Installing a clay beam-and-block floor

It is a partly precast floor. Some of the structure arrives on site ready-made from the factory, and the rest is completed in place by pouring concrete. The result is a single reinforced concrete slab with a clay soffit underneath.

The main parts of the system

• Clay-concrete beams have a clay shell at the bottom with a steel lattice cast into it. The lattice sticks up above the beam, and once the concrete is poured it ties the beam into the slab. The beams come in different lengths to suit the span of each room.
• Infill blocks are hollow clay blocks that sit between the beams. They come in several depths, and the structural engineer chooses the depth according to span and load.
• The concrete topping is the layer of concrete above the blocks, usually roughly 4 to 6 cm thick. A welded steel mesh goes into it. The concrete also fills the gaps between the blocks above the beams and forms ribs.
• The perimeter ring beam is a reinforced concrete band running round the edge of the floor on the loadbearing walls. It ties the floor to the masonry and stiffens the whole building.
• Temporary props are adjustable steel props with timber or steel bearers that hold the floor up until the concrete has gained enough strength.

Why clay

The clay soffit is the same material as the brickwork, so plaster on the ceiling bonds in much the same way as on the walls. The floor is lighter than a solid concrete slab of the same depth, which makes it easier to size the walls and foundations. The hollow blocks also help a little with airborne sound from the room next door. Impact noise from footsteps still needs a floating floor on top.

A clay beam-and-block floor works well for simple house layouts with ordinary spans. The beams are light enough for two people to carry and place by hand. On site you only need a mixer, or a ready-mix truck with a concrete pump for the pour.

Compared with other floor types

• A cast in-situ reinforced concrete slab handles complex shapes, longer spans, cantilevers and balconies. However, it needs formwork over the whole area, more reinforcement and more concrete. The slab is heavier and takes more labour.
• Prestressed hollowcore planks go in quickly, often in a single day, and you can walk on them straight away. They need a crane, good access to the plot and accurately built walls. They are less suited to irregular plans.
• Lattice girder slabs (often called filigree slabs) are thin precast panels with reinforcement, and the rest of the depth is poured on top. They give a smooth concrete soffit and also need a crane.
• A timber joist floor is light and quick. It makes sense in timber-frame houses or over the top storey under the roof, but it is less effective against noise and is not as stiff as a concrete slab.

Where the system reaches its limits

For large open spaces without intermediate support, the engineer will often advise against it, or specify deeper blocks and more closely spaced beams. Cantilevered balconies cannot be formed with this system directly. In practice they are cast in situ and connected to the rest of the floor. In a renovation where an old timber floor is replaced with a clay beam-and-block floor, someone has to check that the existing walls can carry the new, heavier structure. Do not go ahead without an assessment by a structural engineer.

A floor is part of the loadbearing structure. The crew builds it to the design, not by eye. Most problems start before the first beam is even laid.

Design and approvals

Every floor of this type needs a laying plan. It sets out the type and length of the beams and where each one goes, the depth of the blocks, where beams are doubled and where trimming is needed around openings. It also specifies the ring beam reinforcement and the concrete strength class. The structural engineer prepares the laying plan, or the manufacturer's technical department does it from the building design. Without it, the manufacturer usually cannot tell which beams to supply.

A new house and a floor replacement in an existing house both involve the loadbearing structure. Check with your local building authority what approval your project needs under local building regulations. Do not assume that a simple notification will be enough. In a block of flats, any work on a floor must also be agreed with the building management or owners' association, because the floor is a shared part of the building.

Site readiness

• The loadbearing walls must be finished to the design height and level. If the wall heights differ, the beams will sit unevenly and the floor will be out of level.
• The surface under the floor, whether a ground slab or the floor below, must be able to carry the props. On soft fill without sole plates, props sink in.
• The materials should be on site and stored so they do not get damaged. Store beams flat on bearers, not leaning against a wall.
• There must be enough props and bearers. The number depends on the spans and the manufacturer's recommendations.
• The electrician and plumber should know in advance where light fittings, drainage penetrations, ventilation ducts and the chimney will go.

