Project Overview

The subfloor is a fundamental part of any building structure. It can seem like a big job, but we’ll show you how you can do it yourself. You’ll learn how to install bearers and joists for your subfloor. You will also see how to level and reinforce the structure. Plus we explain how to tie a floor down and laminate your bearers when you need extra strength.
Continue to step-by-step instructions
This D.I.Y. Advice is part of a series How to build a subfloor

Step by Step Instructions

1 Laminate the subfloor bearers
2 Install the subfloor bearers
3 Level the subfloor bearers
4 Stagger the joins in the subfloor joists
5 How to join a subfloor joist
  • Step 1. Laminate the subfloor bearers

    Subfloor bearers need to be strong enough to hold the rest of the floor up. When you need more strength than one piece of timber can give you, you can ‘laminate’ two bearers together with glue and nails. Apply a generous amount of construction glue on the surface of one bearer. Place a second bearer on top of the first and slide it around to spread the glue. Then line the two pieces up so their edges are flush and double nail them along the whole length.

  • Step 2. Install the subfloor bearers

    Sit each bearer in place on your foundations. If your foundations are brick or concrete, keep the bearers protected from moisture by putting a piece of waterproof flashing in-between the timber and the foundations. Make sure the bearers stay clear of any other brickwork or concrete to help keep them dry. Tie down the bearer next to the end wall with hoop iron, which is preinstalled in the brickwork.
  • Step 3. Level the subfloor bearers

    Put a spirit level on top of each bearer to check that it is sitting level. If a bearer is uneven and needs adjustment, use cement sheet offcuts as packers to build up one end. Place the packers under the bearer where it is sitting on the foundations.
  • Step 4. Stagger the joins in the subfloor joists

    Sometimes a joist needs to be longer than one piece of timber. In that case, each joist will have a join in it. To improve the strength of the subfloor, these joins should be staggered. Install the short end of the joist on one side for the first joist, then swap sides for the second joist. Continue this pattern across the subfloor. 
  • Step 5. How to join a subfloor joist

    There are two types of joins you can use to connect timber in a floor joist. When your bearer is wide enough, join the two pieces of joist together end-to-end. This is called a butt joint. When your bearers are thinner, it is better to overlap the pieces side by side. This is called a lap joint.

Tools and Materials

Tools

  • Caulking gun
  • Circular saw
  • Combination square
  • Ear muffs
  • Hammer
  • Measuring tape
  • Nail gun
  • Pencil
  • Safety glasses
  • Trimming knife

Materials

  • Bituminous flashing
  • Cement sheet packers
  • Construction adhesive
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