Laying a Concrete Base

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Plan, work out and lay a durable concrete base for a shed, patio, garden room, driveway, garage or workshop. This complete UK 2026 guide covers excavation, sub-base, DPM, formwork, ready-mix volume, concrete grade, finishing, curing and realistic cost planning.

Use this guide to estimate the ready-mix concrete volume, slab area, concrete weight, material allowance and indicative 2026 cost for a simple rectangular concrete base. It is designed for planning shed bases, garden room bases, garage bases, patios, paths, bin-store pads and small driveway slabs.

Planning note:the guide estimates rectangular bases only. For L-shaped, stepped, sloped or reinforced slabs, split the base into simple rectangles, add the volumes together, then confirm with your ready-mix supplier or contractor before ordering.

Good Concrete Starts Below the Slab

The most common concrete base failures are not caused by the concrete alone. They usually come from poor excavation, soft spots, weak sub-base compaction, missing DPM, bad drainage, under-ordering concrete or rushing the curing stage.

Laying a concrete base looks simple: dig out the ground, add hardcore, set timber forms and pour concrete. In reality, a long-lasting base depends on a sequence of small decisions. The thickness must suit the load. The sub-base must be compacted in layers. The formwork must be level, square and strong enough to resist wet concrete pressure. The damp proof membrane must be overlapped and protected. The concrete grade must match the job. The surface must be finished at the right time, then cured instead of being left to dry too fast.

This 2026 UK guide is written for homeowners, landscapers, handymen, builders, self-builders and small contractors who need a practical method for ordinary domestic bases. It covers shed bases, garden offices, log cabins, hot tub bases, patio slabs, bin-store pads, garage floors, driveways and light workshop slabs. It is not a substitute for structural design where the base supports a building, retaining wall, heavy machinery, unusual ground, trees, poor drainage or a load-bearing structure that needs Building Regulations approval.

The main aim is to help you avoid the expensive mistakes: ordering too little concrete, pouring onto uncompacted soil, ignoring water, choosing the wrong concrete grade, setting forms out of level, failing to plan truck access or walking away before the slab has been cured. Concrete is forgiving in some ways, but once the truck has arrived and the drum is turning, there is limited time to correct poor preparation.

Use the table below as a planning guide only. Ground conditions, reinforcement, drainage, exposure, edges, loading and local building requirements can change the correct design.

  • Base Type: Path or light patio; Typical Concrete Thickness: 75–100mm; Common Grade: C15/20 to C20/25; Sub-base Guidance: 50–100mm compacted granular layer; Notes: Use falls for drainage; avoid feathered thin edges.
  • Base Type: Shed base; Typical Concrete Thickness: 100mm; Common Grade: C20/25 or C25/30; Sub-base Guidance: 100mm MOT Type 1 compacted; Notes: DPM recommended under timber sheds and storage buildings.
  • Base Type: Garden room / office; Typical Concrete Thickness: 100–150mm; Common Grade: C25/30; Sub-base Guidance: 100–150mm compacted sub-base; Notes: Check supplier base requirements and insulation design.
  • Base Type: Hot tub base; Typical Concrete Thickness: 100–150mm; Common Grade: C25/30 to C30/37; Sub-base Guidance: Well-compacted, level sub-base; Notes: Full hot tubs are very heavy; keep the slab level and supported.
  • Base Type: Domestic driveway; Typical Concrete Thickness: 100–150mm; Common Grade: C25/30 or C30/37; Sub-base Guidance: 150mm+ compacted sub-base; Notes: Consider reinforcement, joints and frost/de-icing exposure.
  • Base Type: Garage base; Typical Concrete Thickness: 100–150mm+; Common Grade: C25/30 or C30/37; Sub-base Guidance: 100–150mm+ compacted MOT Type 1; Notes: Heavier vehicles may require 150–200mm and engineer measurement.

Step 1 — Confirm the base purpose, load and size

Start with the end use. A small garden shed base has different requirements from a garage slab, hot tub pad or driveway crossing. Measure the structure, then add enough margin around it for overhangs, fixings, rainwater run-off and installation tolerance. Many shed and garden building suppliers ask for the concrete base to be slightly larger than the building footprint, but not so large that rainwater sits against timber walls. Record the length, width and thickness in metres so your concrete volume is easy to work out.

Think about what will sit on the base after installation. Storage loads, motorbikes, ride-on mowers, gym equipment, hot tubs, vans and workshop machines can all increase the demand on the slab. Where a base supports a permanent habitable building, structural frame, masonry wall or garage, ask a competent person whether Building Regulations, planning restrictions, drainage rules or structural design checks apply.