Safety

The work takes place at height, above open space. The floor edges need fall protection, and stair openings must be covered or guarded. Temporary site electrics, especially the site supply, should only be handled by a qualified electrician. This is not a job for amateurs. It is a loadbearing structure, so this page deliberately contains no DIY guide. On a self-build you can help as labour, but experienced people and a site supervisor should run the method and do the checks.

Levelling and preparing the bearings

Where the beams will rest, a bed of mortar goes on top of the wall to even out small irregularities. Each beam must sit on its full bearing area, not just on the edge of a brick. The manufacturer and engineer specify the bearing length, usually roughly 12 to 15 cm at each end.

Setting up the props

The props go up either before the beams are laid or straight afterwards. Bearers run at right angles to the beams and sit on the props. The manufacturer sets the spacing, which is usually around 1.5 to 1.8 m. On longer spans, the props in the middle are set slightly higher so that the floor settles level once loaded. The design sets this pre-camber and its size. The crew should not guess it.

Laying the beams

The beams are laid at centres that match the block width. The first two go down, one block is slotted between them at each end as a spacer, and the position of the rest follows from that. Partitions running across the beams need no special measures. Partitions running parallel to the beams often need doubled beams, as specified in the design. Around openings for stairs, a chimney or a service shaft, the beams are trimmed. This means they are cut short and their load is transferred to strengthened beams on either side or to a cast in-situ beam.

Placing the blocks

The blocks are laid dry between the beams, without mortar, tight against each other. The end blocks at the ring beam are closed off so concrete cannot run into the hollows. Blocks with a cracked bottom face do not belong in the floor, because they could later fall away together with the plaster. Nobody walks directly on the blocks. The crew lays boards or planks to spread the load onto the beams.

Ring beam formwork and reinforcement

The outer face of the ring beam is either shuttered, or special clay edge blocks are used with insulation placed behind them. The insulation prevents a cold bridge at the ring beam. Reinforcement goes into the ring beam to the design, usually four longitudinal bars tied with links. The mesh is laid over the blocks on spacers so that it ends up inside the concrete, not at the bottom of it.

Services and penetrations

This is the last chance to fit electrical boxes for ceiling lights into the blocks, cable conduits in the topping and sleeves or pipes for penetrations. Anything forgotten now will have to be drilled through the finished floor, and drilling into reinforced ribs always carries a risk.

Inspection before the pour

Before the concrete goes in, the site supervisor or structural engineer should inspect the reinforcement and bearings. It is good practice to record the inspection in the site diary. Once the concrete is poured, nothing can be corrected.

The pour

Just before pouring, the blocks are soaked thoroughly with water. Dry clay would draw water out of the concrete, and the concrete in the ribs would not cure properly. Concrete of the specified class is placed evenly, not dumped in one heap. It is compacted with a poker vibrator, especially in the ribs and ring beam where air pockets easily form. The surface is then screeded level with a straightedge.

Curing

The first few days after the pour decide the quality. The surface is kept damp by spraying or by covering it with sheeting, usually for at least a week, and longer in hot or windy weather. Below roughly 5 °C, concreting should not go ahead without cold-weather measures.

The difficulty, and so the amount of labour, can vary a great deal between projects, even when the floor area is the same.

Easier

• A simple rectangular plan with equal spans and one beam type
• Accurately built, level walls that need no extra levelling
• Good access to the plot for a ready-mix truck and concrete pump
• Materials delivered on time and stored close to the building
• A finished laying plan and clear positions for all services

Harder

• An irregular plan, angled walls and many different beam lengths
• Lots of penetrations, trimmed openings and doubled beams
• A tall storey that needs longer props and scaffolding
• Combination with cast in-situ parts such as a balcony or a downstand beam
• A narrow or sloping plot where getting the concrete in is awkward
• A renovation where the floor goes into old masonry, often with an uneven wall top and demolition work
• Bad weather, heat or cold, which calls for extra curing measures

When you compare quotes, check what the per-square-metre item actually includes. Some cover only laying the beams and blocks. Others also include props, ring beam, reinforcement and the pour. The difference in scope is often bigger than it first looks.

