Yes, installing a flush bolt on a double door is generally something a competent DIYer can tackle in New Zealand — no licensed trade is required for this type of hardware fitting. That said, getting it right matters for both security and the smooth operation of your doors, so it pays to understand the process before you start.
What Is a Flush Bolt and Why Do Double Doors Need One?
A flush bolt (sometimes called a concealed bolt) is a sliding bolt that sits recessed into the edge or face of a door, keeping it flush when not in use. On a double door (also called a French door or bi-fold entry), one leaf is typically the active door (the one you open daily) and the other is the inactive door. The inactive leaf needs to be secured at the top and bottom — that's exactly what flush bolts do, shooting into the door frame head and the floor or threshold below.
Without properly fitted flush bolts, the inactive door can flex, rattle, or even be forced open, which is a real security concern.
Is This Restricted Work in NZ?
Fitting a flush bolt is not restricted building work and does not require a building consent under Schedule 1 of the Building Act 2004 — it is considered routine hardware maintenance or replacement. It is also not electrical, gas, or plumbing work, so no licensed professional is legally required.
However, if your double doors are part of a fire-rated assembly (common in commercial buildings or some multi-unit dwellings), the hardware must comply with the fire rating requirements and you should check with your local council or a qualified tradesperson before proceeding. For residential homes, this is rarely a concern, but it's worth confirming. You can find guidance on what requires consent at MBIE Building Performance.
Tools and Materials You'll Need
- Flush bolt set (top and bottom) — available from hardware stores
- Tape measure and pencil
- Combination square
- Sharp wood chisel (25 mm is handy)
- Mallet
- Power drill with spade bits and a flat drill bit matching your bolt barrel diameter
- Screwdriver (or drill driver)
- Strike plate and strike cup (usually included in the kit)
- Masking tape (to protect the door finish while chiselling)
You'll find flush bolt hardware at Bunnings or Mitre 10. Choose a bolt with a stainless steel or brass finish if your doors are exposed to NZ coastal conditions — salt air accelerates corrosion on cheaper zinc alloy fittings.
Step-by-Step Installation Guide
1. Mark the bolt position
With the inactive door closed and latched, mark the centre line of the door edge at the top and bottom where you want each bolt. Flush bolts are typically positioned about 150–200 mm from the top and bottom of the door.
2. Rout or chisel the recess
Hold the flush bolt faceplate against the door edge and trace around it with a pencil. Using a sharp chisel, carefully pare out a recess so the faceplate sits perfectly flush with the door edge. Take your time — a clean recess makes all the difference.
3. Drill the barrel channel
Using a spade bit that matches the bolt barrel diameter (check your hardware specs), drill straight into the door edge along the centre line to the required depth. Keep the drill level — a drill press or a drill guide helps here.
4. Fit the bolt body
Slide the bolt barrel into the drilled channel, position the faceplate in its recess, and screw it firmly into place. Test that the bolt slides smoothly.
5. Mark and fit the strike plates
With the bolt extended, close the door gently to transfer the bolt position onto the door frame head (top) and the floor or threshold (bottom). Drill and chisel recesses for the strike cups, then screw them in. In NZ homes with concrete floors, you may need a masonry bit and a suitable anchor.
6. Test and adjust
Open and close the door several times, operating both bolts. The bolt should engage smoothly without forcing. If it binds, check your alignment and adjust the strike cup position slightly.
Tips for NZ Conditions
- Timber movement: NZ homes — especially older villas and bungalows — can have timber doors that swell in winter and shrink in summer. Allow a small amount of clearance in your strike cups to account for seasonal movement.
- Aluminium doors: Many modern NZ homes have aluminium double doors. Flush bolts for aluminium doors are a different product (surface-mounted or frame-integrated) and may require specific tools. Check the manufacturer's instructions carefully.
- Healthy Homes compliance: If you're a landlord, door security and weathertightness are relevant to the Tenancy Services Healthy Homes standards — properly secured doors contribute to a compliant rental property.
When to Call a Professional
Consider hiring a qualified locksmith if:
- Your doors are aluminium-framed and you're unsure which hardware is compatible
- The door frame or floor is concrete or masonry and you're not confident with masonry drilling
- The doors are part of a fire-rated or security-rated assembly
- You want the work done to a guaranteed standard for insurance or tenancy purposes
- The existing door or frame is damaged and needs repair before hardware can be fitted
A professional locksmith will have the right tools and experience to get the job done cleanly and correctly the first time. You can find qualified local locksmiths on tradedirectory.co.nz.
