How to Lay a Concrete Base — Step-by-Step UK Guide

What You'll Need

Before You Start

• Check for underground services — use a CAT scanner before digging. Contact utility providers if you are unsure of service routes.
• Check if you need building regulations approval. Bases for outbuildings under 15m² are usually exempt, but garages and habitable structures require building control sign-off.
• Plan the pour — for anything over 2-3m³, consider ordering ready-mix concrete. Mixing by hand or in a small mixer for large pours is exhausting and gives inconsistent results.
• Use our Concrete Volume Calculator to work out how much concrete you need and our Concrete Mix Calculator for site-mix ratios.
• Check the weather — you need 48 hours of frost-free conditions after pouring. Avoid pouring in heavy rain.

Step-by-Step Instructions

Step 1: Mark Out and Excavate

Mark out the base area using string lines and pegs. Add 50-100mm to each side for the formwork. Square up the corners using the 3-4-5 triangle method. Excavate to a depth of 200-300mm below your desired finished level (100mm sub-base + 100mm concrete typical). Remove all topsoil and organic material.

Step 2: Compact the Sub-Grade

Compact the exposed earth with a whacker plate. The base must be firm and stable. If the ground is soft or has been recently disturbed, excavate deeper and add more sub-base to compensate.

Step 3: Lay and Compact the Sub-Base

Spread Type 1 MOT crushed stone to a depth of approximately 125mm (it compacts to about 100mm). Compact in layers with the whacker plate — do not try to compact more than 75mm at a time. The finished sub-base should be level and firm.

Step 4: Install Formwork

Build formwork around the perimeter using straight timber boards nailed to stakes driven into the ground on the outside. The top of the formwork should be at your finished concrete level. Check the formwork is level (or has the desired fall for drainage) using a spirit level. Brace the formwork well — wet concrete exerts significant pressure.

Step 5: Lay DPM

Lay a 1200-gauge polythene DPM over the sub-base, turning up the edges against the formwork. Overlap sheets by 300mm and tape the joins. The DPM prevents ground moisture from rising through the slab and also stops the sub-base from drawing water out of the wet concrete.

Step 6: Place Reinforcement Mesh

Lay steel mesh reinforcement over the DPM, raised on spacer chairs to sit at mid-depth in the slab (e.g. 50mm up for a 100mm slab). Overlap mesh sheets by at least one full square and tie together with wire. Cut mesh to fit around formwork edges, leaving 50mm cover from the sides.

Step 7: Pour the Concrete

Pour or barrow the concrete into the formwork, starting at the far end and working back. Spread roughly with a shovel or rake. Work the concrete around the mesh to avoid voids — use a poker vibrator on larger pours or tamp firmly with a rake. Keep the pour going — cold joints (where fresh concrete meets partially set concrete) are weak points.

Step 8: Level and Tamp

Place a screed bar (straight edge) across the formwork and drag it towards you with a sawing motion. This levels the concrete to the top of the formwork. Fill any low spots and pass the bar again. Repeat until the surface is flat and level.

Step 9: Finish the Surface

For a rough finish (suitable for laying slabs or flooring on top), a wooden float finish is sufficient. For a smooth finish (e.g. a garage floor), use a steel trowel once the concrete has stiffened enough to support your weight on a knee board. For a non-slip finish, brush the surface with a stiff broom.

Step 10: Cure the Concrete

Cover the slab with polythene sheeting or spray with a curing compound within 2-4 hours of finishing. Keep the concrete damp for at least 7 days — curing is essential for full strength. Do not allow the surface to dry out or be exposed to direct sun in the first week. Remove formwork after 24-48 hours and backfill the edges.

Common Mistakes to Avoid

• No sub-base: Pouring concrete directly onto soil leads to cracking and settlement. Always lay and compact a sub-base first.
• Weak formwork: Formwork that moves during the pour results in an uneven slab. Brace thoroughly and check levels before pouring.
• Too-wet mix: Adding too much water makes concrete easier to work but dramatically reduces its strength. The mix should slump 50-100mm, not flow like liquid.
• No curing: Concrete that dries out too fast is weak and cracks. Cover and keep damp for at least 7 days.
• Mesh on the bottom: Reinforcement mesh sitting on the DPM at the bottom of the slab does nothing. It must be raised on spacer chairs to mid-depth.

Cost Estimate (2026 UK Prices)

Use our Concrete Volume Calculator and Concrete Mix Calculator for exact quantities for your project.