How to lay a concrete garage floor

To pour a concrete garage floor, lay a compacted hardcore sub-base, add sand blinding and a damp proof membrane, then pour a C25 to C30 slab at least 100mm thick with a slight fall to the door. Level it, float it smooth, and keep it damp for a week while it cures. That gives you a dry, hard floor that will take a car.

Slab thickness by use

For most concrete garages, the floor and the base are the same thing. The slab you pour is both the foundation the building sits on and the surface you drive and walk on. So when people search for how to pour a concrete garage floor, they usually mean how to pour the base itself.

There are two other situations though. One is pouring a new floor inside an old garage that has a broken or missing slab. The other is pouring a screed or topping over an existing slab to level it or smooth it out. The materials and the curing are much the same, but the thickness and the edges change. A fresh full-depth slab does the structural work. A thin topping over sound concrete does not.

This article covers the full-depth slab, because that is what nearly everyone needs. One thing to be clear about from the start: a concrete garage always sits on a poured concrete slab. Paving slabs, gravel, timber bearers and plastic grids are fine under a light timber shed, but they are never an option under a concrete building. The weight and the way the panels bolt down need a solid, flat, poured surface.

Use a C25 or C30 mix for a garage floor. In old money that is roughly a 1:2:4 ratio of cement, sharp sand and 20mm aggregate, though ready-mixed by strength is easier to get consistent. C25 handles a car and general storage. Step up to C30 if you will park something heavy, run a workshop, or want extra margin.

For anything but the smallest floor, ordering ready-mixed concrete on a wagon is the sensible choice. It arrives at a known strength, it saves a lot of mixing by hand, and it means the whole slab goes in wet on wet with no cold joints. A local supplier will quote by the cubic metre. Work out your volume by multiplying length by width by depth in metres, then add about ten per cent for waste and uneven ground.

Mixing on site is fine for a small floor if you have a mixer and a couple of pairs of hands. The risk is running out of daylight or energy halfway through and leaving a join line through the slab. If you do mix on site, keep the water down. A wet, sloppy mix is easy to spread but ends up weaker and more likely to craze and dust on the surface. Aim for a mix that holds its shape when you form it into a ball, not one that slumps into a puddle.

A concrete garage floor should be at least 100mm thick. That is the standard we work to for a single or double garage on normal, stable ground, and it is what most builders pour. It takes a car, a workbench and the weight of the building itself without trouble.

If the ground is soft, made-up or newly disturbed, or if you will drive a heavier vehicle on and off, go up to 125mm or 150mm. At those thicknesses it is worth adding a layer of steel mesh, A142 for lighter duty or A193 for more, set on chairs so it sits in the lower third of the slab. Mesh does not make a thin slab strong, but it holds a thick one together and stops small cracks opening up. There is a fuller explanation of when it earns its place in our reinforcement guide.

One optional upgrade is worth knowing about. Thickening the outside edge of the slab to 150mm gives the perimeter, where the building bolts down and where loads concentrate, a bit more to bite into. It is better than a flat slab, but it is optional, and most builders pour a flat 100mm slab unless you ask for the thickened edge. Do not treat it as the default. A flat, properly compacted 100mm slab on good ground is a perfectly sound garage floor.

A damp proof membrane (DPM) is optional, but it is something we would generally recommend if you want to reduce the chance of moisture coming up through the garage floor.

A DPM is a heavy-duty polythene sheet, usually 1000 or 1200 gauge, which is laid beneath the concrete slab. It creates a barrier between the ground and the concrete, helping to prevent ground moisture from passing through the floor. This can be particularly worthwhile if you intend to use the garage for storage, a workshop, or plan to add a floor covering later.

If you choose to have a DPM, it should be laid over the sand blinding rather than directly onto sharp hardcore, which could puncture it. Any joints should be overlapped and taped, with the membrane turned up around the perimeter to create a continuous barrier beneath the concrete.

You do not have to include a DPM for your garage floor, but adding one is a relatively simple way of providing extra protection against ground moisture and creating a drier internal environment.

No. We recommend that the garage floor is laid level, without any slopes or falls.

The concrete slab should be poured to a consistent finished level across the garage. There is no need to create a fall towards the garage door or in any other direction.

