How to Prepare Your Subfloor for Hybrid Flooring

📖 14 min read

Here's an uncomfortable truth: the most common reason hybrid flooring fails isn't the product – it's what's underneath. I've seen beautiful floors develop hollow spots, clicking noises, and even failed click-locks within months, all because the subfloor wasn't prepared properly.

The good news? Subfloor preparation isn't complicated. It's mostly about getting three things right: clean, dry, and flat. Miss any of these, and you're setting yourself up for problems. Nail all three, and your hybrid flooring will perform exactly as it should for 15-20+ years.

This guide covers everything you need to know – whether you're dealing with concrete slabs, timber subfloors, or existing flooring you want to lay over.

Why Subfloor Preparation Actually Matters

Hybrid flooring is a floating floor – it sits on top of your subfloor rather than being glued or nailed down. The planks click together and "float" as a single unit over the surface beneath. This installation method is brilliant for DIYers, but it means the subfloor condition directly affects how the floor feels and performs.

Here's what happens when subfloor prep is skipped or done poorly:

The frustrating part? These problems often don't appear immediately. You install the floor, it looks great, and then three months later you notice that annoying hollow sound in the hallway or the gap forming between two planks. By then, fixing it means lifting the floor.

The Three Requirements: Clean, Dry, Flat

Every subfloor – regardless of type – needs to meet three criteria before hybrid flooring goes down:

Let's break down each requirement in detail.

Preparing Concrete Subfloors

Concrete slabs are the most common subfloor in Australian homes. They're generally stable and flat, but they have one major challenge: moisture.

Step 1: Clean Thoroughly

Start with a completely clean surface:

• Remove all debris: Sweep and vacuum thoroughly. Even small pebbles or dried paint drips create bumps.
• Scrape off adhesive: If old flooring was glued down, remove all adhesive residue. A floor scraper or grinder works for stubborn areas.
• Clean oil and grease: Degrease any contaminated areas – common in garages or near kitchen areas.
• Remove paint drips: Scrape or sand smooth any paint spatters from previous work.

The concrete should be bare, clean, and free of anything that would create a bump under your flooring.

Step 2: Check and Correct Flatness

Use a long straight edge (a 2-metre level or straight timber) to check for high and low spots:

• Place the straight edge on the floor and look for gaps underneath
• Move it across the entire area, noting any spots where gaps exceed 3mm
• Mark high spots with chalk or tape
• Mark low spots that need filling

For high spots: Grind down using a concrete grinder. These can be hired from equipment rental places like Kennards or Bunnings.

For low spots: Fill with self-levelling compound (covered in detail in the levelling section below).

Step 3: Moisture Testing

This is critical and often skipped. Concrete can hold moisture even when it feels dry to the touch. The Australian Standard AS 1884 specifies moisture testing requirements for resilient floor coverings.

Methods for testing concrete moisture:

• Relative humidity test (preferred): In-slab probes measure actual moisture content. Readings should be below 75% RH for most flooring.
• Plastic sheet test (basic): Tape a 1m² plastic sheet to the concrete for 24-48 hours. Moisture underneath indicates a problem.
• Electronic moisture meter: Quick surface readings, but less accurate than in-slab testing.

If moisture levels are too high, you'll need a moisture barrier or to wait for the slab to dry further. New concrete slabs typically need 4-6 weeks minimum curing time before flooring installation.

Step 4: Apply Moisture Barrier (If Required)

Most quality hybrid flooring comes with pre-attached IXPE underlay that includes some moisture protection. However, for slabs with elevated moisture readings, you may need additional protection:

• Polyethylene sheet: 200μm (0.2mm) plastic sheeting overlapped by 200mm at joins
• Liquid-applied membrane: Painted or rolled onto concrete for severe moisture issues
• Specialised underlay: Some underlays have built-in moisture barriers rated for higher moisture levels

Check your hybrid flooring manufacturer's specifications for moisture barrier requirements. Our underlay guide covers this in more detail.

Preparing Timber Subfloors

Timber subfloors are common in older homes and elevated "Queenslander" style houses. They present different challenges than concrete.

Step 1: Check Structural Integrity

Before anything else, ensure the timber subfloor is structurally sound:

• Walk the entire floor: Feel for soft spots, excessive bounce, or movement
• Look for damage: Check for rot, termite damage, or water-damaged boards
• Check joists: If accessible from below, inspect floor joists for damage or sagging
• Address ventilation: Ensure subfloor ventilation is adequate to prevent moisture buildup

Damaged boards or structural issues need repair before flooring installation. The Queensland Building and Construction Commission (QBCC) provides guidance on subfloor ventilation and maintenance for Queensland homes.

