Hardboard provides a smooth, even surface over the top of floorboards. You can then lay carpet, vinyl sheet, floor tiles or laminate flooring. Use plywood panels to support heavier floor tiles, such as slate or terracotta.
Ensure there is plenty of ventilation under the floorboards by checking there is an airbrick opening into the room. If not, install an airbrick. Check for any signs of damp or mould and cure before fitting the hardboard.
Prepare the old boards by punching any protruding nail heads below the surface with a hammer. You may need to use a power plane to level any very uneven boards. Screw down any creaking or loose boards.
Bring the 6mm hardboard into the room and brush a litre of water over the rough side of each panel. Leave for at least 24 hours to allow the boards to adjust to the humidity of the room. This will cause the boards to expand a little. When they dry out, they will pull tight and lay flat. If you don't do this, and the room gets a bit damp, the hardboard will swell and 'pucker up'.
It's worth taking up one or two of the old floorboards and noting the positions of any wiring or pipes. Use a felt pen to mark the wire and pipes on the hardboard's surface. Fit narrow strips of hardboard along these lines so they can be removed to repair any faults later.
Laying the hardboard
Lay the sheets rough side up, across the floorboards. If you have one long straight wall, you can start by laying boards along this. Remember, if there's room, to tuck the boards under the skirting a little. Ensure that the panels fit squarely to one another and that the edge for the next row is straight.
If you don't have a long square wall, measure the width of each end of the room, divide by two and mark the centre. Use a chalk line held between these points, to strike a line along the floor. Now, lay the first row of hardboard panels along this line.
Continue, laying the next row, and stagger the joints and fix with diamond-headed panel pins every 100mm around the edge and at 150mm centres in the middle. Make sure the nail heads are flush with the hardboard surface.
Slide the boards just under any skirtings to give a smooth edge.