DIY: Tips for Building a Subfloor

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DIY: Tips for Building a Subfloor

Courtesy of Smartnail

A finished wood floor installed over a proper subfloor.

What is a subfloor?

The subfloor is the base for the finished flooring which is usually built on top of joists diagonally or at right angles. It is covered with an underlayment and then the finished flooring. They can also be built on top of concrete slabs which is common for basements. Subfloors are generally made of plywood or OSB which are attached using either nails or screws. Screws can be more expensive but dramatically reduce floor squeaks. Often, an adhesive is used to attach the floor panels to the joists as well as your chosen fastener. The Appliance Analysts offer a good synopsis of different adhesives if you are interested. Your subfloor panels can come in different thicknesses which is usually determined by the type of finished flooring that you will be placing on top. The stiffness or rigidity of your subfloor panels also matter depending on what is on top. Heavy tile shouldn’t be placed on a subfloor that is somewhat flexible which can lead to cracking.

 

What are the regulations for subfloor installation?

A power drill with a bit next to blueprints and screws for a home construction project.

It is important to know the building code requirements for your jurisdiction during any construction project. The Ontario building code section 9.23.14 covers regulations for subflooring. The code outlines when a subfloor is required as well as the materials, thicknesses, and fasteners required. It is important to ensure that you choose the correct material at the correct thickness. See the tables below for an example:

Table 9.23.14.5.A.
Thickness of Subflooring
Forming Part of Sentences 9.23.14.5.(1) and 9.23.15.7.(1)

Item

Column 1

Maximum Spacing of Supports (mm)

Column 2

Minimum Thickness (mm) Plywood & OSB, O-2 Grade

Column 3

Minimum Thickness (mm) OSB, O-1 Grade, & Waferboard, R-1 Grade

Column 4

Minimum Thickness (mm) Particleboard

Column 5

Minimum Thickness (mm) Lumber

1. 

406

15.5

15.9

15.9

17.0

2. 

508

15.5

15.9

19.0

19.0

3. 

610

18.5

19.0

25.4

19.0

 

Table 9.23.14.5.B.
Rating for Subfloor when Applying CSA O325
Forming Part of Sentences 9.23.14.5.(1) and 9.23.15.7.(1)

Item

Column 1

Maximum Spacing of Supports (mm)

Column 2

Panel Mark

Subfloor

Column 3

Panel Mark

Subfloor Used with Panel-Type Underlay

1. 

406

1F16

2F16

2. 

508

1F20

2F20

3. 

610

1F24

2F24

 

How to Install your Subfloor:

The subfloor is the second layer of flooring which is laid upon trusses/joists or a concrete slab. It must be level as the rest of the flooring will be relying on the foundation provided by the subfloor. So it’s important that your joists or slab are level, clean, and dry. The underlay material and finished flooring will reside on top. If the joists have any uneven spaces that can’t be remedied, that is a good spot to use an adhesive, which will add rigidity and reduce squeaking.

Next, you need to lay your plywood or OSB according to the building code standards. The sheets of OSB or plywood need to have a ⅛” gap so that expansion can occur. Then you need to fasten your sheets to the joists at every 8” or according to the building code. If you are choosing screws, Smartnail suggests using the Grabber Screw Gun Deck and Flooring Kit to save your back and speed up your work. Try to stagger the end joints of the sheets so they don’t line up. Avoid having 4 sheets meet at a single point because you will lose rigidity and have increased squeaking. 

The basic concept is to use a thick enough material so when you lay a flooring on top of it, it will carry the load without cracking or squeaking.  One of the problems that has reared its ugly head is when people put down too light a subfloor and then put an underlay over it and put tile on top of the underlay.  The procedure is correct but when the subfloor is too light, it tends to flex under load. Repeated flexing leads to cracking the tile on top. Of course the other problem is when the screws holding the subfloor and the underlay in place are screwed in with too much distance between them, the subfloor will flex and squeak. If the underlay is screwed with more than 3” between the screws, it also will flex and squeak and crack tile.

Subfloor Installation Tips:

A worker using a power drill to install a subfloor or deck.

Here is a summary of the most important tips discussed in this article:

  • Choose the paneling (usually OSB or plywood) for your subfloor based on the type of finished flooring that will be on top
    • Match the rigidity to the heaviness of the material on top
    • Thickness of the paneling is determined by spacing of the joists, the type of finished flooring, and the quality of paneling material
  • Consult the building code for your province
    • They will provide standards for materials, thickness, fasteners, and how the panels need to be placed
  • When installing your subfloor, ensure the that joists or slab are level, dry, and clean
  • Never have four panels meet in one point and generally install them with ⅛” gaps
  • To reduce squeaking, consider using screws as fasteners with an appropriate screw gun for speed
    • Ensure the spaces between the screws are no more than 3” to prevent flex, squeaking, and cracking


If you have any questions, always feel free to contact us at Smartnail.