Placing and screeding a concrete subfloor slab

A subfloor slab is not a structural slab in the way a suspended floor or a roof slab is. Its job is to create a firm, level and continuous surface on the ground. If you laid waterproofing sheets straight onto hardcore, the stones would puncture them and the floor would flex underfoot.

So why does flatness matter when a screed will cover it anyway? Waterproofing needs a smooth base without sharp edges or hollows. Rigid insulation boards rock and crack on a bumpy slab. And every dip or hump ends up in the screed thickness: in some places the screed is too thin and cracks, in others it is needlessly thick and takes ages to dry. A well-levelled base slab saves work and material in every layer above it.

The same idea also applies to the thin layer of concrete poured under strip foundations or a raft, usually called blinding. Blinding is normally a lower strength class and thinner, and its main purpose is to keep the reinforcement out of the mud. This article focuses on the floor base slab, but the principle of placing and striking off is the same.

Pouring concrete is fast work that cannot be paused. When the truck mixer arrives, everything else has to be finished. In practice it often happens that the crew stands waiting because a drain sleeve is missing or nobody has marked the levels.

Ground and compaction

• Soil dug out to the required depth, with no topsoil, roots or building rubble left
• Hardcore sub-base (crushed stone or well-graded gravel) placed in layers, each layer compacted with a plate compactor, usually in lifts of roughly 15 cm
• Top of the sub-base levelled without hollows, so the concrete is not twice as thick in places
• On clay or waterlogged ground, advice from a structural engineer or designer on whether a geotextile or a thicker sub-base is needed

Services under the floor

Everything that runs under the floor must be laid, checked and backfilled before the pour. That mainly means drainage pipes, the water supply, ducts for electrical cables and, in some areas, radon sump or venting pipes. Pipes that will stick up out of the concrete should be capped and fixed firmly so they do not shift during compaction. Breaking out a fresh slab afterwards is wasted effort and a common source of cracks.

Levels, edges and access

• A datum marked on the walls, usually a line one metre above finished floor level, from which the top of the concrete is measured
• Edge insulation strip fixed around the perimeter walls to separate the slab from the masonry
• Access for the truck mixer or a concrete pump, or a place where the concrete can be tipped and moved by wheelbarrow
• Water and power for the vibrator and for washing tools
• Suitable weather: no frost and no hot sun combined with wind

In a new build or an extension, the floor slab is part of the building and falls under local building regulations. Whether your project needs approval, an inspection or only a notification depends on where you live, so check with your local building authority before work starts rather than assuming nothing is required. If you are redoing the floor of a ground-floor flat, agree it in advance with the building management or the owners' association.

Setting the levels

First the crew decides where the top of the concrete will be. Screed rails, also called guide rails, are set on the sub-base. These can be steel sections, timber battens or strips of concrete laid the day before. In smaller rooms, level pads or pegs set with a laser are enough. The spacing between rails depends on the length of the straightedge, usually up to roughly 2 to 3 metres.

Membrane and reinforcement

A polythene sheet is laid over the hardcore if the design calls for it. It stops the dry stone from sucking water out of the concrete and also reduces friction, so the slab cracks less as it shrinks. If the design includes reinforcement, welded steel mesh is laid with proper laps and raised on spacers or chairs. Mesh lying flat on the polythene does almost nothing, because it ends up at the bottom of the slab.

Placing the concrete

Concrete is placed starting from the far corner and working towards the exit, so nobody has to wade back through finished areas. It is spread with shovels or rakes so it sits slightly above the rails. It should not be dropped from a great height or dragged long distances across the floor, because the mix separates and the stone parts from the cement paste.

Compacting and striking off

For the usual thickness of a base slab, a vibrating screed board is common. It compacts the concrete and strikes it off to rail height in one pass. For thicker slabs or dense reinforcement, a poker vibrator is added. On small areas the concrete is tamped by hand and struck off with an aluminium straightedge, moved in a sawing motion while being pulled forward. Wherever a hollow remains after the pass, more concrete is added and the area is struck off again.

Finishing the surface

After striking off, the surface is smoothed with a bull float on a long handle to remove marks and small pits. Under waterproofing the surface is usually left with a fine texture, not polished smooth. If the screed rails are removed, the channels they leave are filled with concrete straight away and levelled.

Curing

As soon as the concrete allows, it is covered with polythene or kept damp by spraying. The first few days decide whether the surface ends up dusty and cracked. More on curing times below.

Strength class and consistence

For a floor base slab, concrete of class C16/20 or C20/25 is usual, with a higher class in garages or under heavier loads. Blinding under foundations is often a lower class. The design decides, and you should state the class clearly when ordering ready-mixed concrete. The consistence should let the crew spread and strike off the concrete easily. Watering down concrete on site to make it runnier is a mistake: it places more easily but loses strength and cracks more.

Thickness

A typical base slab in a house is roughly 10 to 15 cm thick. In a garage, workshop or under heavier loads it is made thicker and reinforced. The exact figure comes from the design. In an older house, thickness is often limited by door heights and thresholds, so plan the whole floor build-up before you start breaking out the old floor.

Steel mesh, fibres or no reinforcement

• No reinforcement: used on smaller areas on a well-compacted sub-base where the slab only carries light layers. It is more prone to cracking, so it needs movement joints.
• Steel mesh: the most common solution. It holds the slab together and limits how far cracks spread. It must be properly raised on spacers.
• Fibres: polypropylene or steel fibres are mixed into the concrete at the plant. They save the time spent laying mesh, but they should be in the design or agreed with the designer.

