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How to install a subfloor

A subfloor of treated timber bearers and joists

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'll 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.

Steps

1Laminate 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.

A person applying adhesive to a timber bearer

2Install 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.
A person lifting a bearer onto some packing sitting on a course of bricks

3Level 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.
A person angle nailing a joist to a bearer using a nail gun

4Stagger 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. 
A subfloor of treated timber bearers and joists

5How 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.
A subfloor of treated timber bearers and joists
Health & Safety

Please make sure you use all equipment appropriately and safely when following the advice in these D.I.Y. videos. You need to be familiar with how to use equipment safely and follow the instructions that came with the equipment. If you are unsure, you may feel it is safest to consult an expert, such as the manufacturer or an expert Bunnings Team Member.

Grave health hazards are linked to asbestos, which may be in homes built up to 1990. Health hazards may result from exposure to lead-based paints in older materials and copper chromium arsenic (CCA) treated timber. For information on the dangers of asbestos, lead-based paint and CCA treated timber and tips for dealing with these materials contact your local council's Environmental Health Officer. You can also use a simple test kit from Bunnings to indicate the presence of lead-based paint.