Concrete Base for a Garage, Shed or Extension in North Devon: How a Proper Base Is Built
A concrete base looks like the simplest job on any building project. Flat, grey, finished in a day. Yet it is the part that decides whether a garage door still closes in ten years, whether a shed floor stays dry, and whether an extension cracks along the wall. This guide explains how a concrete base for a garage, shed or extension is designed and built in North Devon, what depths and materials are typical, and where the common mistakes happen.
Crabb Contractors Ltd has prepared ground and poured bases around Ilfracombe since 1962. We know what slate, shillet and clay under a steep garden do to a slab, and we build for it.
Quick Answer: What Does a Good Concrete Base Need?
Short answer: A good concrete base sits on firm, level, compacted ground, with a compacted layer of hardcore, a damp proof membrane, the right thickness of concrete for the load, reinforcement where needed, and proper falls and drainage around it. Thickness depends on use: a light shed base is thinner than a garage slab that carries a car, and a foundation for a habitable extension is designed separately under Building Regulations.
Typical build up by use
Use | Typical concrete thickness | Hardcore layer | Reinforcement |
Garden shed or summerhouse | About 100mm | About 100mm compacted | Often not needed |
Greenhouse or workshop | 100 to 125mm | 100mm compacted | Mesh recommended |
Single garage for cars | 125 to 150mm | 100 to 150mm compacted | Mesh recommended |
Driveway or hardstanding | 125 to 150mm | 100 to 150mm compacted | Mesh recommended |
Extension, conservatory or habitable room | Designed by engineer or building control | As specified | As specified |
These are general guides only. The ground, the loads and the structure all affect the final design, which is why a site visit matters.
Base, Slab or Foundation: What Is the Difference?
People use these words interchangeably, but they mean different things on site.
A base is a flat concrete platform for something light and freestanding, such as a shed or greenhouse.
A slab is a thicker, usually reinforced floor for a garage or workshop, sometimes with thickened edges.
A foundation carries the walls of a building. Strip foundations, trench fill and raft foundations are designed for structures that must stay stable for decades, and they fall under Building Regulations.
If you are building a habitable extension, the foundation is not something to guess. It needs depth, width and reinforcement chosen for the ground, the loads and nearby trees, and it will be inspected by building control. Our groundwork contractors prepare sites for foundations, and work to the engineer's or designer's drawings.
Planning Permission and Building Regulations
Check the rules before you order concrete.
Sheds and outbuildings.
Many can be built under permitted development, with limits on height, footprint and distance from boundaries. A building close to a boundary has tighter height limits, and rules differ in conservation areas and for listed buildings.
Garages.
A detached single storey garage with a small floor area is often exempt from Building Regulations if it is built mainly from non combustible material or sits a set distance from the boundary. Bigger or attached garages are treated differently.
Extensions and conservatories.
Almost always need building control approval for the foundations, and may need planning permission depending on size and position.
Always confirm the current rules with North Devon Council before committing. If your plan involves a new driveway or vehicle access to the garage, read our guide to driveway planning permission and dropped kerbs in Devon.
Step 1: Site Survey and Setting Out
Every good base starts with measuring, not digging.
We check the footprint, the finished floor level, the position of boundaries, and what is nearby: drains, pipes, trees, walls and slopes. Pegs and string lines mark the corners, and levels are taken so the base sits at the right height relative to the house, the door thresholds and the ground outside.
This is also when we look for services. A hidden pipe or cable under the plot should be found before the digger arrives, not after.
Step 2: Excavation
The ground is stripped of topsoil, turf and roots to reach firm subsoil, and then dug to the depth needed for the full build up: hardcore, any blinding, membrane and concrete.
In Ilfracombe the depth to firm ground varies a lot. Some plots are shallow over rock, others sit on made up ground or soft clay pockets. Soft or organic soil must be removed, never built on. Our guide to excavation for driveways and groundwork describes what actually happens during this stage, and why decisions made here shape the finished result.
Tools and machines. For a garden shed base, hand digging may be enough. For a garage, an extension or a hardstanding, a mini digger is far faster and more accurate. We can supply an operated digger for a day or longer, and our article on plant hire with operator in Devon explains when an operator is worth having.
