Yes, reglazing a timber window sash is well within reach for a capable DIYer. It requires no licensed trade qualifications, and with the right tools, materials, and a bit of patience, you can replace a broken or deteriorated pane yourself and save on call-out costs. That said, there are situations — such as large panes, heritage joinery, or double-glazed units — where calling a professional glazier is the smarter move.
What Is Reglazing?
Reglazing means removing an old or broken pane of glass from a window sash and fitting a new one, bedded and sealed with glazing putty (also called linseed oil putty or window putty). Timber sashes are traditionally glazed this way, and the technique hasn't changed much in over a century. The putty holds the glass in place, seals out weather, and gives that classic angled profile you see on older NZ villas and bungalows.
Tools and Materials You'll Need
- Hacking knife or old chisel — for removing old putty
- Glazier's hammer or pin hammer
- Glazier's hacks (spring clips/glazing sprigs) — small metal pins that hold the glass while the putty cures
- Putty knife — for applying and smoothing putty
- Tape measure and glass cutter (if cutting your own glass — see note below)
- Linseed oil putty — the standard choice for timber sashes
- Primer — to seal bare timber before puttying
- Safety gloves and eye protection — essential when handling broken glass
- Replacement glass — have it cut to size at a glass merchant or hardware store
Step-by-Step: How to Reglaze a Timber Sash
1. Remove the Sash (If Possible)
Where practical, take the sash off its runners and lay it flat on a workbench. This makes the job much easier and safer.
2. Remove Broken Glass Safely
Wear heavy gloves and safety glasses. Carefully remove loose shards, then use a hacking knife or chisel to chip away the old putty. Pull out the old glazing sprigs with pliers.
3. Clean Up the Rebate
The rebate is the stepped groove the glass sits in. Remove all old putty, paint flakes, and debris. Sand lightly if needed.
4. Prime the Bare Timber
This is a step many DIYers skip — don't. Bare timber will suck the oil out of your putty, causing it to crack and fail prematurely. Apply a coat of linseed oil or a suitable timber primer and let it dry before continuing. Resene and Dulux both stock suitable primers.
5. Measure and Order Your Glass
Measure the rebate opening carefully — height and width — then deduct 3 mm from each dimension to allow for clearance and thermal movement. Have the glass cut to size at a glass merchant; most will cut standard 3 mm or 4 mm float glass while you wait. Cutting glass yourself is possible but takes practice.
6. Bed the Glass in Putty
Knead a small amount of linseed oil putty until it's pliable. Press a thin, even layer (about 3 mm) into the rebate — this is called the back putty or bedding layer. Press the glass firmly into this bed, applying pressure at the edges (never the centre).
7. Secure with Glazing Sprigs
Hammer glazing sprigs into the timber every 150–200 mm around the perimeter, sliding them flat against the glass face. These hold the glass while the front putty cures.
8. Apply the Front Putty
Press putty into the rebate at a 45° angle using your putty knife, creating a neat chamfered bead. Smooth it off so it sits just below the sight line (the inner edge of the rebate) — this keeps it invisible from inside and allows paint to seal it properly.
9. Paint Over the Putty
Once the putty has skinned over (usually 1–2 weeks, depending on temperature), paint over it with an exterior-grade paint, lapping slightly onto the glass to form a weathertight seal. Wattyl and Resene both offer suitable exterior trim paints.
Key Tips for a Long-Lasting Result
| Step | Common Mistake | Best Practice |
|---|---|---|
| Priming | Skipping primer | Always prime bare timber first |
| Glass size | Cutting too tight | Deduct 3 mm each dimension |
| Putty | Using cold, stiff putty | Warm it in your hands first |
| Painting | Painting too soon | Wait for putty to skin (1–2 weeks) |
| Sprigs | Forgetting them | Always use — putty alone won't hold |
Safety Notes
Broken glass is a serious hazard. Always wear cut-resistant gloves and safety glasses, wrap broken glass in newspaper before disposal, and never put loose shards in a recycling bin. Check your local council's guidelines for glass disposal. For work at height, use a stable ladder and follow safe working practices as outlined by WorkSafe NZ.
When to Call a Professional
While reglazing a standard single-glazed timber sash is DIY-friendly, there are situations where you should engage a qualified glazier:
- Large panes (over roughly 0.5 m²) — heavy and awkward to handle safely alone
- Double-glazed units (IGUs) — these are factory-sealed and cannot be reglazed with putty; the whole unit must be replaced
- Heritage or character homes — specialist glass types (e.g. obscure, tinted, or heritage cylinder glass) may be required
- Overhead or high-level glazing — safety glass requirements may apply under the NZ Building Code
- Any glazing in a safety glazing zone — areas such as beside doors, low-level panels, and bathrooms may require safety glass under NZS 4223. Check with your local council or MBIE Building Performance if you're unsure.
A licensed glazier will know the relevant standards and can certify the work where required. Find a qualified local glazier on tradedirectory.co.nz.