Yes, adding a second skin of plywood over an existing weak or springy floor is a well-established technique used by NZ carpenters — and in many situations it's a straightforward DIY project. That said, the success of the job depends heavily on diagnosing why the floor is weak in the first place, choosing the right plywood, and fixing it correctly.
First, Diagnose the Problem
Before you reach for the drill, work out what's causing the weakness. A floor can feel soft or bouncy for several different reasons:
- Undersized or widely spaced joists — common in older NZ homes built before modern span tables under NZS 3604
- Existing subfloor sheeting that is too thin, delaminated, or damaged — particularly in homes with particle board that has swollen from moisture
- Rot or borer damage to joists or existing subfloor
- Inadequate fixing — the original sheets may simply have never been glued or nailed properly
If you find rot, significant borer, or structural joist damage, stop and call a licensed building practitioner. Overlaying new plywood over a compromised structure will not fix the underlying problem and could mask a safety hazard. You can find qualified local carpenters on tradedirectory.co.nz.
Does This Work Need a Building Consent?
In most cases, overlaying a second skin of plywood to an existing floor is exempt from building consent under Schedule 1 of the Building Act 2004, as it is like-for-like repair or maintenance work. However, if you are changing the floor structure (for example, sistering joists or altering load-bearing elements), consent may be required. Always confirm with your local council before starting — rules can vary. Check MBIE Building Performance for general guidance on what requires consent.
Choosing the Right Plywood
For structural subfloor overlays, use structural plywood rated to NZS 3603 or equivalent — not decorative or furniture-grade ply. Key points:
- Thickness: A minimum of 12 mm is generally recommended for an overlay; 15 mm or 17 mm gives a noticeably stiffer result, especially over joists at 600 mm centres
- Bond type: Use Type A bond (exterior grade) plywood, which is moisture-resistant — important in NZ's varied climate and in areas like kitchens or bathrooms
- Sheet size: Standard 2400 × 1200 mm sheets are the most practical
You can source structural plywood from Bunnings, Mitre 10, or PlaceMakers. Always check the grade stamp on the sheet before buying.
Preparation Is Everything
A poorly prepared base will cause the overlay to squeak, flex, or fail prematurely.
- Fix any existing squeaks — drive screws through the existing subfloor into joists to pull down any loose areas
- Check for high spots — use a long straightedge and sand or plane down any ridges so the new layer sits flat
- Mark joist positions on the walls before you start, so you can hit them when screwing through two layers
- Sweep and vacuum the surface thoroughly — debris under the new sheet will create hollow spots
- Check moisture levels — if the existing floor feels damp or smells musty, investigate the underfloor ventilation before proceeding. BRANZ has useful guidance on underfloor moisture management
How to Fix the Overlay
The best results come from using both construction adhesive and mechanical fixings.
| Method | Details |
|---|---|
| Adhesive | Apply a structural flooring adhesive (such as a product from Sika or similar) in a serpentine bead across the existing floor |
| Screws | Use 50–65 mm coarse-thread screws at 150–200 mm centres along joist lines, and 300 mm centres in the field |
| Nails | Ring-shank nails can be used but screws are preferred — they pull the sheet down firmly and resist squeaking |
Stagger the sheet joints so they don't line up with the joints in the existing subfloor — this is critical for stiffness. Leave a 2–3 mm expansion gap between sheets and at walls to allow for seasonal movement.
Thickness Considerations for Finished Flooring
Adding 12–17 mm to your floor height will affect door clearances, skirting boards, and transitions to adjoining rooms. Measure carefully and plan for:
- Trimming the bottoms of doors (or lifting them on their hinges)
- Scribing or replacing skirting if it sits on the floor
- Transition strips at doorways where floor heights change
Bathrooms and Wet Areas — Extra Care Required
If the weak floor is in a bathroom or laundry, be aware that NZ Building Code clause E3 (Internal Moisture) sets requirements for wet area floors. Any overlay must be compatible with your intended waterproofing system and tile adhesive. Check with your tile or waterproofing supplier, and consider consulting a licensed tradesperson for wet area work.
When to Call a Professional
Call a Licensed Building Practitioner (LBP) or qualified carpenter if:
- You find rot, borer, or structural damage to joists
- The floor deflection is severe or the joists are undersized for the span
- The work is in a wet area requiring waterproofing
- You are unsure whether building consent is needed
- The existing subfloor is particle board that has significantly swollen or delaminated — it may need full removal rather than overlay
You can verify a tradesperson's LBP licence at Licensed Building Practitioners and find local carpenters on tradedirectory.co.nz.
