Your flooring is only as good as what’s underneath it. A poorly prepared subfloor causes clicks to fail, planks to bounce, joints to open, and warranties to void. Here’s everything you need to know about subfloor prep and choosing the right underlay — whether you’re laying SPC hybrid, engineered timber, or hardwood.
In This Guide
1. Why Subfloor Prep Matters
3mm
Max Deviation / 3m
75%
Max RH (Concrete)
#1
Cause of Install Failures
Voids
Warranty if Skipped
An uneven subfloor is the single biggest cause of flooring installation failures. When the floor isn’t flat, click-lock joints can’t engage properly, planks rock and bounce underfoot, and gaps open between boards over time. Every flooring manufacturer requires subfloor flatness within 3mm over 3 metres — exceed that and your warranty is void from day one.
Warranty Warning
Every major flooring manufacturer — including all products sold by Hippo Floors — requires proper subfloor preparation as a warranty condition. If your floor fails because the subfloor wasn’t flat, dry, or clean enough, the warranty claim will be rejected. This step is non-negotiable.
2. Subfloor Types & How to Prep Each One
Concrete Slab
The most common subfloor in Australian homes. Concrete slabs need to be:
- Flat: Max 3mm deviation over 3 metres. Use a long spirit level or straight edge to check.
- Dry: Moisture content below 75% relative humidity. Test with a digital hygrometer or calcium chloride test. New concrete needs at least 90 days to cure before flooring.
- Clean: Remove all paint, adhesive residue, plaster drips, and debris. Scrape and vacuum thoroughly.
- Smooth: Fill cracks and holes with concrete patch compound. Grind down high spots with a concrete grinder (hire from Bunnings for ~$80/day).
Moisture in Concrete: The Silent Killer
Excess moisture in concrete causes flooring to buckle, cup, and grow mould underneath. New slabs can take 3–6 months to fully cure. If moisture is high, apply a moisture barrier (epoxy or polyethylene) before laying underlay. Never rely on underlay alone as a moisture barrier for slabs over 75% RH.
Timber Subfloor (Floorboards)
Common in older Australian homes with stumps or bearers and joists:
- Fix squeaks: Screw down loose boards to the joists. Use 50mm timber screws — not nails, which work loose over time.
- Check flatness: Same 3mm over 3m rule. Sand high spots or pack low spots with shims between joists and boards.
- Replace damaged boards: Rotten, split, or severely warped boards must be replaced. Water-damaged boards will transfer problems to your new floor.
- Ventilation: Ensure subfloor ventilation is adequate (crucial for homes on stumps). Poor ventilation = moisture = mould.
Existing Tiles
You can float new flooring directly over tiles if they’re:
- Firmly adhered: Tap each tile — hollow-sounding tiles may be loose. Re-glue or remove them.
- Flat: Grout lines deeper than 2mm should be filled with self-levelling compound.
- Clean and dry: Remove all grease, wax, and cleaning product residue.
Existing Vinyl or Linoleum
You can float over old vinyl as long as it’s stuck down flat with no bubbles, tears, or curling edges. If the vinyl is in poor condition, remove it. Warning: Vinyl installed before the 1980s may contain asbestos — if in doubt, have it tested before removing it.
| Subfloor Type | Can Float Over? | Key Concern | Prep Difficulty |
|---|---|---|---|
| Concrete slab | Yes | Moisture and flatness | Moderate |
| Timber boards | Yes | Squeaks and flatness | Moderate |
| Tiles | Yes (if flat and firm) | Deep grout lines | Easy |
| Vinyl/lino | Yes (if stuck flat) | Bubbles and edges | Easy |
| Carpet | No — must remove | — | — |
| Cork | No — must remove | Too soft/compressible | — |
3. How to Level a Subfloor
1
Find the High and Low Spots
Lay a 1.8m spirit level or straight edge across the floor in multiple directions. Mark any spots where the gap under the level exceeds 3mm. Use a pencil to circle high spots and a different colour for low spots.
