How to Replace a Suspended Floor with Concrete
Replacing a suspended floor with concrete
There may be several reasons for replacing a suspended floor with concrete. One of the most likely scenarios is when the original floor has suffered from rot or infestation and will have to be replaced. In older houses unprotected by a damp-proof course a concrete floor might be installed as part of the overall damp-proofing measures in the property.
Tools for the Job
- tape measure & pencil
- spirit level
- hand saw
- bolster & chisel
- small cement mixer
- tamping board
- plasterer’s trowels
- Before stripping out the old flooring, make a pencil mark 1m (3ft) up the wall. Draw a line around the room at this height with a spirit level — this is your datum mark.
- Strip out the old floorboards and joists. Joists are easier to handle cut into sections, especially on older floors where they will need to be pulled out of wall sockets. Ensure all traces of the old joists have been removed, this is especially important if has been present.
- Shovel in a layer of hardcore so that its top level is 1.3m (4ft 3in) below the level of the datum mark on the wall. Use a punner to tamp it down and break up any large lumps.
- The next stage is to lay on the sand blinding. Shovel builder’s sand on top of the hardcore to a depth of 100mm (4in), all the time checking the height against the datum marks on the wall. You may find it helpful to cut a batten to 1.2m (3ft 11 in) rather than keep measuring with the tape measure. Smooth it down with the back of your shovel as you go and use a long board to make certain that the sand is flat.
- Lay a sheet of plastic damp- proof membrane on top of the sand, taking care not to puncture the sheet. Ensure that it lips up the wall by about 300mm (1ft). Use some strips of duct tape to hold the membrane against the wall.
- Reinforcing bars strengthen the concrete and prevent it from cracking. Place the bars raised up on concrete garden slabs. Make a criss-cross pattern, at 400mm (1ft 4in) centres, tying any joints with binding wire. Keep the bars at least 100mm (4in) clear of any wall. Once finished, the grid should be fairly rigid and should not sag if you walk on it.
- Provided another person helps, for a small room it is perfectly feasible to mix the concrete yourself with a small mixer. Use a mix of 1 part ordinary Portland cement, 1 part building sand and 3 parts aggregate. There is no need to be too precise, just measure each of the components out by the shovel-full. Do not make the concrete overly wet — add just enough water to mix the components into a grey creamy consistency.
- Rather than simply spreading the concrete in a random fashion, start in one corner of the room and work back towards the doorway. Use the shovel to work the concrete down between the reinforcing bars, forcing out any trapped air.
- Check the top level of the concrete against the datum with a batten cut to 1m (3ft), to ensure the finished floor will not be lower than adjacent rooms.
- Tamp the concrete surface with a board or piece of 18mm (11/16in) thick ply. Starting at one end of the room, work the top of the concrete with the tamp edge. Do not apply great force but simply stipple the surface slightly. If the room is over 2.5m (8ft) wide you will need a helper. If you work towards the doorway tamping as you go, the holes left by your boots will refill.
- After a couple of hours water will have come to the surface before it starts to sink back into the concrete. At this point you can trowel the surface to a smooth finish with metal plasterer’s trowels. Spread your weight on two ply boards about 800mm (2ft 7in) square. Gently lift the leading edge of the trowel to stop it digging in as you drag it across. Work back towards the doorway, moving the boards as you go.
- You can walk on the concrete the next day, but leave at least three days before trimming off the plastic and fitting skirting. Wait at least three weeks if laying carpet as moisture remaining in the concrete will rot the backing.
Keep cement away from skin and eyes. Wash off splashes as soon as possible and if you get any in your flush it with plenty of water and seek medical advice.
Tips of the Trade
When trowelling the concrete surface it helps to have two trowels, one for smoothing and the other to spread the weight of your other hand so you do not damage the surface.