The floor is where many trades meet. The better they agree things in advance, the less cutting into the finished structure there will be later.

• The bricklayer must build the walls to exactly the height in the design. Floor level affects the staircase, room heights and window heights on the storey above.
• The structural engineer prepares the laying plan and inspects the reinforcement before the pour.
• The electrician fits the light boxes and conduits before the pour.
• The plumber and heating engineer mark penetrations for drainage, water and heating so nothing needs drilling later.
• The chimney installer specifies the opening size and the clearances from combustible materials.
• The concrete supplier delivers concrete of the right class and consistency at the agreed time. The pouring crew must be ready before the truck arrives.
• After the floor come the walls of the next storey or the roof structure, and later the floors and the ceiling plaster. Plaster goes straight onto the clay, so a level floor saves the plasterer work.

The same mistakes come up again and again in practice. Most come from rushing or from trying to save on props.

• Too few props or props too far apart. The beams bend under the weight of wet concrete, and the floor stays permanently sagged. You see it as an uneven ceiling and cracks in the plaster.
• Short or uneven bearings on the wall. A beam can twist or break off the edge of the masonry. Later, cracks appear in the corner between wall and ceiling.
• Dry blocks during the pour. The concrete in the ribs loses water, cures poorly and ends up weaker. Hairline cracks form on the surface.
• Mesh lying at the bottom of the concrete. Without spacers, the mesh sinks onto the blocks and does nothing. The floor surface may then crack.
• Poorly compacted concrete. Honeycombing and voids remain in the ribs and ring beam. The structure is weaker and the reinforcement may corrode.
• End blocks left open. Concrete runs into the hollows, the floor becomes unnecessarily heavy and concrete is missing where it should be.
• Props removed too early or the floor loaded too soon. Putting pallets of bricks on a young floor, or striking props after a few days, leads to sagging and cracks.
• Forgotten penetrations. Drilling afterwards can cut through the reinforcement in a rib. A larger opening needs an engineer's assessment.
• A partition parallel to the beams without strengthening. The beam under the partition gradually deflects and a crack appears in the partition.

Laying the beams and blocks itself is quick. On an ordinary house, an experienced crew usually lays one storey in one to two days. Ring beam formwork and reinforcement, mesh and services take another day or two. The pour for one storey is usually done in a day.

The waiting takes longest. Concrete reaches its design strength after roughly 28 days, and the props usually come out then, unless the engineer or manufacturer says otherwise. Some methods allow gradual easing earlier, but only on the design's instructions. In cold weather concrete hardens more slowly, so the time gets longer. You can walk carefully on the floor after a few days. Do not store heavy materials on it until the conditions in the structural calculations are met.

If you want to start the walls of the next storey, ask the engineer when that is possible and whether the props must stay in place. Building on a propped floor is common, but it must be agreed.

When you accept the work, focus on things that are hard to fix later. Some checks happen before the pour, others after the props are removed. It is a good idea to have the site supervisor with you.

• Beam type, lengths and positions match the laying plan
• Beam bearings on the walls have the specified length and sit on a mortar bed
• Doubled beams and trimmed openings are where the design shows them
• Blocks are undamaged, tightly laid, and end blocks are closed
• Ring beam reinforcement and mesh are placed to the design and sit on spacers
• The pre-pour reinforcement inspection is recorded in the site diary
• Light boxes and penetrations are in the agreed positions
• The concrete delivery note confirms the specified strength class
• The topping surface is level, without larger cracks or honeycombing
• The soffit is flat after the props are removed, with no visible sag
• There are no cracks where the floor meets the walls
• The props were removed no earlier than the design or engineer allowed