2
Grind High Spots
Use a concrete grinder (hire from Bunnings ~$80/day) to shave down high spots on concrete. For timber subfloors, use a belt sander. Wear a dust mask and safety glasses. Re-check with the spirit level after grinding.
3
Fill Low Spots
For small dips (under 10mm), use a concrete patch compound applied with a trowel. For larger areas, use self-levelling compound — pour it over the area and it flows to create a flat surface. Follow the product’s drying time (usually 24–48 hours).
4
Final Check
After grinding and filling, re-check the entire floor with your spirit level. The floor should be within 3mm over any 3-metre span. Vacuum the entire area to remove dust and debris before laying underlay.
4. Choosing the Right Underlay
Underlay sits between the subfloor and your flooring. It provides acoustic insulation, minor levelling, and (on concrete) a moisture barrier. The right underlay depends on your subfloor type and flooring product.
| Underlay Type | Thickness | Best For | Price/m² |
|---|---|---|---|
| IXPE foam | 1.5mm | SPC hybrid on any subfloor. Most common choice. | $2–$4 |
| Cross-linked PE foam | 2mm | SPC hybrid and engineered timber. Better acoustic. | $3–$5 |
| Cork underlay | 2–3mm | Engineered timber and hardwood. Premium acoustic. | $5–$10 |
| Rubber underlay | 2–3mm | High-traffic areas, apartments. Best sound reduction. | $6–$12 |
| Combination (foam + foil) | 2mm | Concrete subfloors. Built-in moisture barrier. | $3–$6 |
Pre-Attached Underlay
Some premium SPC hybrid products come with underlay pre-attached to the plank (usually IXPE or cork). If your flooring has this, do NOT add additional underlay — double underlay causes bounce and click failures. Check the product specifications before buying separate underlay.
Key Rules for Underlay
- Never overlap: Butt underlay edges together and tape seams with aluminium tape. Overlapping creates ridges.
- Silver side down on concrete: The foil/aluminium layer faces the concrete to act as a moisture barrier.
- Don’t exceed manufacturer’s thickness: Most SPC hybrid requires 1.5–2mm underlay. Thicker isn’t better — it creates instability.
- Cover the entire floor: No gaps, no exposed subfloor. Trim neatly at walls.
5. Subfloor Prep Checklist
Is Your Subfloor Ready?
0% completeInspection
Preparation
Underlay
6. FAQs
How flat does my subfloor really need to be?
Maximum 3mm deviation over 3 metres. This is the industry standard across all flooring types and is specified by every major manufacturer. Check with a 1.8m spirit level laid across the floor in multiple directions. Even small bumps and dips outside this tolerance will cause problems over time.
Can I lay flooring over old tiles?
Yes, as long as the tiles are firmly adhered, flat, and clean. If grout lines are deeper than 2mm, fill them with self-levelling compound first. Lay underlay over the tiles before installing your flooring. Loose or hollow-sounding tiles should be re-adhered or removed.
How do I test moisture in concrete?
The most accessible method is a digital hygrometer: tape a piece of plastic sheeting to the concrete, seal the edges, and leave for 24–48 hours. Place the hygrometer underneath and read the relative humidity. Below 75% is acceptable. For more precise results, use a calcium chloride test kit (available from Bunnings for ~$30).
Do I need underlay if my flooring has it pre-attached?
No. If your SPC hybrid or engineered timber has underlay pre-attached (usually a thin layer of IXPE or cork on the bottom of each plank), do NOT add additional underlay. Double underlay creates too much cushion, which causes the click-lock joints to flex and eventually fail. You may still need a separate moisture barrier on concrete, but check the manufacturer’s instructions first.
What’s the cheapest way to level a concrete floor?
For small areas, a concrete angle grinder (~$80/day hire) handles high spots, and a bag of concrete patch compound (~$15) fills low spots. For larger areas with multiple dips, self-levelling compound (~$25–$40 per 20kg bag, covers ~4m² at 5mm depth) is the most efficient option. A professional can level a typical room for $300–$800.
Ready to Lay Your Floor?
Once your subfloor is prepped, browse our full range of hybrid, engineered timber, and hardwood flooring. Free delivery Australia-wide.
