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.
Cutting and trimming edges
Once the bulk area has been laid, you will need to cut pieces of hardboard to fit the edges. For rectangular pieces, simply measure the width between the last board and the skirting, and mark up a piece accordingly.
Cut the boards with a sharp general purpose knife, using a straight metal edge as a guide.
Alternatively, if you have a jig saw, you will find that this will make very light work of cutting boards.
Cutting hardboard round frames
The easiest way to cut awkward shapes is to make a paper template first. Use old newspaper or a piece of lining paper. Lay its square edge against the last board and, using scissors, cut the other edge so that the paper can be contoured to the awkward shape. Fold the strips back so that the paper fits the shape perfectly, and trim to this shape. Now lay the paper over the piece of hardboard and mark and cut this shape.
Lay exterior grade plywood panels in the same way as hardboard but leave an expansion gap of at least 10mm all the way around the edge of the room.