Subfloor Preparation Guide

The subfloor is everything under your new floor: the concrete slab, timber boards, plywood or existing tiles it sits on. Most flooring problems, from lifting boards to bumps showing through vinyl, start with a subfloor that was not flat, dry or sound. This guide explains subfloor preparation for each type of flooring and what happens at a measure.

The three rules: flat, dry and sound

• Flat: high spots and dips cause floating floors to flex and click, and show through thin glued floors like vinyl planks.
• Dry: moisture rising from a slab can damage timber and laminate, and break down adhesives under glued floors.
• Sound: the base must be solid, clean and free of loose material, with no movement or bounce.

Every maker’s installation guide sets out its own rules, and the warranty depends on following them.

Moisture limits by floor type

Concrete slabs hold moisture, especially new ones. Installers measure it as relative humidity (RH), and each floor has a limit. Some examples from the ranges we sell:

If a slab is too damp, the options are to wait for it to dry, apply a moisture barrier system, or choose a floor that tolerates more moisture.

Moisture barriers under floating floors

Floating floors such as hybrid, laminate and engineered timber go over a moisture barrier on concrete. For our Royal hybrid and timber ranges, the installation guide requires a 200 µm moisture barrier, with joins taped, on every subfloor, or a moisture barrier coating before a glued-down installation. Some underlays have a barrier built in. Our guide to flooring over concrete covers this in more detail.

Preparing a concrete slab

• Test moisture, especially on a new slab, which needs time to dry before flooring goes down.
• Grind high spots and fill dips, cracks and holes with a suitable patching compound.
• Apply a levelling compound (screed) where the floor is out of flat over a wider area.
• Remove old adhesive, paint, plaster and other residue so glue or underlay sits on a clean surface.
• Vacuum thoroughly before installation.

Preparing a timber subfloor

• Check for loose or squeaky boards and fix them down.
• Replace damaged or rotten boards, and find and fix the cause of any moisture.
• Check the space under the house has good ventilation.
• Sand or plane high joins, or lay sheet flooring such as plywood over uneven boards where the floor calls for it.
• For glued vinyl over timber, a smooth sheet underlay is often needed so board joins do not show through.

Laying over existing floors

Some floors can go over what is already there. Rigid core products such as Karndean Van Gogh Rigid Core go over most existing hard floors, but the base still needs to be flat. Tiles need to be firmly bonded and grout lines may need filling. Heartridge’s guide says carpet, cushioned floors and existing floating floors are not suitable bases for its hybrids. Our flooring over tiles guide explains what to check.

Subfloor needs by floor type

What happens at the measure

At a free measure we check the subfloor’s level and condition, test moisture where it matters, and include any preparation needed in your written quote. That way the cost of levelling or a moisture barrier is known up front rather than found on installation day. See what is included in your quote. For what follows on installation day, see timber flooring installation, step by step.

If your floor is already lifting or moving, our guide to why hybrid floors lift or buckle explains how subfloor problems play a part. To start a project, book a free measure and quote or visit a showroom in Penrith, Campbelltown, Hornsby or Logan.

Frequently asked questions

What does subfloor preparation involve?

Making the base flat, dry, clean and sound. That can mean grinding high spots, filling dips and cracks, applying levelling compound, removing old adhesive, fixing loose boards or adding a moisture barrier.

How dry does a concrete slab need to be?

It depends on the floor. Several glued vinyls and Heartridge engineered timber need concrete at or below 75% RH, Heartridge laminate 80% RH or less, and Polysafe QuickLay can go over substrates up to 97% RH.

Do I need a moisture barrier under hybrid flooring?

On concrete, yes. For our Royal hybrid ranges the installation guide requires a 200 µm moisture barrier on every subfloor.

Can new flooring go over old tiles?

Often, if the tiles are firmly bonded and the surface is flat. Grout lines may need filling, particularly under thin glued vinyl.

Is subfloor preparation included in the quote?

We check the subfloor at the free measure and include any preparation needed in the written quote, so there are no surprises on the day.