The subfloor is the layer of structural sheathing applied directly to the joists that provides a base for all the finish floors to come. Some types of flooring, such as carpet or traditional hardwood, can be installed directly on top of the subfloor. Others—such as tile, vinyl, and some engineered wood products require an additional layer called underlayment before they are installed.
The most important function of a subfloor is to create a structural diaphragm that helps to distribute wind and seismic loads through the house frame. That’s the main reason subflooring has a code-specified nailing schedule (every 6 in. on edges parallel to joists, and 12-in. spacing in the field). But perhaps the most obvious reason that proper subfloor installation is important is to minimize floor squeaks down the road. If subflooring panels can move against the joists or abutting sheets, they will squeak. Gluing the panels down in addition to nailing them is considered best practice for eliminating squeaks, although it’s not included in the structural calculations, nor is it required by code. Some engineered floor specifications, however, may require the use of adhesive. Whenever working with engineered floor systems, be sure to follow the manufacturer’s instructions to the letter.
New materials solve old problems
Most subflooring used these days is either 3⁄4-in. plywood or 3/4-in. OSB, one long edge of which has a tongue and the other long edge a matching groove. Tongue-and groove subflooring was pretty new to the market when I started framing in the 1980s. Prior to that, the standard was regular old 5/8-in. or 3/4-in. CDX plywood. To support the edges of the sheets so that they didn’t sag between the joists, carpenters installed blocking where the long edges of the sheets would meet. This required extra material for the blocking as well as a fair amount of extra work.
The introduction of tongue-and-groove subflooring eliminated the need for the extra work of installing the blocking because the sheets support each other when their edges interlock. Getting the sheets to go together could sometimes be challenging, however, particularly if they’d been exposed to much moisture prior to installation. The solution with the commodity sheets I saw in years past was to beat them together with a sledgehammer, using a length of 2x to cushion the blows.
Even after subflooring has been installed, extended exposure to the elements often can lead to trouble. Plywood subflooring can delaminate, and OSB is famous for its edges swelling (sometimes to the point where the joints must be sanded down prior to the finish flooring being installed). The market responded to these problems with premium grades of subflooring, such as the Huber AdvanTech used here. The company promises that AdvanTech can be exposed to the weather for 500 days with no edge swelling severe enough to require sanding, and says that carpenters should never need to use more than a few taps with a block and a framing hammer to drive the sheets home.
Types of subfloor
Plywood is the most common material used for subfloors. Plywood has several properties that make it a useful and popular for this purpose, including:
- High strength.
- High panel shear.
- Moisture resistance.
- Chemical and fire resistance.
- Impact resistance.
The thickness of a plywood subfloor depends on the distance between joists. Thinner plywood can be used when joists are close together, but slightly thicker plywood is recommended when there is more than 40cm between joists.
Subfloors can also be made from oriented strand board (OSB). OSB has similar properties to plywood and is suitable for load-bearing applications. It can be more cost-effective than plywood. Impermeability to water can be achieved through the use of additional membranes.
Another type of subfloor is high-performance engineered panels. Like plywood and OSB, these panels are designed for load bearing applications. They have built in moisture resistance and are manufactured with special resins to reduce other issues – such as swelling associated with some subfloors.
- Caulking gun
- Circular saw
- Combination square
- Ear muffs
- Measuring tape
- Nail gun
- Safety glasses
- Trimming knife
- Bituminous flashing
- Cement sheet packers
- Construction adhesive
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.
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.
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.
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.
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.
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