The fastest way to waste a premium flooring budget is to install over a subfloor that was not properly prepared. Most floor failures in NZ homes (gapping boards, cupping, hollow sounds, swollen joints, mould smell after the first wet winter) trace back to one or two checks that got skipped before the floor went down.
This is the pre install checklist we walk Power Dekor customers through before any laminate or engineered hardwood install starts. Concrete and timber subfloors get separate sections because the failure modes are different.
Planning a flooring install in the next 8 weeks?Talk to the Power Dekor team for an on site subfloor assessment before you order materials. Fixing a subfloor problem at quote stage costs a fraction of fixing it after the floor is laid.
Why subfloor prep matters more than the floor itself
Modern laminate and engineered hardwood are floating systems. The boards click together and sit on top of an underlay, not on the subfloor itself. That means everything underneath the boards (moisture, flatness, structural movement) shows up directly in the finished floor.
A great floor on a bad subfloor will:
- Make hollow sounds when walked on
- Develop gaps between boards over the first dry summer
- Cup or crown if moisture migrates up from the subfloor
- Wear unevenly because boards flex over high spots
- Often lose warranty cover, because manufacturer terms require subfloor compliance
None of this is the floor’s fault, and replacement of the floor will not fix it. The cause sits below.
The four checks every NZ subfloor needs
Before you order materials or book installers, every subfloor in NZ (concrete or timber) gets the same four diagnostic checks:
- Moisture: is the substrate dry enough for a floating floor?
- Flatness: are dips and humps within the manufacturer tolerance?
- Structural integrity: is the substrate sound, with no movement, rot or cracks beyond hairline?
- Cleanliness: is the surface free of dust, debris, old adhesive, paint splatter and oils?
Fail any of the four and you have prep work to do before any underlay or board lands.
Concrete subfloor checklist
Most new NZ builds and many renovations sit on a concrete slab. Concrete is stable and strong, but it is also a moisture reservoir and rarely as flat as it looks.
1) Moisture test
Concrete always releases moisture, even years after pouring. The two tests that matter:
- Surface moisture meter: a non destructive reading taken at multiple points across the slab. Most flooring manufacturers require under 4 percent moisture by weight, or under 75 percent relative humidity in slab
- Calcium chloride test: a 72 hour test that measures vapour emission. The threshold for floating floors is typically under 3 pounds per 1,000 square feet per 24 hours (in metric, under 12 micrograms per square metre per second)
Rule of thumb for new slabs: 60 plus days of drying per 25mm of slab thickness, with active ventilation. A 100mm slab needs about 4 to 5 months minimum, often longer in a damp NZ winter.
2) Flatness test
Lay a 2m straight edge across the slab in multiple directions. The deviation under the edge should be no more than 3mm over 2m. Anything more needs correction before the floor goes down.
Common fixes:
- Self levelling compound: for small to medium variations. A liquid mix poured over the slab that flows to a level finish, cures overnight
- Mechanical grinding: for raised humps, dried concrete blobs, or old adhesive ridges
- Patching compound: for small dips, drill holes, and edge gaps
3) Cracks and structural condition
Hairline cracks under 1mm wide are usually cosmetic and can be filled and covered with no impact on the floor above. Wider cracks, especially ones with vertical movement between sides, need structural assessment. A floor laid over a moving crack will eventually show a corresponding line of damage.
4) Cleanliness and old finishes
Vacuum thoroughly. Remove any of these before underlay goes down:
- Old adhesive residue (mechanical removal or scraper)
- Paint or sealant splatter
- Oil or solvent stains (these can leach into the underlay and discolour boards)
- Loose efflorescence (the white crystalline deposit that shows up on damp slabs)
5) Vapour barrier and underlay choice
Concrete subfloors almost always need a vapour barrier under the underlay, or an underlay with an integrated barrier. The barrier overlaps need to be taped at every seam, including up the wall edges by 25 to 50mm. For the full underlay decision tree, see our laminate flooring underlay guide.
Timber subfloor checklist
Older NZ homes (pre 1980s) usually sit on timber joists with tongue and groove or particleboard sheeting. Newer builds increasingly use engineered timber subfloor systems. Either way, the failure modes differ from concrete.
1) Structural integrity
Walk every room and listen. Note any boards that:
- Feel spongy or deflect noticeably underfoot
- Squeak at the joist line (a sign of nail loosening or board separation)
- Show visible rot, water staining, or insect damage
- Have lifted or warped sections, especially near plumbing or external walls
Squeaks must be fixed at this stage. Once the new floor is down, every squeak below it will transmit through. Use construction screws into the joist (not nails) and add structural adhesive where the subfloor meets the joist.
2) Underfloor moisture and ventilation
If the home has a crawlspace or piles, the underfloor area needs:
- Cross flow ventilation through the perimeter vents (clear, not blocked by garden mulch or insulation)
- A ground vapour barrier (polythene sheet) covering the soil if subfloor humidity is high
- No standing water, no signs of repeated dampness on the underside of the floor
Coastal homes and older Auckland villas often have weak underfloor ventilation, which then drives moisture up into the new floor. Fix the ventilation first.
