Pre-drilling kwila (also sold as merbau) is essential — skip it and you risk splitting this dense, oily hardwood or snapping screws. As a general rule, use a drill bit 0.5–1 mm smaller than the shank diameter of your screw for the pilot hole, paired with a countersink bit to recess the screw head neatly flush with the surface.
Understanding Kwila and Why Pre-Drilling Matters
Kwila is one of the most popular decking timbers in New Zealand thanks to its durability and natural oils that resist rot and insects. However, those same properties make it extremely dense and hard. Driving screws into kwila without a pilot hole almost always results in split boards, stripped screw heads, or broken screws — all of which are frustrating and costly to fix.
Pre-drilling relieves the compression stress in the timber fibres, allowing the screw to pull the board down cleanly without forcing the grain apart.
Choosing the Right Drill Bit Size
The correct pilot hole size depends on the shank diameter of the screw you are using, not the thread diameter. For typical kwila decking installations in NZ:
| Screw Type | Screw Gauge | Shank Diameter | Recommended Pilot Bit |
|---|---|---|---|
| Decking screw (standard) | 10-gauge | ~4.8 mm | 3.5–4.0 mm |
| Decking screw (heavy duty) | 12-gauge | ~5.5 mm | 4.0–4.5 mm |
| Coach screw / structural | 8 mm shank | 8.0 mm | 6.5–7.0 mm |
These are general guides. Always check the screw manufacturer's recommendations, as thread profiles vary between brands. The pilot hole should grip the threads but allow the shank to pass through without binding.
Countersinking
After drilling the pilot hole, use a countersink bit (or a combination pilot/countersink bit) to create a recess for the screw head. This lets the head sit flush or just below the surface, which looks tidy and prevents water pooling around the head. A countersink angle of 82° is standard for most flat-head decking screws.
Combination Bits
Combination pilot/countersink bits are widely available at Bunnings and Mitre 10 and can save time by drilling the pilot and countersink in one pass. Make sure the bit is sized to match your specific screw.
Drilling Technique for Kwila
- Use a sharp bit. Kwila's density blunts bits quickly. HSS (high-speed steel) bits work, but cobalt or carbide-tipped bits last significantly longer in hardwood.
- Drill at moderate speed. High RPM generates heat that can harden the timber surface and dull your bit faster. A steady, moderate speed with firm pressure works best.
- Clear the flutes regularly. Kwila's oily nature can cause sawdust to pack into the bit. Withdraw the bit periodically to clear debris.
- Drill perpendicular to the board. A drill guide or steady hand keeps holes straight, ensuring screws pull down evenly.
- Pre-drill through the decking board only, not into the joist below, unless you are using a specific structural fixing that requires it.
Screw Selection for Kwila
Kwila's natural tannins and oils are corrosive to standard zinc-plated screws, which will rust and stain the deck orange-brown over time. Always use stainless steel (Grade 316) or hot-dipped galvanised screws rated for use with hardwood decking. This is especially important in coastal NZ environments where salt air accelerates corrosion. Check BRANZ guidance on corrosion zones if you are unsure what fixings are appropriate for your location.
Building Code and Consent Considerations
Most residential deck builds in NZ require a building consent, particularly if the deck is more than 1.5 m above ground or attached to the house. Some lower decks may qualify as exempt work under Schedule 1 of the Building Act, but you should always confirm with your local council before starting. The MBIE Building Performance website has useful guidance on when consent is required. Structural fixing requirements for decks are covered under NZS 3604 for timber-framed construction.
When to Call a Professional
Pre-drilling and fixing decking boards is generally DIY-friendly work, but there are situations where a qualified deck builder adds real value:
- Structural framing and posts — getting the substructure right is critical for safety and longevity.
- Consent documentation — a Licensed Building Practitioner (LBP) can carry out and sign off restricted building work. Find out more at Licensed Building Practitioners.
- Large or complex decks — elevated decks, decks over 1.5 m, or those attached to the house benefit from professional assessment.
- If you are unsure about fixing patterns or joist spacing — incorrect spacing can lead to bounce, movement, or structural failure over time.
You can find qualified, local deck builders on tradedirectory.co.nz who are familiar with kwila and NZ building requirements.
Licensed Building Practitioner (LBP)
