Can I Add Blocking Between Floor Joists Myself to Reduce Bounce?

Yes, in most cases a homeowner can add blocking between floor joists themselves — it's one of the more accessible DIY structural tasks and can make a noticeable difference to a springy or bouncy floor. That said, there are a few important things to understand before you grab your saw, including when the work might require a building consent or a licensed professional.

Why Floors Bounce and How Blocking Helps

Floor bounce (sometimes called floor deflection or springiness) happens when joists flex under load. This is common in older NZ homes where joists are long, undersized by today's standards, or simply dried out and shrunk over time. Blocking — also called dwangs, noggings, or solid bridging — is timber fitted tightly between joists to transfer load sideways, stiffen the floor system, and reduce that annoying give underfoot.

Blocking works best when:

• Joists are long and unsupported in the middle of their span
• The subfloor feels bouncy in a localised area
• Joists are in good condition but just need lateral stiffening

If your floor is bouncy because joists are rotted, cracked, undersized, or sagging, blocking alone won't fix the problem — you'll need a builder to assess and potentially sister or replace the joists.

Is a Building Consent Required?

For straightforward blocking between existing joists, this work typically falls under Schedule 1 of the Building Act 2004 as routine maintenance or repair, meaning a building consent is usually not required. However, this depends on the scope of work and your local council's interpretation.

Always confirm with your local council before starting, especially if you're doing more extensive work like adding new joists, altering the structure, or working in a heritage zone. You can find general guidance at MBIE Building Performance.

What You'll Need

Timber

Use H1.2 treated timber (the minimum treatment level for interior framing in NZ) that matches the depth of your existing joists — commonly 190×45 mm or 240×45 mm in older homes, though this varies. Measure your joists carefully before buying. Treated framing timber is available from Bunnings, Mitre 10, and PlaceMakers.

Tools

• Tape measure and pencil
• Hand saw or circular saw
• Hammer or framing nail gun
• 90 mm framing nails (or structural screws)
• Safety glasses, dust mask, and knee pads
• Torch or work light (subfloor spaces are dark)

Step-by-Step: Adding Solid Blocking

1. Access the subfloor. Most NZ homes on piles have a crawl space. Locate your access hatch — usually in a hallway, laundry, or outside. Check for adequate ventilation and that the space is dry before entering.

2. Identify the bouncy area. Have someone walk on the floor above while you watch from below to pinpoint which joist span is flexing the most.

3. Measure the joist spacing. Standard NZ spacing is 450 mm or 600 mm centres, but older homes vary. Measure the clear distance between joists for each block — they won't all be identical.

4. Cut your blocking. Cut each piece to fit snugly (a tight fit is better than a loose one). Blocking should be the same depth as the joist and sit flush with the top and bottom edges.

5. Install in a staggered row. Position blocking at mid-span or at thirds for longer spans. Stagger alternate blocks slightly (offset by about 40 mm) so you can nail through the joist face into each end of the block — two nails per end minimum.

6. Nail off securely. Drive 90 mm framing nails through the joist face into the block end. If access is tight, toe-nail (angle the nail) from the block into the joist.

7. Check your work. Walk the floor above — you should notice a reduction in movement immediately.

Tips for NZ Conditions

• Subfloor moisture is a common issue in NZ, particularly in older homes and wetter regions. If you notice damp ground, pooling water, or mould while you're under there, address the moisture problem first — otherwise new timber will deteriorate quickly. BRANZ has good guidance on subfloor ventilation and moisture control.
• Check that existing subfloor ventilation meets requirements (NZ Building Code clause E3) — blocking should not obstruct airflow paths.
• If your home is on a concrete slab, floor bounce is a different problem (usually related to the flooring material itself) and blocking won't apply.

Safety Note

Crawl spaces can be confined, dusty, and may contain old insulation, rodent droppings, or even asbestos in homes built before the 1990s. Wear a P2 respirator, safety glasses, and gloves. If you suspect asbestos-containing materials (common in older pipe lagging or some older insulation), do not disturb them — contact a licensed asbestos assessor. See WorkSafe NZ for guidance on working safely around hazardous materials.

When to Call a Professional

Call a qualified builder if:

• Joists are rotted, cracked, notched incorrectly, or visibly sagging — this is structural repair work
• The bounce is severe or affects a large area of the floor
• You're unsure whether the floor system complies with NZS 3604 (the timber-framed buildings standard)
• You're uncomfortable working in a confined crawl space
• Your home is earthquake-prone or has unusual construction

A Licensed Building Practitioner (LBP) can assess the floor properly and carry out any restricted building work. You can find qualified local builders on tradedirectory.co.nz or check credentials at Licensed Building Practitioners.

Sources & Further Reading

• MBIE Building Performance – Schedule 1 exempt building work
• BRANZ – Subfloor moisture and ventilation
• WorkSafe NZ – Working safely in confined spaces and around hazardous materials
• Licensed Building Practitioners – Find or verify an LBP
• PlaceMakers – Framing timber and building supplies
• Bunnings – Timber and fasteners