Before you get started

A subfloor provides a base for a floor covering and is applied to the surface of an existing floor, although in some cases the floor itself may be an appropriate base for the new floor covering. All subfloors must be flat, even, clean and completely dry.

If laying flooring on a new concrete floor, make sure it is completely dry. Do not lay flooring onto recently laid concrete. The thickness of the concrete will determine the drying time, but in all cases a minimum of two months is required for concrete to dry thoroughly. Allow one day per mm for the first 50mm and then 1½ days for each additional mm of thickness thereafter.

Check existing screeds and concrete for moisture. This can be carried out using a moisture meter or alternatively sheets of polythene approximately 1m2 can be taped to the screed and a heavy weight placed on top for 24 hours. The screed will discolour or moisture will appear under the polythene if there is any present. If this happens you should stop and seek expert advice.

The most other common subfloors are made of plywood (laid on a wooden floor), hardboard (also laid on wood) and self-levelling compound, overlaid to smooth and level a concrete subfloor.

These subfloors will raise the floor height and stop doors from opening or closing as normal, but don't trim doors until the completed floor covering is laid. It may be possible to disguise slight differences in floor levels between rooms with a threshold strip.

Use suitable 18mm plywood as a subfloor for hard tiles and thinner ply to provide a smooth surface for floor coverings such as vinyl, carpet and soft tiles.

Hardboard is generally laid smooth side up, especially when floor coverings are to be stuck down, such as with vinyl tiles. Check which side up a manufacturer recommends for its particular floor covering. For example, some sheet vinyl manufacturers recommend laying the hardboard rough side up. Always use hardboard that has been graded, or conditioned, for flooring use.

The order in which plywood or hardboard is laid is not important, but the joints between rows should be staggered to avoid long joins, which might create a weakness.

Fix loose floorboards

1

Loose, damaged or creaking boards are common. Use a detector to help avoid cables and pipes and find a safe fixing position.

2

When sure that no services are below the boards, drill a pilot hole on one side of the loose board, through into the joist below.

3

With the right length screw to go through the board and into the joist, screw the board down securely until it is flush with the adjacent board.

4

If the board is to be left exposed, use nails to hold the board down. As with the screw, ensure the nail is long enough to go through to the joist.

5

Use a nail punch to knock the nail head below the surface and repeat, if necessary, on the other side of the board.

Lay hardboard

1

Stapling is the quickest way to secure hardboard, although ring-shank nails (15-20mm ideal, shown here) can be equally efficient. Fix at 15cm intervals.

2

Butt the second board tightly up against the first sheet. As you place each board keep them as neatly aligned as possible.

3

Work across the room in rows and stagger the joints between the hardboard sheets on subsequent rows until only edge gaps are left.

4

Measure and cut pieces for the edges. To cut a straight line in hardboard, use a steel rule and a retractable knife. Score the smooth side of the board.

5

Turn board over, lay rule along the scored line, bend the board up along the line and separate with a retractable knife. Fill in edges with cut pieces.

Lay plywood

1

When laying the plywood, make sure that no floorboards are loose, as a subfloor must be laid on a surface that is as rigid as possible.

2

Start in a corner and lay the first piece of plywood as tightly as possible. Secure with screws every 15cm along the edge.

3

Also fix with screws at 15cm centres, in other words, at 15cm intervals around the edge and in lines across the centre of the sheet as well.

4

The screws should go into the floorboards but not through them. Butt joint the sheets tightly and continue laying them across the floor.

5

If sheets of ply need to be cut to fit, use a panel saw for straight cuts and a profile gauge and jigsaw to make cuts around architraves or pipes.

Level a concrete floor

1

Prepare the floor by sweeping it thoroughly to remove all dust and debris before you use the self-levelling compound.

2

Prime the floor surface as directed by the self-levelling compound manufacturer. Allow the primer to dry before continuing.

3

Mix self-levelling compound as directed by the manufacturer. A drill/ driver with a mixing paddle is ideal. Be sure to wear safety glasses.

4

When you are happy that the compound is smooth and contains no bubbles, pour it onto the floor and allow it to settle to its own level.

5

Mix further compound until the floor is covered. Use a plastering trowel to smooth any lumpy areas. Leave to dry. Work quickly as it will set swiftly.