How do I prepare a subfloor for ceramic tiles?

Find out everything you need to know to prepare your subfloor for ceramic flooring!

00:06 Materials and tools required
00:12 PREPARING A CONCRETE SUBFLOOR
00:15 Moisture measurement
00:34 Checking surface for conformity and unevenness
00:48 Rectifying irregularities and cleaning up
00:55 Installing adhesive tape around the perimeter of the room
00:59 Preparing, applying and drying undercoat primer
01:09 Preparation, application and drying of self-levelling compound
01:19 Check surface for unevenness
01:24 PREPARING A PLYWOOD SUBFLOOR
01:28 Material and thickness of plywood required
01:53 Fastening the plywood
01:58 Rectifying surface irregularities and cleaning
02:05 Checking surface for unevenness
02:10 Use self-levelling compound for large irregularities
02:13 Use self-levelling compound for small irregularities
02:22 Removal of final irregularities and clean-up

Store our underlayment primer, self-levelling and self-levelling patching compound

Materials and tools required

Subfloor reinforcement materials

• Exterior-grade plywood
• Wood screws or floor screws (11/4” long or longer, depending on subfloor thickness)

Levelling materials

• Underlayment primer

• Self-leveling

• Self-leveling patching compound

• Uncoupling membrane (if required)

Tools for subfloor preparation

• Hammer and nails or screwdriver

• Circular saw

• Spirit level or long ruler

• Trowel

• Bucket

• Mixer

• Vacuum cleaner or broom

• Sponges

• Sander

This list covers the essential materials for preparing the subfloor, but you may need to adapt certain items to suit the specifics of your project (type of subfloor, moisture, general condition of the floor, etc.).

Preparing a concrete subfloor

Measuring moisture content

If humidity levels are too high, wait until the concrete has fully cured. In some cases, moisture-control products are available.

We recommend measuring the actual humidity level of the slab before installation, using tools such as :

• In situ relative humidity probes (ASTM F2170) that measure at 40% of the total depth of the slab, or at 20% using two probes (top and bottom).

• Non-destructive moisture meters based on the electrical impedance of concrete.

• The calcium chloride moisture test (ASTM F1869) measures the rate of water vapor escaping from the concrete surface over a 24-hour period.

Compliance check and surface unevenness

Check that the floor surface is solid, clean and dry, with a maximum unevenness of 1/8” (3 mm) over a distance of 10 ft (3 m). If this is the case, apply a self-levelling compound to rectify the surface.

Deformations must not exceed 3/64” (1.1 mm). To check surface flatness, use a 10' (3 m) metal straightedge.

The surface is considered level if the maximum progressive deviation does not exceed 1/4” (6 mm) in 90% of the samples checked.

Rectifying irregularities and cleaning

The subfloor surface must be smooth: rectify slight irregularities and remove roughness with a scraper or sander.

Remove dust and residues with a vacuum cleaner or broom.

Installing tape around the perimeter of the room

Install tape around the perimeter of the room to prevent direct contact between the products and the wall.

Preparing, applying and drying undercoat primer

Mix the underlay primer according to the manufacturer's instructions.

Then apply the underlayment primer with a roller to the concrete surface and allow to dry according to the manufacturer's instructions.

Preparing, applying and drying self-levelling compound

Mix your self-levelling compound according to the manufacturer's recommendations.

Apply self-levelling compound evenly with a trowel and allow to dry according to manufacturer's instructions.

Check surface for unevenness

Once the self-levelling compound has dried, check that your surface is level.

Preparing a plywood subfloor

Material and thickness of plywood required

Check what your subfloor is made of If the subfloor is made of particleboard, it will have to be replaced, as this material is not stable enough for ceramic tiles.

The subfloor must be rigid and stable; a subfloor that flexes can cause cracks in the ceramic.

For a ceramic floor, the subfloor must be at least 11/4“ or 1 1/2” (32 to 38 mm) thick.

If the subfloor is too thin, add a second layer of 5/8” (16 mm) plywood or use a suitable uncoupling membrane.

Plywood used for underlayment must be exterior grade, Group 1, Class CC or better, conforming to APA classification and U.S. PS 1-95, or CANPLY “Select” or “Select Tight Face” certified to CSA-0121 for Douglas fir. It should be suitable for direct glue-down applications on interior floors and countertops, in dry conditions, for residential or light commercial use.

Attaching plywood

Secure plywood with screws to prevent movement.

Screws should be spaced approximately 4” (10 cm) apart.

Rectifying surface irregularities and cleaning

The subfloor surface must be free of all residues (glue, paint, plaster, etc.).

Rectify slight surface irregularities with a sander or scraper as required.

Remove residues and dust with a vacuum cleaner or broom.

Check surface for unevenness

The subfloor should be as flat as possible, with a maximum undulation of 1/8” (3 mm) over 10 ft (3 m).

Use a long ruler or spirit level to check for bumps and dips.

Using a self-levelling compound for large irregularities

For large irregularities, use a self-levelling patching compound (as seen previously in the concrete subfloor option).

Using a self-levelling compound for small irregularities

If your plywood has cracks or small irregularities, be sure to smooth the surface by applying a cement-based undercoat finish.

Depending on the condition of the plywood, one or two coats may be necessary.

Start by covering joints and screws with a first coat, then apply a second coat over the entire surface for a uniform finish.

Allow to dry according to the manufacturer's instructions (drying times may vary according to ambient room humidity).

Remove any remaining irregularities and clean upIt may be necessary to use a scraper to remove any irregularities.

Once the surface is smooth and flat, sweep or vacuum for a thorough clean.