Laying a plywood subfloor

Summary: Learn how to lay a plywood subfloor, lay a subfloor and cut plywood.

A subfloor will provide a smooth, level surface upon which to lay a floor covering. The type of wood board you use will depend on what the existing floor is made from and the type of floor covering you want to lay. The most common form of wooden subfloor is plywood. For heavy flooring, such as hard tiles the subfloor should be thick plywood, but for soft flooring (vinyl, carpet and soft tiles) thin plywood or even hardboard with the smooth side facing up will be sufficient.

Plywood

Plywood comes in a variety of thicknesses: 4mm (1/6in), 6mm (1/4in), 9mm (2/3in), 12mm (1/2in) and 18mm (3/4in). It is also available in different size sheets. If you are laying a subfloor over a large area, the larger the sheet the quicker you will cover the floor. 

A plywood subfloor will raise the floor’s height and prevent doors from opening smoothly, so be aware that you may need to make alterations to any doors that open into the room.

Preparation

  • As fitting a plywood subfloor will involve hammering nails into the existing floorboards ensure the nails you use will not penetrate the underside of the floorboard and risk damaging underfloor pipes and cables.
  • Inspect the floor for any protruding nails or screws, punch or countersink those you find below the surface.
  • A subfloor must be laid on a rigid surface, so ensure that all the floorboards are secured firmly.
  • Sweep and vacuum the floor to remove any dust and debris.
  • As this project involves working in a kneeling position you may want to use kneepads.
Ensure you stagger the joints, leaving an expansion gap between the edge of the board and the skirting.

Laying the subfloor

  • Select a corner of the room from which to start that is as square as possible, but leave expansion gaps of 10mm-15mm (about 1/2in) between the edge of the board and the skirting. The distance of the gap can be maintained by inserting spacers made of off-cut timber. These can be removed and the gaps filled in later with small strips of cork.
  • Fix the plywood to the floorboards with ring shank nails every 15 mm (6in) around the edge of the board.
  • Lay the second sheet of plywood, tightly butting the edges of the two sheets together to create a butt-join. Leave a gap the size of a credit card between the two sheets.
  • Continue laying the first row of boards in the same fashion across the room. When you are ready to begin the second row ensure you stagger the joints.
  • Lay all the whole sheets first and then you can fill in the areas that need cut sheets.
Mark the shape of the obstacle on the frayed paper and transfer this to your board.

Cutting plywood

  • Measure the areas you need to fill and mark the dimensions onto the plywood. Use a panel saw to cut straight lines.
  • To fit plywood around irregular shaped objects, such as the pedestal of a bathroom sink, you will need to make a template. This is easily done by taking a piece of paper or thin card cut to the exact size of the piece of plywood you are looking to fit. 
  • Cut a series of slits along the edge of the paper that faces the obstacle. Then fit the piece of paper in the same position where the plywood is to go. Press the frayed edge of the paper up against the obstacle and mark the shape of the base of the obstacle on the paper.
  • Cut along the line and transfer the paper to the board. Mark the shape onto the board and then use a jigsaw to cut the shape out of the board. Nail the cut piece of plywood into place.

Tools needed

  • Nail punch
  • Hammer
  • Panel saw
  • Jigsaw

Materials needed

  • Ring shank nails

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