Step 2: Secure Loose Boards

Squeaky or loose floorboards will telegraph through your new flooring. Fix them now:

• Re-screw loose boards: Use 50mm timber screws through existing boards into joists
• Address squeaks: Squeaks usually mean movement – add screws either side of the squeak point
• Check nail heads: Punch down any protruding nail heads below the surface

Step 3: Create a Flat Surface

Timber floors often have gaps, cupping, or uneven boards. You have several options:

Option A: Sand the floor flat

• Best for floors with minor unevenness and cupped boards
• Hire a floor sander and sand to a consistent level
• Time-consuming but creates an excellent base

Option B: Overlay with plywood or fibre cement sheet

• Best for floors with significant unevenness or wide gaps
• 6mm plywood or fibre cement screwed to existing floor
• Creates a new, flat surface
• Adds height – check door clearances

Option C: Use self-levelling compound (for minor issues)

• Fill low spots and gaps with flexible levelling compound
• Not suitable for major unevenness

Step 4: Clean and Check

• Vacuum all dust and debris (especially if you've sanded)
• Run your straight edge across the floor to confirm flatness
• Address any remaining high or low spots

Laying Over Existing Flooring

One of hybrid flooring's advantages is that it can often be installed over existing hard flooring. Here's what works and what doesn't:

✅ Can Lay Over

❌ Must Remove First

• Carpet: Always remove carpet and underlay. Cannot lay over.
• Loose or damaged tiles: Re-adhere or remove entirely
• Vinyl with asbestos backing: Requires professional removal – common in homes built before 1990. Safe Work Australia provides guidance on identifying and handling asbestos.
• Uneven or bouncy flooring: If the existing floor has issues, they'll transfer to your new floor

Preparing Existing Tiles for Overlay

If laying over tiles:

1. Check all tiles are firmly adhered (tap and listen for hollow sounds)
2. Re-adhere any loose tiles with tile adhesive
3. Fill deep grout lines with flexible filler if they might telegraph through (usually only an issue with very thick grout lines)
4. Clean thoroughly – remove any wax, sealer, or cleaning product residue
5. Check height transitions – adding flooring over tiles increases floor height significantly

How to Level Your Subfloor

If your flatness check revealed issues, here's how to fix them:

For Low Spots: Self-Levelling Compound

Self-levelling compound is a liquid-like cement that flows into low spots and cures flat. It's the standard solution for concrete subfloors.

Steps:

1. Clean and prime: Vacuum the area thoroughly. Apply primer if required by the compound manufacturer.
2. Dam the edges: For larger areas, create a barrier at doorways to contain the compound.
3. Mix the compound: Follow manufacturer instructions precisely. Consistency matters.
4. Pour and spread: Pour onto low areas and use a spreader to help it flow. It self-levels, but may need encouragement.
5. Allow to cure: Typically 4-24 hours depending on product and thickness. Check manufacturer specs before walking on or installing over.

Self-levelling compound is available from Bunnings, hardware stores, and flooring suppliers. Brands like Ardex, Dunlop, and Bostik are commonly used in Australia. The Housing Industry Association (HIA) recommends following manufacturer specifications for best results.

For High Spots: Grinding

High spots on concrete need to be ground down:

1. Hire a concrete grinder from equipment rental
2. Work the high spot with the grinder until level
3. Check frequently with your straight edge
4. Vacuum thoroughly when complete

For timber subfloors, high spots can be sanded with a belt sander or floor sander.

The 3mm Rule

The industry standard for floating floors is 3mm tolerance over 3 metres. This means:

• Place a 3-metre straight edge on the floor
• No gap underneath should exceed 3mm
• No high spot should create more than 3mm deviation

Thicker hybrid flooring (like our 9.5mm range) is more forgiving of minor imperfections than thinner products, but you should still aim for this standard.

Moisture Testing and Barriers

Let's go deeper on moisture – it's the hidden problem that causes the most grief.

Why Concrete Moisture Matters

Concrete is porous. Moisture from the ground beneath your slab migrates upward through the concrete (this is called "rising damp" or "moisture vapour transmission"). Even when the surface feels dry, there can be significant moisture moving through the slab.

While SPC hybrid flooring itself is water-resistant, issues can still occur:

• Moisture can migrate through expansion gaps at walls and around the perimeter
• High humidity underneath the floor can create condensation issues
• In extreme cases, moisture can affect adhesive in glue-down applications
• Mould can develop in trapped moisture environments

When Moisture Testing is Essential

• New concrete: Any slab less than 6 months old
• Ground-level slabs: Especially without proper damp-proof membrane
• Known moisture history: Previous water damage or flooding
• Humid climates: Queensland, coastal NSW, tropical areas
• Below-grade installations: Basements or sunken rooms

Acceptable Moisture Levels

For SPC hybrid flooring over concrete:

• Relative humidity (RH): Below 75% is generally acceptable
• Above 75% RH: Moisture barrier required
• Above 85% RH: Investigate moisture source before proceeding

If you're unsure about moisture levels, engage a professional to test. It's cheaper than replacing flooring later.

Final Preparation Checklist

Before you start laying your hybrid flooring, run through this checklist:

Common Mistakes to Avoid

Here are the errors I see most often:

Ready to Start Your Project?

Proper subfloor preparation takes time, but it's the foundation of a successful flooring installation. Get it right, and your hybrid flooring will look and perform beautifully for years. Rush it, and you'll be dealing with problems down the track.

If you're unsure about your subfloor condition or have questions about preparation, give us a call on 0431 311 633 or send us a message. We can advise on your specific situation and help you avoid common pitfalls.

Last updated: December 2025 · Written by the team at Hybrid Floors Australia