A bouncy or springy floor is almost always a structural issue — most commonly joists that are too small for their span, spaced too far apart, or lacking adequate mid-span support. The good news is that several effective remedies exist, ranging from straightforward DIY work to jobs that require a licensed building practitioner.
Understanding Why Floors Bounce
Floor joists are the horizontal timbers that carry your floor load. Under NZS 3604:2011 (the standard for light timber-framed buildings), joist size, spacing and span are all tightly related. When any one of those is out of balance — perhaps because the original build used undersized timber, or a wall was removed that previously provided mid-span support — the floor deflects more than it should under load, giving that uncomfortable bounce.
Before you start any remedial work, it pays to understand the cause:
- Undersized joists — the timber depth or grade is insufficient for the span.
- Excessive joist spacing — joists spaced at 600 mm centres where 400 mm is needed.
- Lack of mid-span blocking or bridging — joists can twist and deflect independently.
- Notching or drilling damage — previous trades cutting into joists incorrectly.
- Subfloor settlement — piles or bearers have dropped, increasing the effective span.
If you're unsure of the cause, have a licensed builder or structural engineer assess the subfloor before you start work. You can find qualified local builders on tradedirectory.co.nz.
Check Whether a Building Consent Is Required
Structural repairs to floor framing can require a building consent depending on the scope of work. Under Schedule 1 of the Building Act 2004, some like-for-like repairs are exempt, but adding new structural members or significantly altering the framing may not be. Always confirm with your local council before starting. MBIE Building Performance has clear guidance on what triggers consent.
Fix 1 — Sistering New Joists
Sistering is the most effective and widely used remedy. It involves fixing a new, correctly sized joist hard up against each existing undersized joist, running the full length of the span.
How it works
- Access the subfloor (most NZ homes have a crawl space beneath).
- Measure the existing joist depth, spacing and span, then calculate the correct replacement size using NZS 3604 span tables or get an engineer to specify.
- Cut new joists to length from structural-grade timber (typically SG8 or SG10 H1.2 treated for subfloor environments).
- Apply construction adhesive along the mating face, then clamp the new joist tight against the old one.
- Nail or bolt through at regular intervals (your engineer or the span tables will specify fixings).
Sistering is generally considered restricted building work if it forms part of the primary structure, meaning it should be carried out or supervised by a Licensed Building Practitioner (LBP).
Fix 2 — Mid-Span Blocking and Bridging
If the joists are adequately sized but simply lack lateral restraint, adding solid blocking or herringbone bridging between joists at mid-span can dramatically reduce bounce by making the floor system act as a unit.
- Cut solid blocks from the same timber size as the joists.
- Stagger them slightly so you can end-nail from both sides.
- Space blocking rows at no more than 1.8 m apart along the span.
This is generally within the capability of a competent DIYer with good carpentry skills, but confirm with your council whether consent is needed.
Fix 3 — Adding a Mid-Span Bearer and Pile
Where joists are significantly overspanned, the most robust solution is to introduce a new mid-span bearer supported on a new pile, effectively halving the joist span.
- This is a structural alteration and will almost certainly require a building consent.
- A new concrete or timber pile will need to meet the ground conditions and NZS 3604 requirements.
- Engage an LBP builder and, depending on complexity, a structural engineer.
Fix 4 — Stiffening the Subfloor Sheathing
If your floor is sheeted with thin particle board or an older 19 mm tongue-and-groove, upgrading to 20 mm or 22 mm structural plywood (glued and screwed) can add meaningful stiffness. This works best in combination with one of the joist fixes above rather than as a standalone remedy. PlaceMakers and Mitre 10 stock structural flooring-grade plywood.
Timber Treatment in the Subfloor
New timber used in the subfloor must be appropriately treated for the hazard zone. In most NZ subfloor situations, H1.2 treatment is the minimum for framing not in contact with the ground; H3.2 is required for timber in ground contact or exposed to wetting. Check the BRANZ guidelines or your timber supplier for the correct specification.
A Quick Comparison of Fixes
| Fix | Best for | DIY-friendly? | Consent likely? |
|---|---|---|---|
| Sistering joists | Undersized or damaged joists | Partial — LBP recommended | Possibly |
| Mid-span blocking | Adequate joists, poor lateral restraint | Yes, with care | Unlikely |
| New bearer and pile | Severely overspanned joists | No — LBP required | Yes |
| Upgraded floor sheathing | Thin or deteriorated sheeting | Yes | Unlikely |
When to Call a Professional
Call a Licensed Building Practitioner if:
- The bounce is severe or has worsened quickly.
- You can see cracked, split, notched or visibly sagging joists.
- You suspect pile or bearer settlement.
- The work involves adding new structural members.
- You are unsure whether consent is required.
A structural engineer may also be needed if the floor framing falls outside the scope of NZS 3604 (e.g., longer spans, heavier loads, or unusual configurations). Find qualified local builders and engineers on tradedirectory.co.nz.
Safety note: Working in a subfloor crawl space carries risks including confined spaces, biological hazards (rodent droppings, mould) and awkward working positions. Wear a P2 respirator, gloves and eye protection, and never work alone in a confined space.