A Step-by-Step Guide to Pouring a Concrete Slab for a Garage or Outbuilding

If you're planning a new garage, workshop or outbuilding, the base is where the project either succeeds or starts causing problems. Get it wrong and you're looking at cracking, damp or a floor that won't take the weight of a car.

Pouring a concrete slab for a garage or outbuilding involves more steps than most people expect and knowing what's involved before you start makes it far easier to plan your budget, your timeline and who you'll need on site. Here's what goes into getting it right, from the ground up, so you know what to expect when you hire a professional.

What Goes into a Garage Concrete Base Specification

From the very start, the most important thing to consider is the specification. A proper specification for a garage base covers five things:

• Sub-Base
• Damp-Proof Membrane
• Concrete Mix
• Slab Thickness
• Reinforcement (where needed)

Miss one of these out and you're building on a compromise, even if the finished slab looks fine on the day.

Think of the specification as a checklist rather than a single decision. The sub-base carries the load before the concrete does its job. The membrane keeps moisture out. The mix and thickness need to match what the building will be used for, whether that's parking a car or storing garden equipment. Finally, reinforcement adds resistance to cracking under load or ground movement.

Pouring a concrete slab for a garage or outbuilding without covering all five is how you end up with a floor that looks fine for a year or two and then starts letting you down.

Preparing the Sub-Base for Your Concrete Slab

The first rule is to never compromise on the sub-base. For concrete slabs, it is the layer that does the real work of supporting everything above it. Skip this stage or rush it, and no amount of good concrete will save the floor from settling unevenly.

Getting it right generally means:

• Excavating to the Correct Depth: Allow for the sub-base, membrane and slab together, typically 150 to 200mm for the sub-base alone.
• Laying and Compacting Hardcore: Use well-graded crushed stone or MOT Type 1, laid in layers and compacted with a plate compactor rather than dumped in one go.
• Checking Levels Throughout: Use a laser level or string line to confirm the sub-base is flat, with a slight fall away from the building where drainage requires it.
• Blinding the Surface: A thin layer of sand fills voids before the membrane goes down.

This is how to pour a concrete slab that stays flat and stable for decades, not just the first winter.

Damp-Proofing and Reinforcement Options

Once the sub-base has been compacted and levelled, the next stage is to limit moisture rising through the slab and provide any reinforcement required by the design. A damp-proof membrane is commonly installed below the concrete, typically using at least 1200-gauge polythene, equivalent to approximately 300 microns. It should be laid on a smooth, non-damaging surface, with joints properly lapped and sealed. Where the slab forms part of a building, the membrane should also be detailed so that it creates a continuous moisture barrier with the wall damp-proof course.

Reinforcement depends on the slab’s size, ground conditions and expected loads. Where steel mesh is specified, it should be supported on suitable spacers so that it remains at the designed position within the concrete rather than resting on the sub-base.

Fibre reinforcement can help control shrinkage cracking and, where an appropriate macro-fibre system has been specified, may provide a structural function. However, it should not automatically be treated as a replacement for steel mesh. Fibres may be suitable for some lightly loaded outbuildings, but a vehicle garage or workshop carrying concentrated loads should have its reinforcement specified according to the design requirements.

Choosing the Right Garage Floor Concrete Mix and Slab Thickness

The concrete specification should reflect the building’s intended use, the ground conditions and the loads likely to be applied. For a domestic garage, a strength class such as C25/30 or C30/37 may be suitable, but neither should be treated as a universal requirement. The appropriate specification may also depend on factors such as exposure conditions, the intended finish and any possible contact with oils, chemicals or de-icing salts.

Ground-bearing garage slabs are often around 100mm to 150mm thick, although the appropriate thickness depends on the sub-base, soil conditions, reinforcement, slab size, joint layout and expected loading. A 100mm slab may be suitable for some domestic vehicle use where the ground has been properly prepared, while heavier vehicles, machinery or uncertain future use may require a thicker slab or a structural design.

Reducing the thickness to save on materials can be a false economy if the space may later be used as a garage or workshop; NHBC guidance gives 100mm as a minimum for unreinforced ground-bearing concrete floors, but this is not a universal specification for vehicle-bearing slabs.

Pouring a durable concrete slab for a garage or outbuilding is therefore about coordinating the DPM, sub-base, concrete strength class, reinforcement, thickness, joints, finishing and curing, not simply choosing a stronger mix or a thicker slab.

Ordering Ready-Mix Concrete for Your Garage Floor

Once the specification is settled, it's time to work out volume. Calculate your garage concrete slab area in square metres, multiply by the slab thickness in metres, and you've got your cubic metres of concrete needed. Always add a small margin for uneven sub-base levels, since running short mid-pour is far more disruptive than having a little extra.

Ready-mix concrete for garage floors is almost always the better choice over mixing on site for anything beyond a very small slab. A ready-mix supplier delivers the exact volume and mix specification you need, batched to consistent quality, arriving on a set schedule so the whole crew is ready to go the moment it lands.

Ordering the right volume of the correct mix, at the right time, is what keeps pour day running smoothly rather than becoming a scramble.

What to Expect on Pour Day

Pour day moves fast once it starts, and knowing the sequence in advance makes a real difference to how smoothly things go. A typical sequence looks like this:

• Final Checks Before Delivery: Confirm the sub-base, membrane and any reinforcement are exactly as specified and free of debris.
• Placing the Concrete: Whether tipped or pumped into position, it needs spreading evenly and working into corners without leaving voids.
• Levelling and Screeding: A straight edge or power screed brings the surface to the correct level across the whole slab.
• Finishing the Surface: Floating and a final trowel pass, where needed, gives the texture the building requires.
• Curing: Covering the slab and controlling moisture loss over the following days gives the concrete its full strength.

This is how to pour a concrete slab for garages that ends up flat and durable, whatever it needs to hold. On sloped or restricted sites, an experienced team often makes the difference.

Getting Your Slab Right the First Time

We're 2 Brothers Concrete & Pumping, a family-run business based in Southampton, and we've spent years helping self-builders across the South get their garage concrete slab poured properly. From advising on mix and thickness to pumping concrete into awkward sites, we handle the parts hardest to get right alone. For a concrete base for outbuildings of any size, call us on 01489 552737 or fill in our contact form.