How the concrete gets there

Concrete reaches the floor straight down the chute of the truck mixer, through a pump, or by wheelbarrow. A pump makes sense where the truck cannot get close or where you are pouring inside a house through a window or door. For small areas concrete is sometimes mixed on site in a cement mixer. Then it is harder to keep every batch the same, and the pour takes longer, so the first areas start to set before the last ones are done.

The same number of square metres can mean half a day of comfortable work or a full, demanding day. These are the main factors:

• Access: a large open area where the truck can reverse right up to the edge is straightforward. Pouring in a cellar, inside an old house or behind a narrow gate needs a pump or a lot of barrowing.
• Layout: many small rooms, nibs, columns and pipe penetrations mean more hand levelling around obstacles.
• Sub-base quality: level, well-compacted hardcore saves concrete and time. Hollows and bumps use more concrete and make an even thickness harder to achieve.
• Flatness requirements: if the floor finish or underfloor heating goes straight onto the slab without a levelling layer, tighter level control and more careful finishing are needed.
• Reinforcement: laying and raising mesh is a job of its own, and moving through mesh during the pour is slower.
• Weather: in heat the concrete stiffens quickly and there is less time to strike off. In cold weather the slab needs protection and longer waits.
• Volume in one go: a steady supply from the plant without long gaps between trucks makes the work much simpler. If the second truck is late, a cold joint forms where the batches meet.

How long it takes

An experienced crew can usually place and strike off a typical house floor in one day, often in a few hours. Preparation such as compaction, setting rails and laying mesh usually takes longer than the pour itself.

Setting and curing

You can usually walk carefully on the slab the next day, depending on temperature. The concrete should be kept damp for at least roughly a week by spraying or covering with polythene, especially in hot and windy weather. Below roughly 5 °C, concrete should not be placed without precautions, because frost in the first days seriously damages the surface and the strength. Concrete usually reaches its design strength after about 28 days.

What comes next and what to coordinate

Waterproofing usually follows the base slab. Bituminous membranes go onto a dry, primed surface, and with torch-on membranes you have to wait until the concrete is dry enough. The membrane manufacturer sets the timing. In many regions radon is also a concern, so agree with your designer beforehand whether the membrane must act as a radon barrier and whether venting needs to be prepared. Local guidance or radon maps tell you whether your area is affected. After the waterproofing come the insulation, underfloor heating pipes and the screed.

Talking to the other trades before the pour pays off. The plumber should confirm drain positions, the electrician the cable ducts and the heating installer the height of the floor build-up. If internal walls are still to be built, agree whether they will stand on the base slab or on their own foundations.

• Poorly compacted sub-base: the slab settles over time, especially along walls and over backfilled drain trenches. It shows as cracked tiles and a hollow sound underfoot.
• No edge separation at the walls: as the concrete shrinks and moves with temperature it pushes against the masonry, and cracks appear in corners and at doorways.
• Water added on site: the concrete spreads more easily, but the surface is dusty, soft and cracks more. Waterproofing bonds poorly to it.
• Mesh at the bottom: mesh trodden down onto the polythene holds nothing together, and cracks spread freely.
• Wrong height: a slab that is too high steals space from the insulation or screed. Too low means a thicker screed and longer drying. Often the floor layer is the one who finds out.
• Poor curing: the surface dries too fast, and hairline cracks and dust appear. In summer this can happen on the first day.
• Forgotten penetrations: breaking out later damages the slab, the polythene and the waterproofing, which then needs awkward repairs.
• Pouring in frost: the surface flakes and the concrete never reaches the strength expected of it.

You can handle a small area yourself, such as the floor of a garden shed, a small cellar or a workshop. You need at least one helper, a cement mixer or a ready-mix delivery, and an aluminium straightedge. For larger areas, for living spaces with a complex floor build-up and wherever the slab is part of the load-bearing structure, leave the work to professionals. If electrical cables run under the floor, only a qualified electrician should install and connect them.

1. Dig out the soil to the required depth and remove topsoil, roots and rubble.
2. Spread the hardcore in layers and compact each layer with a plate compactor, which you can hire.
3. Mark the height of the top of the concrete on the walls and fix the edge strip around the perimeter.
4. Lay the polythene sheet with generous overlaps and, if needed, place steel mesh on spacers.
5. Set screed rails in concrete pads or on pegs and check their height with a spirit level or laser.
6. Prepare concrete of the required class, follow the mix recipe or the instructions on the dry mix bag, and do not add extra water.
7. Place concrete from the far corner, spread it slightly above the rails and tamp it down.
8. Strike it off along the rails with a sawing, forward motion, fill any hollows and strike off again.
9. Remove the rails, fill the channels with concrete and smooth the surface with a float.
10. Cover the slab with polythene and keep it damp for roughly a week.

Check the base slab before the waterproofing covers it, because mistakes are hard to fix later. A two-metre straightedge, a spirit level or laser and the datum line on the wall are all you need. Under waterproofing, the usual expectation is that no gap larger than roughly 5 mm appears under the straightedge, but the design or the manufacturer of the next layer sets the actual tolerance.

• Top of the slab matches the design height at several points across the floor
• Flatness under a two-metre straightedge is within the agreed tolerance
• Surface is continuous, with no hollows, honeycombing or loose stones sticking out
• Edge strip runs around the perimeter and the slab does not touch the masonry
• Pipes and ducts are in the right places and have not shifted
• No reinforcement shows at the surface
• No cracks wider than hairline
• Surface is not dusty and does not crumble when scratched with a key
• Concrete was cured and you know when and for how long
• You have the delivery notes from the concrete plant showing the strength class