3) Flatness
Same 3mm in 2m rule as concrete. Older timber floors often have noticeable dips at joist mid spans and humps at the joists. Sand the humps, fill the dips with a flexible flooring patch compound rated for timber substrates.
4) Existing finishes and old flooring
If you are installing over an existing floor (a common renovation choice), the existing surface needs to be flat, stable and dry. Lift old carpet plus underlay completely. Remove old vinyl that is lifting or has soft spots. Existing laminate or tile that is sound, flat and well bonded can often stay in place under a new floating floor.
5) Plumbing, electrical and access
Before any new floor goes down over a timber subfloor, check that:
- No plumbing inspection points are about to be covered (especially in laundries and bathrooms adjacent to the install)
- Electrical floor outlets are mapped if any are present
- Underfloor heating cables or pipes are documented and located
Acclimatisation: the prep step everyone forgets
Laminate and engineered hardwood are real material products that respond to humidity. Boards delivered straight from a cold truck into a heated home expand or contract within hours. Installing without acclimatisation is one of the most common warranty void reasons in NZ.
The standard acclimatisation procedure:
- Deliver boards to the installation room at least 48 hours before install
- Leave them flat, in original packaging, with airflow around the stack
- Maintain the room at the same temperature and humidity it will operate at long term (typically 18 to 22 degrees Celsius, 35 to 55 percent relative humidity)
- For engineered hardwood, extend to 72 hours minimum in NZ’s variable conditions
Skipping this step is the cheapest way to cause expensive damage.
Quick subfloor decision matrix
| Finding | Action | Typical NZ cost |
|---|---|---|
| Concrete moisture above threshold | Wait, ventilate, retest | Delay, not dollars |
| Concrete out of flat (more than 3mm over 2m) | Self levelling compound | NZD 18 to 35 per square metre |
| Wide cracks with movement | Structural assessment required | NZD 250 to 600 assessment, repair varies |
| Squeaky timber subfloor | Screw and glue at joists | NZD 200 to 600 per room |
| Sponginess in one or more boards | Replace damaged sheeting | NZD 80 to 200 per square metre of repair |
| Inadequate underfloor ventilation | Add or clear vents, install vapour barrier on ground | NZD 500 to 2,000 for retrofit |
| Old vinyl adhesive residue on slab | Mechanical grind or scrape | NZD 12 to 25 per square metre |
What goes wrong when prep is skipped
The same five failure modes appear over and over when subfloor prep is cut short:
- Moisture cupping: board edges rise above the centre after the first wet winter, because slab moisture migrated up
- Hollow sounding boards: the boards bridge over a dip in the subfloor with nothing to support them in between
- Squeak telegraphing: every original squeak in the timber subfloor now sounds louder through the new floor
- Gapping at joints: boards that did not acclimatise contract once installed, opening joints across the floor
- Warranty rejection: manufacturer warranties require subfloor compliance, and a failed claim almost always traces back to prep
Every one of these is preventable. None of them are cheap to fix after the floor is laid.
Next step: get a subfloor assessment before you order
Whether you are installing AquaRepel waterproof laminate in a kitchen, or engineered hardwood in a living area, the subfloor checks are the same and they happen first.
The Power Dekor team can run an on site subfloor assessment as part of your initial quote. We measure moisture, check flatness, document any prep work needed, and quote the floor with the prep transparent up front. No surprises at install day.
Book a subfloor assessment here.
FAQs
How long does a new concrete slab need to dry before flooring?
Roughly 60 days per 25mm of slab thickness, with active ventilation. A 100mm slab typically needs 4 to 5 months minimum. Always finish with a moisture meter or calcium chloride test before installing any floating floor.
Can I install laminate or engineered wood over a tile floor?
Often yes, if the existing tile is flat, stable, well bonded, and dry. Lippage between tiles needs to be ground flat. Soft or loose tiles need to be removed or replaced. A flat, sound tile floor is one of the better substrates for a floating install.
What is the maximum allowable dip in a subfloor?
The industry standard is 3mm of deviation over a 2m straight edge. Anything beyond that needs correction, usually with self levelling compound on concrete or patching on timber.
Do I need a moisture barrier on timber subfloors?
Not always at the underlay level, but you usually need a polythene ground vapour barrier in the crawlspace below if underfloor humidity is high. Check ventilation first, then add a ground barrier if the underfloor area shows persistent damp.
How do I fix squeaky boards before installing a new floor?
Locate the joist below the squeak (a stud finder helps), then drive construction screws through the existing subfloor sheet into the joist. Apply structural adhesive at the joist line if the sheet has separated. Avoid nails, which loosen over time.
What happens if my installer skips moisture testing?
You bear the risk if the floor fails. Manufacturer warranties almost universally require documented moisture compliance, and an installer who did not test typically cannot prove the slab was dry on the day. Insist on documented readings.
Can I prep the subfloor myself?
Light cleaning and minor patching are realistic DIY tasks. Self levelling compound and structural repairs are usually better done by a contractor, because the prep is more critical than people expect. The cost of getting prep wrong almost always exceeds the cost of paying someone to do it once.
What underlay should I use after subfloor prep?
That depends on the subfloor type, the room and any heating. For the full breakdown of underlay options matched to subfloor and use case, see our laminate flooring underlay NZ guide.