sheds on decent ground, it's the route we point most people to.
You can add a base kit to any building when you order from the shop or spec one alongside your building in the Build Studio — it travels on the same wagon as your shed.
Read your base before the building arrives
Signs of a good base
- Sits slightly proud of the ground, so water runs away
- A spirit level on a straight edge reads level in every direction
- Both diagonals measure the same — the base is square
- Matches the building's footprint, not a foot bigger all round
- Solid underfoot: no movement, rocking or soft spots
- Room around it to walk, fit and maintain
A base that will bite you
- Slabs that rock when you stand on the corners
- A fall to one side — the doors will catch within weeks
- A surface below ground level that ponds after rain
- Laid straight onto grass or soil, wicking damp into the floor
- Concrete poured yesterday, carrying a building today
- Hard against a fence with no room to reach the back wall
Get base-ready before delivery day
Our installation teams build your shed on its base the day it arrives — which means the base must be finished, level and fully cured before the wagon turns up. They can't level ground or wait for concrete to cure on the day. Everything about the day itself is on our delivery page.
While you're setting out, leave working room around all four sides — enough to walk round with a paintbrush. It makes fitting easier and keeps the annual maintenance possible without dismantling a fence. Not sure your site is ready? Send us a photo — we'd rather check it a week early than on the day.
Ready-for-delivery checklist
Run through this the week before delivery.
- Base is firm, level and slightly proud of the surrounding ground
- Spirit level checked on a straight edge, in both directions
- Diagonals measured and equal — the base is square
- Base matches the building's footprint
- Concrete cured for at least 48 hours (longer in cold weather)
- No rocking slabs, soft spots or standing water
- Clear access from the road to the base for carrying sections
- Working room left around all four sides
Shed base questions, answered straight
Do I need a concrete base for a shed?
Not always. Concrete is right for large or heavy buildings and workshops, but paving slabs on a sharp sand and cement mix suit most sheds, and a pressure-treated timber base kit is quicker still on level ground. Every option must be firm, level, well-drained and slightly proud of the ground.
Can I put a shed on grass?
No — never straight onto grass or soil. Damp wicks up into the floor, the building settles unevenly, the doors rack and the floor rots from underneath. The quickest proper route is a pressure-treated timber base kit on firm, levelled ground — it lifts the building clear so nothing of it touches earth.
How level is level enough?
Lay a long straight edge across the base with a spirit level on top and check several directions, not just one. The bubble should sit centred every time. Even a fall you can't see by eye will twist the frame — it's the number one cause of doors catching.
Does the base need to be bigger than the shed?
No — match the building's footprint. A slab much larger than the shed collects rain and splashes it back at the cladding, keeping the bottom boards wet. Size the base to the building and let the ground around it drain.
Can you deliver before my base is finished?
The base must be ready before delivery day. Our teams build on it as soon as the sections come off the wagon and can't fix groundwork on the day. If your base has hit a delay, tell us early and we'll rearrange the slot.
Do paving slabs need cement, or just sand?
Bed them on a sharp sand and cement mix, not loose sand. Loose sand washes out and lets slabs rock — and a rocking slab under one corner shows in the doors within weeks. A semi-dry sand and cement bed holds each slab flat for good.
Base sorted? Design the building
Size, style, doors and windows — priced live as you go. Made to order in our West Midlands workshop, delivered free to the UK mainland.
Open the Build StudioReady to put it into practice? Every building is made in our West Midlands workshop with free mainland delivery.
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