Keeping the floor level provides a flat, practical surface throughout the building, making it suitable for parking vehicles, storage, shelving, workbenches and other typical garage uses. It also makes things easier if you decide to add a floor covering or change how you use the garage in the future.

Any rainwater or surface water outside the garage should be dealt with through the surrounding ground levels and suitable external drainage rather than by introducing a slope into the garage floor itself.

Good preparation is most of the job. Dig out the topsoil and any soft ground until you reach firm subsoil. Topsoil is full of organic matter that rots and settles, so none of it should stay under the slab. Then build the sub-base: 100mm to 150mm of compacted MOT Type 1 hardcore, whacked down in layers with a plate compactor. Hire a compactor for the day if you do not own one; it is cheap and it makes the difference between a slab that stays put and one that sinks.

On top of the hardcore goes a thin layer of sand blinding to fill the sharp voids and give the DPM a smooth bed. Then the membrane, then any mesh on chairs. Set your shuttering last. The shuttering forms the edges of the slab and holds the wet concrete to the right shape and level, so it needs to be straight, well pegged and set to your finished heights including the fall. Our shuttering guide walks through how to set it up square and solid.

Remember the golden rule on size. For a sectional concrete garage the slab should be 150mm wider and 150mm longer than the overall building, giving a small margin all round the panels. Get the dimensions from the building's spec sheet, not from the door opening, and mark them out with pegs and string before you dig. There is more detail in our base laying pages if you want the full method.

Pour the concrete in from one end and work backwards so you are never standing in the wet mix. Spread it roughly with a rake or shovel to just above the level of the shutters, then screed it off. Screeding means dragging a straight board across the top of the two shutters with a sawing, side to side motion, pulling the excess back and filling any hollows as you go. Two passes usually gets it flat and to the fall you set.

Once it is screeded, leave it to firm up a little, then float the surface. A bull float or a hand float knocks down the ridges and brings a smooth, closed surface up. Do not overwork it and do not throw dry cement or water on the top to make it easier, both of which weaken the surface and cause dusting later. For a garage you want a smooth but not glassy finish. A very slightly textured float finish gives grip underfoot when the floor is wet, whereas a polished power-floated surface can be slippery.

Get the whole slab in, screeded and floated in one session. Concrete waits for no one, so have your tools ready, your access clear and enough people to keep up with the wagon. A ready-mix driver will not want to sit around while you find another wheelbarrow. If you are hand mixing, this is exactly where slabs go wrong, so be honest about whether you can lay the volume before it starts to go off.

Give a garage floor a week to cure before you put a building on it or drive anything on it. Concrete is walkable within a day or so, but it is nowhere near full strength. It keeps gaining strength for weeks, and the first seven days are when it does most of that work. Rushing it is how you get cracks and a soft surface.

Curing is not just waiting, it is keeping the slab damp so the chemical reaction can finish. If the surface dries out too fast, especially in wind or sun, the top skins over while the inside is still green and you get crazing and dusting. Cover the fresh slab with polythene sheeting, or spray it with a proprietary curing compound, or in warm weather keep it lightly damp for the first few days. In cold weather protect it from frost, because concrete that freezes before it sets is ruined.

After a week the slab will comfortably carry the building and normal traffic. If you want to seal or paint the floor, or lay any covering, wait longer still, typically four weeks or more, so the residual moisture has come out. A concrete sealer keeps the floor dusting and makes it easier to sweep, and if you plan to use the garage as a warm space it is worth reading about insulating a concrete garage floor before you commit to a finish.

To lay a concrete garage floor, dig out the topsoil, compact 100mm to 150mm of MOT Type 1 hardcore, add sand blinding and a damp proof membrane, then set your shuttering to the right levels with a slight fall to the door. Pour a C25 or C30 slab at least 100mm thick, add mesh if the ground is soft or the loads are heavy, screed it flat, float it smooth and cure it under polythene for a week. For a sectional concrete garage the slab is both the floor and the base, so make it 150mm wider and longer than the building. The two things people most often get wrong are leaving out the damp proof membrane and rushing the curing. Get those right, along with a firm sub-base and an honest mix, and you will have a dry, flat, hard-wearing floor that lasts the life of the building.