Step 3: Muckaway and Spoil
Digging out a base creates a surprising amount of waste soil. A single garage footprint produces several tipper loads. If it cannot be spread or reused on site, it must be removed by a licensed waste carrier. Plan for this early. Our guide to muckaway services in North Devon explains how it works, and our piece on tippers, plant and lorries covers the access questions, such as whether a lorry can actually reach your back garden.
Step 4: Compacting the Ground
Once excavated, the exposed ground is levelled and compacted with a plate compactor or roller. If it is soft, wet or inconsistent, extra work is needed: removing and replacing weak areas, adding a geotextile membrane, or using a deeper layer of stone.
Skipping this stage is the most common cause of a cracked slab. Concrete is strong in compression but not in bending, and if ground beneath moves, the slab bends and cracks.
Step 5: Hardcore Sub Base
A compacted layer of clean, graded crushed stone is laid in layers and compacted in stages. This layer spreads loads, stops the concrete from sitting on wet ground, and provides a level working surface.
Common mistakes
Using a thick layer in one go, which cannot be compacted properly
Using mixed rubble with soft material in it, such as old plaster or soil
Failing to compact the edges
Leaving the stone surface uneven, so the concrete is thicker in some places than others
A thin layer of sand or fine material, called blinding, is often added on top to protect the membrane from sharp edges.
Step 6: Damp Proof Membrane
For any floor that will be used as a room, workshop or storage area, a damp proof membrane is laid over the blinding. This is a heavy duty polythene sheet with overlapped, taped joints, lapped up the edges. It stops moisture rising through the concrete.
For sheds and storage buildings, damp rising from the floor ruins tools, stored boxes and timber frames. In North Devon's damp climate, a membrane is a sensible detail even where Building Regulations do not require it.
Step 7: Formwork and Edges
Timber or steel shuttering is set around the perimeter, held firmly with pegs and braces, and set exactly to the finished level. The shuttering controls the shape of the slab, the thickness at the edges, and the finished height.
For garages, the slab often needs a slight fall towards the door, usually small, so water that comes in on car tyres runs out rather than pooling. For a hardstanding, falls take water away from buildings.
Step 8: Reinforcement
Steel mesh is placed inside the slab on spacers, so it sits in the middle or lower third of the thickness, not on the ground. Mesh controls cracking and holds the slab together if cracks do form. It is not a magic fix for poor ground, and it has to be in the right place to do anything.
Garage slabs, workshop floors and any base carrying vehicles or heavy equipment should be reinforced. Light shed bases often do not need it, though mesh adds little effort when the slab is being poured anyway.
For thicker slabs, thickened edges or ground beams may be used around the perimeter.
Step 9: Pouring and Finishing the Concrete
Concrete is ordered to a mix that suits the use. For most domestic bases, a standard designated mix is used, supplied ready mixed to site. Access matters. A ready mix lorry needs a route close enough for its chute, or a pump or barrows are needed, which adds time and labour.
On pour day:
The concrete is placed and spread evenly.
It is compacted to remove air pockets.
A screed board is pulled across the shuttering to strike off the surface level.
A float smooths it, and finishing is timed to the weather and how quickly the concrete sets.
For garages and outdoor areas, a brushed finish can provide grip.
Do not add water on site to make concrete easier to work. Extra water weakens the finished slab and increases cracking.
Step 10: Curing
Concrete does not simply dry. It cures through a chemical reaction that needs moisture. Curing it properly makes it stronger and less likely to crack.
Cover the surface with polythene or damp hessian for several days.
In hot or windy weather, keep the surface damp.
In cold weather, protect it from frost.
Avoid heavy loads for at least a week, and take care before parking vehicles on it.
Concrete keeps gaining strength for weeks, and is generally treated as fully cured after about 28 days.
Movement Joints and Cracks
Concrete shrinks as it cures, and some hairline cracking is normal. Movement joints, cut into the surface or formed with inserts, control where cracks happen. Large slabs need them at regular intervals.
Wider or uneven cracks can point to ground movement, weak sub base, tree roots or poor drainage. If a crack is growing, or one side is higher than the other, get it looked at.
