Replacing a single pane of glass in a timber window frame is a job many NZ homeowners can tackle themselves, provided the frame is in good condition and the replacement is straightforward — think a standard single-glazed sash or casement window. It does require care, the right materials, and a healthy respect for broken glass. If the frame is rotten, the window is large, or you're dealing with safety glass or double glazing, it's worth calling in a qualified joiner.
Is This a DIY Job or Restricted Work?
Replacing glass in a residential window is not restricted building work under the NZ Building Act, so you don't need a Licensed Building Practitioner to do it. However, there are a couple of important caveats:
- Safety glazing requirements: Under NZS 4223 and the NZ Building Code, certain locations require safety glass — for example, low-level panels, doors, bathrooms, and areas near stairs. If your broken pane is in one of these locations, the replacement glass must also be safety-rated (toughened or laminated). Check with your local council or MBIE Building Performance if you're unsure.
- Building consent: A simple like-for-like glass replacement in an existing frame generally falls under Schedule 1 exemptions and won't need a consent — but always confirm with your council before starting.
Tools and Materials You'll Need
- Glazing putty (linseed oil-based for timber frames is traditional; modern acrylic glazing compounds also work well)
- Hacking knife or old chisel for removing old putty
- Glazier's points or spring clips to hold the glass in place
- Putty knife or filling knife
- Tape measure and glass cutter (if cutting your own glass — though most glaziers and hardware stores will cut to size)
- Replacement glass — measure carefully (see below)
- Primer for bare timber before puttying
- Safety gear: heavy leather gloves, safety glasses, and sturdy footwear
You can find many of these supplies at Bunnings or Mitre 10.
Step-by-Step: How to Replace the Glass
1. Remove the Broken Glass Safely
Wear heavy gloves and safety glasses. Carefully remove any loose shards, then use a hacking knife or old chisel to chip away the old putty around the frame rebate. Pull out the glazier's points with pliers. Remove the remaining glass pieces and dispose of them safely — wrap in newspaper and label clearly.
2. Prepare the Rebate
Clean out all old putty and debris from the timber rebate. Sand back any rough spots. If bare timber is exposed, apply a coat of timber primer and let it dry fully — this stops the wood from drawing oil out of the new putty, which would cause it to crack prematurely.
3. Measure for the New Glass
Measure the rebate opening (the inner dimensions of the frame where the glass sits) in both width and height. Subtract 3 mm from each dimension to allow for expansion and fitting tolerance. Double-check your measurements before ordering — measure twice, cut once.
Take your measurements to a local glazier or hardware store to have the glass cut. For most standard windows, 3 mm or 4 mm float glass is typical, but confirm the thickness of the original pane.
4. Bed the Glass in Putty
Roll a thin rope of glazing putty and press it into the rebate all the way around — this is called the back putty or bedding layer. Press the new pane of glass firmly into the rebate, starting from one corner and working around. Apply even pressure so the glass beds into the putty, leaving a thin, consistent layer behind it.
5. Secure with Glazier's Points
Press glazier's points (small metal clips) into the timber frame every 150–200 mm around the perimeter using a putty knife or a flat screwdriver. These hold the glass in place while the front putty cures.
6. Apply the Front Putty
Press glazing putty firmly into the angle between the glass and the frame. Use a putty knife held at a consistent 45-degree angle to create a neat, smooth bevel. Trim away any excess putty from the glass face with the knife.
7. Paint and Finish
Leave the putty to skin over for at least 7–14 days before painting. When ready, paint over the putty and slightly onto the glass (about 2 mm) to seal the joint and protect the putty from weathering. Resene and Dulux both offer suitable exterior timber paints and primers.
Common Mistakes to Avoid
| Mistake | Why It Matters |
|---|---|
| Skipping the timber primer | Putty dries out and cracks within months |
| Glass cut too large | Cracks under thermal movement |
| Too few glazier's points | Glass can shift or rattle |
| Painting before putty skins | Paint traps moisture, putty won't cure |
| Using the wrong glass type | Safety glazing rules may apply |
When to Call a Professional
Some situations are best left to a qualified joiner or glazier:
- Rotten or damaged timber frames — the frame needs repair or replacement before re-glazing
- Large panes or overhead glazing — heavy glass is a serious injury risk
- Double-glazed units (IGUs) — these are factory-sealed and cannot be re-glazed at home; the whole unit must be replaced
- Safety glazing locations — if you're unsure whether safety glass is required, get professional advice
- Heritage or leadlight windows — specialist skills are needed
A qualified joiner will also be able to assess whether the window frame itself is worth saving or whether replacement is more cost-effective. You can find experienced local joiners and glaziers on tradedirectory.co.nz.
A Note on Safety
Glass injuries can be serious. Always wear cut-resistant gloves and safety glasses when handling glass. Never lean a pane of glass unsupported against a wall — it can slide and shatter. Dispose of broken glass responsibly, wrapped securely so it can't cut through rubbish bags.
For guidance on working safely, refer to WorkSafe NZ.