Step 2 — Check levels, falls, drainage and access

Concrete bases should rarely be planned in isolation. Water must drain away from buildings and should not be directed towards neighbouring property, damp walls or low thresholds. For external slabs, a gentle fall is usually better than a perfectly flat surface, especially for patios, bin stores and driveways. A common practical fall is around 1:60 to 1:80, but the correct fall depends on the finish, location and drainage route.

Before digging, confirm how materials will reach the working area. Can a ready-mix truck get close enough? Is a wheelbarrow route clear, firm and wide? Do you need a concrete pump? Where will excavated soil go? Can bulk bags be delivered safely? Planning these details saves money because delays during delivery may create waiting-time charges, rushed placing and poor finishing.

Step 3 — Mark out the base accurately

Use pegs, string lines, marker paint and a tape measure to mark the exact footprint. Check diagonals to make sure the rectangle is square. If the diagonals match, the corners are square. Set the top level of the finished slab using a laser level, spirit level or water level. Mark both the finished concrete level and the excavation level, allowing for concrete depth, sub-base depth, bedding or blinding, membrane and any insulation if used.

For bases close to a house, garage or boundary, leave sensible working space for shuttering and compaction equipment. Do not undermine existing foundations, retaining structures, drainage runs, inspection chambers, service ducts or garden walls. If there is any doubt about underground services, investigate before excavation.

Step 4 — Excavate to the correct depth

Dig out vegetation, topsoil and soft ground until you reach firm, stable material. For a 100mm concrete slab over a 100mm compacted sub-base, the excavation will usually be at least 200mm below finished concrete level, plus any allowance for sand blinding, DPM protection or insulation. Do not leave organic material, roots or soft pockets under the slab. These can rot, compress and create settlement.

For larger excavations, unstable ground, nearby structures or trenches, safety becomes a major issue. Sides can collapse, services can be struck and water can collect quickly. Use safe excavation practice, keep people away from unsupported edges and follow competent advice for deeper or risky excavations.

Step 5 — Compact the subgrade

The subgrade is the soil beneath the sub-base. It should be firm, even and compacted. If the ground is soft, wet, clay-heavy or variable, a simple domestic slab may still move unless the weak material is removed or treated. Do not pour concrete over mud. Let wet ground dry where possible, improve drainage, replace weak spots and compact thoroughly before adding aggregate.

Compaction is not just a quick pass with a plate compactor at the end. The base works best when each layer is placed evenly and compacted before the next layer is added. Poor compaction leaves voids and soft patches that later become cracks, dips and rocking shed floors.

Step 6 — Install and compact the sub-base

For most UK domestic concrete bases, a well-compacted granular sub-base such as MOT Type 1 is used below the concrete. Spread it in layers, rake it level and compact each layer with a vibrating plate. For typical shed and patio bases, 100mm compacted sub-base is common. Heavier driveway and garage applications often require more depth and better edge restraint.

The sub-base spreads load, improves stability and creates a more reliable platform for the concrete. It also helps prevent sharp stones from pressing directly into the DPM if a sand blinding layer is used above it. Do not use loose rubble, bricks, plasterboard, timber, mixed waste or ungraded demolition material as a structural sub-base.

Step 7 — Add blinding, DPM and insulation where needed

A thin sand blinding layer can protect a damp proof membrane from puncture. A DPM is strongly recommended where the base supports a shed, garden room, workshop, garage, internal floor, timber structure or stored goods. Overlap membrane joints, tape if required and turn up edges where appropriate so moisture cannot easily bypass the barrier.

Insulation is not needed for every outdoor base, but it may be part of the design for garden offices, habitable outbuildings, garages converted to rooms or heated workshops. If insulation is included, choose the correct compressive strength and follow the building designer’s slab build-up. Do not guess insulation type under heavy loads.

Step 8 — Build strong, level formwork

Formwork, also called shuttering, holds the wet concrete in shape until it hardens. Use straight timber boards, strong pegs and secure bracing. The top of the boards often acts as the screed rail, so take time to set them to the correct level and fall. Weak formwork can bow out during the pour, increasing the volume, spoiling edges and making finishing difficult.

Check that the internal dimensions of the forms match the intended base size. Re-check diagonals, corner levels and falls before ordering concrete. Wet concrete is heavy and exerts pressure, especially on deeper pours, so brace corners and long sides properly.

Step 9 — Choose concrete grade and order volume

For light domestic bases, C20/25 may be suitable. For many shed bases, garden rooms, garage floors and driveways, C25/30 or C30/37 may be specified depending on load, exposure and supplier recommendation. For structural foundations, retaining walls, suspended slabs or unusual loads, the grade should be specified by a competent designer.

