Take a look at our comprehensive guide for all you need to know about laying a concrete garage floor.

Concrete garage floors: a step-by-step guide

Concrete is easy to lay and provides a hard-wearing, durable solution for garage floors, which have to support heavy vehicles, withstand contaminants like petrol and anti-freeze and put up with freezing temperatures. To ensure your garage floor lives up to your expectations, follow our step-by-step guide.

Step 1: do your research

A high quality ready-mixed concrete will last for decades providing you with a maintenance-free, hard-wearing garage floor, but flaking, dusty and cracked concrete slabs are all too common.

The more you know about concrete before you start the better, whether you are planning to carry out the work yourself or employ a builder. 

Check that your concrete supplier is quality assured and its products adhere to all the latest British Standards and make sure you order the correct volume and mix of concrete for your project as well as determining whether you need to hire a concrete pump. Most domestic driveways will break if a fully loaded concrete wagon drives on them (a fully loaded wagon can weigh over 30tonnes).

Step 2: site preparation

The effort you put in to prepare your site before your concrete is poured has a direct impact on the finished results. You will be unsurprised to hear that the better the preparation, the better the finished concrete garage floor will be.

Ground conditions aren’t too much of an issue as even the poorest soils can support a sizeable concrete slab but you do need to make sure they are consistent across the pour area to avoid any settling, which will cause the slab to bend and potentially crack.

Mark out the perimeter of the concrete slab using pegs and string – allowing an extra 75mm to accommodate the formwork, which will hold the wet concrete in place when it is drying – before excavating to the required depth. You need to allow between 100 and 200mm for the depth of the concrete slab (depth will depend on the load it is supporting) plus a minimum of an additional 100mm for the sub base. Then compact. 

Place pegs at regular intervals to mark out the ultimate height of the finished concrete, using a spirit level to keep the top of each peg level. Next add your formwork using 25mm timber planks placed around the edge of the slab and using a spirit level to ensure they line up with your pegs.

Lay the sub base (ideally crushed stone and sand) to a minimum depth of 100mm and compact. The material should ideally be a mix of graded crushed stone and fines as the different shapes and sizes interlock together when compacted.

Concrete volume calculator

Concrete volume calculator

Use our ready-mixed concrete calculator to select the best concrete for your project and work out the volume you require.

Concrete volume calculator

Installing a damp-proof membrane

Finally lay a damp-proof membrane, turning the edges up to form a tray against the form work and ensuring and any joints are overlapped and taped. This will protect the underside of the concrete from rising damp and any chemicals in ground water as well as helping to prevent the poured concrete from drying out too quickly, improving its final strength and reducing the chance of it cracking.

Step 3: Ordering concrete

Now the site is prepared, you are ready to order the concrete. Before you do so, it is important to know which type of concrete you require, and if any reinforcement is needed, based on the ground conditions and how the garage will be used. If you are in any doubt, take a look at our guidance and/or seek advice from a structural engineer. 

You will also need to know how much concrete to order and you may find using a concrete calculator useful for this . It will ask for the basic shape of the area – square/rectangle, right-angled triangle, parts of a circle – and dimensions (length, width and depth) to provide you with an estimate of the volume you need. The depth of concrete you need will depend on the use of the garage: a minimum depth of 100mm will be required but it may need to be 150mm-200mm thick to accommodate heavy vehicles.

Do I need to hire a concrete pump?

Finally, you need to consider ordering a concrete pump. Using a pump is perfect when you are dealing with large volumes of concrete or when time is tight and you don’t have the manpower to use wheelbarrows (you can pump approx. 1m3 of concrete per minute). You will also need to use a pump if the concrete mixer truck can’t get close enough to the pour area or access to your site is restricted, underground, within an existing building or at height.

Step 4: Concrete pour

It is important that you have everything ready – including suitable protective clothing – before your concrete is delivered as it typically starts to go off within two hours of being mixed. It will need to be unloaded and levelled as quickly as possible, so have all the tools you need at hand. A rake or shovel can be used to move the concrete around so that it reaches all the corners before it is levelled in line with the top of the formwork by dragging a straight-edged piece of timber across in a sawing motion. This will help eliminate any pockets of air and also give the finished floor some grip. If you want a smooth finish, a concrete float can be used.

Click here for more information on how to prepare for a concrete pour.

Step 5: Finishing and curing

It is important that the newly-laid concrete doesn’t dry out too quickly as it hardens as this can cause a dusty finish but also flaking and cracking. The easiest was to achieve this is to keep the slab damp by covering with polythene sheeting and is particularly important in temperatures over 20°C or if there is a stiff breeze that could dry the surface. Also, if temperatures are likely to dip below 4°C, a frost blanket or similar should be used to insulate the slab and protect the surface from frost. You could also spay some water on the slab each day while it cures or use a curing compound.

Follow-on work should be possible within 48 hours but it is best to leave the formwork for 72 hours to avoid any possible damage to the edges of the concrete. Concrete takes 28 days to reach its full strength, though specialist concretes are available if you want to the concrete to be in use quickly.

For more information on curing concrete click here.