A concrete slab looks simple: you dig, you build the formwork, you pour. In reality, its lifespan is decided mostly before the pour, in the preparation of the ground and the choice of the right structure. Whether you are preparing a terrace, the base for a garden shed, a garage or a carport, this guide helps you ask the right questions.
It does not replace a professional’s advice: every plot and every use calls for its own answers.
Define the slab’s use before you dig
It all starts with what the slab has to support. Four cases come up most often:
- A terrace: mainly foot traffic, furniture, sometimes a barbecue or a light spa.
- A garden shed: a permanent, concentrated load under the walls and the items stored inside.
- A garage: vehicle loads, so a slab under more stress, with requirements for flatness and bearing capacity.
- A carport: vehicle loads and the supports of the structure, often under the posts.
The use determines the thickness, the reinforcement, the quality of the sub-base and the precision of the finish. A terrace slab copied as it is for a garage is one of the most common mistakes.
Also note from the outset what will go on top: tiles, slabs on pedestals, plain smoothed concrete, a timber or metal structure. This affects the surface finish and the slopes to allow for.
The ground survey, the first step for your slab
Well-proportioned concrete cannot make up for poorly prepared ground. Before ordering anything, look at the plot:
- is the ground backfilled, loose, clayey, or already compact?
- does water stand after rain?
- is there a slope, a wall, a tree or roots nearby?
- do any pipes or cables cross the area?
Ground that moves or holds water causes settlement and cracks. On doubtful ground, a professional can assess the bearing capacity and propose suitable preparation. A visit on site is better than an estimate from a distance.
Before digging, find out about any buried pipes: piercing a pipe while excavating is costly and can be dangerous.
Earthworks and sub-base: the foundation of the slab
Earthworks consist of excavating the area to the required depth, then preparing the sub-base: a layer of draining, compacted materials on which the slab will rest. It spreads the loads and drains the water.
Points to watch:
- The excavation must be even and at the right levels. A variable depth gives a variable concrete thickness, and therefore weak zones.
- Compaction is done in layers. A layer that is not compacted settles over time.
- The slope must let water run away from buildings, without leaving puddles.
- Removal of the soil has to be planned: even a modest excavation produces a large volume.
For a small area, digging by hand is still possible. Once the surface grows or the ground is hard, a machine and a team used to this work save time. Looking at earthworks companies lets you compare this item separately.
Formwork, reinforcement and slab thickness
The formwork
The formwork outlines the slab and fixes its levels. It must be rigid, well wedged and at the right height. Formwork that moves during the pour gives a distorted slab that cannot be corrected afterwards.
The reinforcement
The reinforcement, generally a mesh or bars laid to a plan, helps the slab withstand stresses and limits cracking. It must be held at the right height within the thickness of the concrete, not laid flat at the bottom: there, it is almost useless. Its type and layout depend on the use; a garage or a carport does not call for the same solution as a small terrace.
The thickness
The thickness is chosen according to the use and the ground, and it is a point to have defined by the professional. Above all, remember that a slab too thin for its load cracks, while an oversized slab wastes concrete with no real benefit. Do not copy a neighbour’s thickness without knowing their ground or their use.
Joints and pouring: avoiding cracks
Concrete shrinks as it dries and reacts to temperature changes. A slab without joints ends up cracking where it decides for itself.
Joints help guide these movements:
- Contraction joints are deliberately weakened sections that concentrate cracking in planned places.
- Expansion joints separate the slab from a wall, a post or another structure so that they can move independently.
Their position depends on the shape and surface of the slab. An L-shaped slab or one with inward corners needs particular attention, because these corners are natural cracking points.
At the time of pouring, a few pointers:
- order enough concrete to pour in one go, without interruption;
- spread and vibrate or tamp the concrete to expel the air;
- set the level with a straightedge, then float the surface;
- protect the site from rain, frost and excessive sun.
Pouring goes better with several people: concrete does not wait, and the finishing must follow at the right moment.
Curing time: be patient before loading the slab
Concrete does not really “dry”: it hardens through a chemical reaction that continues for weeks. It is usable well before it reaches its final strength, but not straight away.
What makes this time vary:
- temperature and humidity;
- wind and sunshine;
- the composition of the concrete;
- the thickness of the slab.
Concrete that dries too fast, for example in full sun or in dry wind, risks cracking at the surface. Dampening the slab or covering it for the first few days is a common practice to avoid this. Ask the professional when you can walk on it, put up a structure, then park a vehicle: these deadlines are not the same.
Permits: check before building on the slab
The slab alone is not necessarily affected, but the structure planned on top may be. A terrace, a garden shed, a garage or a carport do not fall under the same rules, and these vary from one commune and canton to another.
Before you start, contact your commune or the competent cantonal office to find out:
- whether a permit application is required for what you are planning;
- whether there are distances to respect from the property boundaries;
- whether any particular constraints apply to your plot.
If you are a tenant, your landlord’s agreement is essential. In co-ownership, the question also arises with the other owners. Pouring first and asking afterwards can force you to demolish: it is better to check before ordering the concrete.
What makes the cost of a concrete slab vary
No amount can be given without knowing your situation. Here is what weighs on the cost:
- the surface and the thickness, and therefore the volume of concrete;
- the condition of the ground and the earthworks needed;
- the removal of excavated soil;
- access to the site: can a machine or a concrete mixer get through easily?
- the reinforcement and the type of surface finish;
- the joints, the slopes and any connections to existing walls;
- the time of year and the availability of companies.
To compare properly, ask for each quote to detail these items. A quote that specifies neither the thickness, nor the ground preparation, nor the removal of soil is hard to compare with another.
Mistakes to avoid before pouring
- Neglecting the sub-base to save time.
- Laying the mesh flat on the ground.
- Pouring without planning for water drainage.
- Forgetting the joints.
- Loading the slab too early.
- Building without checking whether a permit is required.
- Choosing on price alone, without comparing what each quote includes.
When to call in a professional for your slab
A small base slab for a light shed may be within reach of an experienced DIYer. However, it is better to delegate in these situations:
- a garage or a carport, which carry vehicles;
- sloping, damp or backfilled ground;
- a large surface, which requires pouring in one go;
- a slab adjoining an existing building;
- a visible, carefully finished surface.
Depending on the job, you will deal with concrete slab, masonry or earthworks companies, sometimes all three for the same project. To know who to contact first, describe your project and let the companies propose their approach.
Request a quote for your concrete slab
To receive comparable quotes, prepare some information: the intended use, the approximate dimensions, the condition of the ground, access to the site, and what will be built on top. A few photos help a great deal.
You can request a quote by describing your project online: your request is passed on to craftspeople and companies in the region, who get back to you to prepare a free, no-obligation quote. Then compare what each offer includes, not just the total.
Updated 29 October 2026