Drainage Around Your Base
A base is only as good as the water management around it.
The ground around the base should fall away from it, not towards it.
Guttering from a shed or garage should connect to a soakaway or drain.
Hardstanding next to a building needs a channel drain or fall away from walls.
Standing water against a concrete edge softens the ground and erodes support over time.
Our groundwork and drainage guide explains how the two fit together, and our article on soakaways and land drainage in North Devon covers what to do with the water once it is collected.
Sloped Plots: A North Devon Specialty
Flat building plots are rare around Ilfracombe. A garage or shed on a slope may need:
Cutting into the bank to form a level platform
Retaining walls to hold back the cut
A stepped or raised base with filled ground carefully compacted in layers
Extra attention to drainage behind retaining walls
Never build on loose fill. If the base is partly on cut ground and partly on fill, the two parts settle differently and the slab cracks. Our guide to sloped and uneven gardens in North Devon explains how these sites are handled.
Can You Build It Yourself?
A small shed base is a realistic DIY project for a competent person with help. A garage, workshop or anything that takes weight deserves a professional team.
DIY is realistic when:
The base is small and light
Ground is firm and level
You can hire a compactor and get concrete delivered
There are plenty of helpers on pour day
Professional help is wise when:
The base is for a garage, vehicles or an extension
The site is sloped, soft or restricted
Services or drains run beneath the area
You need a mini digger, tippers and a ready mix delivery coordinated
Building control inspections are involved
Concrete gives one chance. Once it is poured, mistakes in level or thickness stay forever.
Choosing Who Builds It
Look for a contractor who will visit the site, ask about ground conditions and drainage, and explain the build up in plain terms. Be wary of anyone who offers to pour straight onto soil, or who cannot say what is going under the concrete. Our guide on how to choose a trusted groundwork contractor in Ilfracombe EX34 lists the questions to ask.
Our team handles excavation, hardcore, shuttering, mesh and pouring, and coordinates the plant, tippers and deliveries needed. For commercial and larger projects, see our article on commercial groundwork and site preparation across North Devon.
Checklist Before Your Base Goes In
Do you know exactly what the base will carry?
Have you checked planning and Building Regulations?
Has the site been surveyed for levels, services and trees?
Is access clear for the digger, tipper and concrete lorry?
Is there a plan for spoil and waste?
Is the ground firm, with soft spots removed?
Is the sub base compacted in layers?
Is a membrane included where needed?
Is reinforcement planned and positioned correctly?
Are falls and drainage designed around the base?
Is there a plan for curing and weather protection?
Related Guides From Crabb Contractors
Keep reading: our guides to block paving repair, cleaning and sealing. For help on site, see our groundwork contractors and brick paving and driveways pages.
Frequently Asked Questions
How thick should a concrete base be for a shed?
About 100mm of concrete on a compacted hardcore layer is typical for a standard garden shed. Heavier buildings need more.
How thick should a garage base be?
Garage slabs are commonly 125 to 150mm thick on compacted hardcore, with reinforcement. The right figure depends on ground conditions and loads, so it should be confirmed on site.
Do I need planning permission for a concrete base?
A base on its own usually does not, but the building on top might. Check the rules for the structure, its size and its position on the plot.
How long before I can put a shed or garage on a new concrete base?
Wait at least a week for light structures and longer for heavy loads, and ideally allow close to 28 days before full use.
Can I pour concrete in winter?
Yes with care. Frost can damage fresh concrete, so cold weather needs protection, and pours may need to be postponed when freezing is forecast.
Can Crabb Contractors build concrete bases in Ilfracombe?
Yes. We excavate, build the sub base, shutter, reinforce and pour bases for sheds, garages, workshops and hardstandings across Ilfracombe and North Devon.
Build It Once, Build It Right
A concrete base rewards patience in the stages nobody sees: digging to firm ground, compacting the stone, laying the membrane, setting the levels. Skip those and the slab cracks. Do them well and it carries a garage, a workshop or an extension for the life of the building.
If you are planning a base in Ilfracombe or anywhere in North Devon, call Crabb Contractors on 01271 500072 or contact the team. We will visit the site, read the ground, and tell you what the base needs before any concrete is ordered.
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