Work out volume using length × width × depth, then add a wastage allowance. Ten percent is a sensible planning allowance for many small bases because ground level, formwork movement, uneven excavation and small measurement errors all affect volume. It is usually better to have a small amount spare than to run short halfway through finishing a slab.

Step 10 — Place, level and compact the concrete

When the concrete arrives, place it as evenly as possible across the base. Do not dump it all in one corner and drag it excessively, because that can make finishing harder and increase segregation risk. Use rakes, shovels and a straightedge to move and level the concrete. For small slabs, tamping with a straight timber or aluminium beam helps settle the concrete and bring paste to the surface.

Avoid adding extra water on site just to make concrete easier to place. Extra water can reduce strength, increase shrinkage and create dusty or weak surfaces. If workability is important, discuss slump and admixtures with the ready-mix supplier before delivery rather than trying to correct the mix on site.

Step 11 — Finish the surface and cut joints if needed

Finish depends on the base use. A shed base or garage floor may need a reasonably smooth trowelled finish. A path, patio or driveway often benefits from a light brush finish for grip. Do not overwork the surface while bleed water is present, and do not repeatedly sprinkle dry cement over wet concrete to hide water or force a finish. That can create a weak, dusty top layer.

For larger slabs, joints help control where shrinkage cracks form. Saw-cut or formed joints may be needed depending on slab size, shape and restraint. As a simple rule, long narrow slabs, L-shaped bases and slabs around fixed objects are more crack-prone. Plan joints before pouring rather than waiting until random cracks appear.

Step 12 — Cure, protect and load at the right time

Curing is the process of keeping concrete moist and protected while it gains strength. Fast drying from sun, wind or frost can damage the surface and increase cracking. Cover the slab with polythene, curing membrane or damp hessian as appropriate, and protect it from rain impact, pets, leaves, foot traffic and freezing temperatures.

Concrete gains strength over time. It may be firm enough to walk on after a short period, but that does not mean it is ready for full load. Many normal concretes are specified by their 28-day strength. Follow supplier guidance before installing heavy sheds, vehicles, hot tubs or machinery. In cold weather, strength gain slows and protection becomes more important.

Costs vary heavily by region, access, excavation depth, disposal, concrete grade, delivery distance, pump hire, reinforcement and finish. Use these ranges for early budgeting, then obtain local quotes.

  • Cost Item: Ready-mix concrete; Typical 2026 Planning Range: Often budgeted around £115–£170+ per m³; What Affects It: Grade, region, short-load fees, delivery distance, admixtures; Guide Link: Ready-mix guide
  • Cost Item: MOT Type 1 sub-base; Typical 2026 Planning Range: Usually estimated by tonne or bulk bag; What Affects It: Depth, compaction, supplier minimums, delivery access; Guide Link: Sub-base guide
  • Cost Item: DPM; Typical 2026 Planning Range: Low material cost, high value protection; What Affects It: Sheet gauge, laps, tape, puncture protection; Guide Link: DPM guide
  • Cost Item: Excavation and waste; Typical 2026 Planning Range: Can exceed concrete cost on awkward sites; What Affects It: Soil volume, skip permits, access, distance to disposal; Guide Link: Truck load guide
  • Cost Item: Labour and finishing; Typical 2026 Planning Range: Often priced per day or per m²; What Affects It: Team size, pour size, finish standard, weather risk; Guide Link: Garage base costs

A durable concrete base is a system, not just a layer of concrete. Each layer has a job. The ground carries the load, the sub-base spreads and stabilises it, the membrane controls moisture, reinforcement controls cracking where specified, and the concrete provides the hard wearing surface. Removing one layer to save money can shorten the life of the whole base.

Subgrade

The natural ground after excavation. It should be firm, trimmed and compacted. Soft spots should be removed and replaced with suitable compacted material.

Sub-base

A compacted aggregate layer, commonly MOT Type 1 for domestic work. It reduces settlement, spreads load and gives the slab a stable platform.

Blinding Layer

A thin sand or fine material layer used to smooth sharp aggregate and protect the DPM. It should not become a soft, loose cushion.

DPM

A damp proof membrane helps resist rising moisture. It is especially important below sheds, garden rooms, garages, workshops and storage slabs.

Reinforcement

Mesh or fibres may be used to control cracking and improve performance. Reinforcement must be correctly positioned, not left lying at the bottom.

Concrete Slab

The finished base layer. Its depth, grade, finish, curing and joints must match the base use and expected loading.

For very small pads, people often try to skip the sub-base and pour concrete directly onto soil. That may work temporarily in sheltered, lightly loaded situations, but it is not good practice for a long-lasting base. Soil expands, shrinks, becomes wet, dries out and varies from one part of the garden to another. A compacted granular sub-base gives the slab a more predictable support.

