Casting a Solid Floor

Filed Under: Do it yourself, Home repair, Remodeling    by: ITC

The first job is to remove the old floor and dig out the ground below to at least 12in below the final floor level. Lever up an old wooden floor and saw through the joists for easy removal. Then demolish the dwarf walls. Break up a concrete floor: the best way is with a rented jack hammer. Keep the rubble for use as a bed for the concrete later. Wear stout boots, thick gloves, overalls and safety goggles.

Find the flashing in the walls; it may be a layer of slate or bitumen, or one or two courses of engineering bricks. If necessary, chip away the plaster to find it.

If the floor in the next room is of suspended wood, lay plastic drain pipes between any airbricks and the inter-connecting door threshold, setting them in place with bits of stone or brick: this is vital to provide ventilation throughout the floor. Build a retaining wall across the threshold with concrete building blocks.

Next make up some datum pegs from 2 x tin sawn softwood marked with the depths of the bed and concrete subfloor layers: 6in and 4in respectively. Cut a point on one end of the pegs then drive a peg into the ground near a given reference point, to indicate the surface of the floor. Drive the other pegs in at 3ft intervals, checking that their tops are level with the first.

Put down the brick and stone bed, leveling it with the marks on the pegs and compacting it well with a purpose-made tamper. Spread a layer of damp builder’s sand over the top to fill any voids.

The concrete for the subfloor should be of 1 part cement: 21/2 parts concreting sand: 4 parts gravel.

Lay the concrete so that it is level with the tops of the pegs, tamping it down well and drawing a stout batten across the tops of the pegs to level it. Fill any hollows with more concrete then tamp again.

When the concrete has cured, lay the cleavage membrane. With bitumen emulsion apply about three coats, taking it up the wall to the flashing. If plastic sheet is used, tack it to the walls above the flashing. Fold the corners and overlap the sheets by 8 to 12in, sealing the join with building adhesive.

Use 1 x 2in battens to divide the floor into 3ft wide bays for the finishing screed. Set them in place with dabs of mortar, level if necessary by packing offcuts underneath: check with a spirit level.

Fill the bays with a 3:1 mortar mix and draw it off level with a straight-edged batten held across the tops of the dividing battens. When two bays have been completed, lift out the batten in between and fill the resulting slot with mortar. Then trowel both bays smooth with a metal trowel. When the mortar has stiffened, give it a final polish with a wetted trowel.

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