Laying a concrete floor

Summary: Learn how to lay a concrete floor and how to mix and lay screed.

A concrete floor is usually laid during renovation work to replace a rotting timber floor. Before laying a concrete floor, you must consult your local Building Control Department as changing a floor can affect the ventilation in other parts of the building and it is possible that insulation will also be required. If infill material of more than 600mm is required a new suspended floor must be fitted instead, as the settlement of the infill material may result in damage to the new concrete floor.

Preparation

Remove the skirting board, take up the old timber floor and joists and remove any doors.
Treat the ground and the walls below the old floor level with fungicide.
When removing the floor joists it is more than likely that recesses will be left in the wall. Fill them with bricks and mortar.
Measure and chalk a line around the room, using a spirit level, to mark the height of the new floor. Label this line A. This line marks the top of the screed. However, if you are planning to lay a thick floor covering on top of the new floor such as quarry tiles, allow for the depth of the tiles.
Measure down from line A 25mm and draw a line to mark the top of the concrete layer, and label this B.
From B measure down 100mm and mark this line C to indicate the top of the damp-proofing and insulation layer.
From C measure down 50mm and mark this line D - the top of the sand layer.
From D measure down another 50mm to mark the top of the hardcore layer which should be marked E.
From E measure down another 100mm to reach the base of your new concrete floor.
The total minimum depth required is 325mm from line A.

Infilling

Before concreting you need to infill the area with layers of different materials.

The first layer is the hardcore, which can be rubble made up of old bricks or tiles, but break up any large pieces with a sledge hammer. Alternatively you can use coarse stone. Remove any wood, plaster or metal from the hardcore as it can have an unfavourable reaction with cement.
When you have reached level E, compact the hardcore with a compacting plate.
Cover the hardcore up to level D with a layer of sand, which should be tamped down and smoothed with the back of a shovel being moved in a circular motion.
Cut and lay a sheet of damp-proof membrane of 1,000 or 1,200 gauge thickness over the sand, ensuring that it also reaches up the wall to just below line A. If more than one sheet is required, overlap the sheets by 200mm (8in) and seal any joins with waterproof tape. Tape the membrane to the wall.
Lay the insulation board over the membrane, taping any joins.
Cut four lengths of insulation board to go around the walls. The lengths should be 100mm (4in) wide to reach line C, and will act as a mould for the concrete.

Laying the concrete

Mix up some concrete and fill the mould. The mix should be:
1 part cement
2 1/2 parts sand
4 parts aggregate
Don't add too much water as the mix needs to be quite stiff.
Starting at the wall furthest away from the door, shovel the concrete on to the floor area. Work across the floor area completing sections about 600mm at a time.
Use a length of 100mm x 50mm timber as a straight edge. Using the top of the insulation board at the sides of the room as a guide, scrape the straight edge forward to level the concrete.
Use a trowel to deposit concrete in any hollows. Once you have the concrete aligned with level C. Smooth the concrete with a plastering trowel.
Begin the next section and continue until the floor area is completed.
Cover the concrete with a polythene sheet to prevent rapid drying and possible shrinkage. Leave for a minimum of three days so the concrete sets.

Laying the screed

A screed (layer of mortar) should be laid over the concrete to produce a smooth surface. The floor screed mix should be:

3 parts sharp sand
1 part cement

Before laying the screed, dampen the concrete floor using water and a bonding agent mixed in equal parts.

Position two or three 38mm battens (depending on the width of the room) on the concrete running in the direction you are working.
As you did when laying the concrete, work across the floor area completing sections about 600mm at a time. Layer the screed up to the level of the top of the battens.
Smooth the screed with a plastering trowel. When you are satisfied the screed is level remove the batten and fill in the gaps.
Smooth over the surface with a float.
Continue across the floor until the whole area is covered.
Cover with polythene sheets and leave to dry for one week. You can then fit skirting boards to cover the damp proof membrane visible above the surface of the floor.
The floor will take a few months to dry out completely. Do not lay an impermeable floor covering during this period.