  • Grade: C10/15; Typical Use: Blinding, non-structural fill; Base Suitability: Not ideal for main bases; Notes: Usually too weak for a finished shed, driveway or garage base.
  • Grade: C15/20; Typical Use: Light paths and basic domestic slabs; Base Suitability: Light use only; Notes: Can suit low-load paths/patios where specified correctly.
  • Grade: C20/25; Typical Use: Shed bases, patios, general domestic pads; Base Suitability: Common choice; Notes: Often used for simple domestic slabs with suitable sub-base.
  • Grade: C25/30; Typical Use: Garden rooms, garages, driveways, workshops; Base Suitability: Stronger domestic choice; Notes: Common where loading is higher or durability is more important.
  • Grade: C30/37; Typical Use: Driveways, heavier garages, reinforced slabs; Base Suitability: Heavy domestic use; Notes: Useful for heavier traffic, but design and joints still matter.
  • Grade: C35/45+; Typical Use: Structural and specialist applications; Base Suitability: Engineer specified; Notes: Do not upgrade randomly without checking workability, finish and design.

For tiny pads and repair jobs, mixing concrete on site can be cost-effective. For larger bases, ready-mix is usually more consistent, faster and easier to place. A 4m × 3m × 100mm base is 1.2m³ before wastage. That represents a lot of cement, aggregate, sand, water control and mixing time if done manually. Once the volume rises above around 1m³, many homeowners find ready-mix or volumetric delivery more practical.

Ready-mix concrete is produced under controlled batching conditions and can be specified by grade, slump, aggregate size and exposure needs. The downside is logistics. You must be ready when the truck arrives, have enough labour, have a wheelbarrow or pump route and understand any short-load or waiting-time charges. Site-mixed concrete gives flexibility but increases labour, variability and manual handling.

Choose ready-mix when…

The base is over about 1m³, you need consistency, the pour must be completed quickly, the grade matters, or you are laying a driveway, garage, hot tub base or garden building slab.

Choose site-mix when…

The job is very small, access is impossible for delivery, timing is flexible, the finish is non-critical and you can manage safe handling of bags, aggregates and water control.

Whether you choose ready-mix or site-mix, the concrete base still needs the same preparation: proper excavation, compacted sub-base, formwork, level control, moisture management, finishing and curing. A good ready-mix cannot compensate for a bad foundation below it.

Small domestic concrete work can still involve real hazards: excavation collapse, hidden services, silica dust, cement burns, heavy manual handling, moving vehicles, slips, trips, vibrating equipment and wet concrete splashes. Wear suitable gloves, eye protection, boots and long sleeves. Wet cement can cause serious skin burns, so wash off splashes quickly and do not kneel in fresh concrete.

For construction work, the Health and Safety Executive provides guidance on excavations, manual handling and general construction site risk control. HSE notes that danger to workers in or near excavations must be prevented and that manual handling risks should be avoided, assessed and reduced. Even on domestic projects, these principles are sensible: avoid unnecessary lifting, use mechanical aids, keep work areas tidy and plan safe access.

Concrete specification in the UK commonly references BS EN 206 and BS 8500. BS EN 206 covers concrete specification, performance, production and conformity, while BS 8500 gives UK-specific complementary guidance. For most simple garden slabs, you may order a common ready-mix grade from a supplier. For foundations, structural slabs, aggressive ground, chemical exposure or building-control work, use a proper specification rather than a guess.

Important:this page is a practical estimating and planning guide. It does not replace a structural engineer, building control officer, architect, surveyor, drainage designer or competent contractor where the base forms part of a regulated or structural project.

  1. Pouring onto topsoil:topsoil contains organic matter and compresses over time. Remove it before installing sub-base.
  2. Using too little sub-base:concrete needs stable support. Thin aggregate over soft ground can still settle and crack.
  3. Skipping compaction:loose hardcore is not the same as a compacted sub-base. Compact in layers.
  4. Forgetting DPM:moisture can rise into sheds, timber floors, garden rooms and storage areas.
  5. Ordering exact volume only:uneven excavation and formwork movement can leave you short. Add wastage allowance.
  6. Adding water on site:extra water may weaken the mix and increase shrinkage. Discuss slump before delivery.
  7. Ignoring weather:hot sun, wind, heavy rain and frost can all damage fresh concrete.
  8. No curing:concrete needs moisture retention and protection while it develops strength.
  9. Bad falls:water should drain away from buildings, doors and timber edges.
  10. No joint planning:larger slabs crack; joints help control where cracks appear.

Before the concrete truck arrives or before you start mixing, confirm every item below. The pour is the wrong time to discover that the wheelbarrow route is blocked or the shuttering is not braced.

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