Rot around the glass in a timber window frame is almost always caused by water getting in through a failed glazing seal — whether that's cracked putty, dried-out glazing tape, or paint that's peeled away and left bare timber exposed. The good news is that if you catch it early, this is a manageable DIY job. Left too long, though, rot can spread deep into the frame and you'll need a professional glazier or joiner to carry out repairs.
Why Timber Rots Around the Glass
The joint between glass and timber is one of the most vulnerable spots on any window. Water sits in the rebate (the groove the glass sits in), and if the seal isn't watertight, it soaks into the end grain of the timber — which absorbs moisture fastest. New Zealand's variable climate, with driving rain in many regions, makes this especially common. UV exposure also degrades putty and paint over time, opening up gaps.
Common causes include:
- Cracked or shrunk glazing putty that no longer bonds to the glass
- Dried or compressed glazing tape that has lost its seal
- Peeling or missing paint leaving bare timber exposed to moisture
- Poor original installation with no back-bedding compound under the glass
- Condensation running down the glass and pooling in the rebate
Assessing the Damage
Before you start, probe the timber with a screwdriver or bradawl. If it sinks in easily, the rot is active and soft. Soft rot that's more than a few millimetres deep may mean the structural integrity of the frame is compromised — at that point, call a professional. Firm timber with surface discolouration is much easier to treat yourself.
Also check whether the glass itself is cracked or the glazing bead (the strip holding the glass in) is loose, as these will need to be addressed at the same time.
Step-by-Step: Resealing and Treating the Frame
1. Remove Old Putty or Glazing Tape
Using a hacking knife or old chisel, carefully remove all cracked or loose putty from the rebate. Take care not to crack the glass. If glazing tape was used, peel it away completely. Clean out any debris.
2. Treat the Bare Timber
Once the rebate is clear, treat any discoloured or mildly rotted timber with a wood hardener (available at Bunnings or Mitre 10). Products like Selleys Wood Hardener or similar consolidants soak into softened fibres and harden them. Allow to dry fully per the manufacturer's instructions.
For small areas of soft rot, you can also use a two-part epoxy wood filler to rebuild the rebate profile before resealing.
3. Prime All Bare Timber
This step is critical and often skipped. Prime every bit of bare timber — including the rebate — before applying new putty or tape. Use a quality exterior primer such as those from Resene or Dulux. Unprimed timber will absorb oils from the putty, causing it to dry out and crack prematurely.
4. Re-bed and Reseal the Glass
Once the primer is touch-dry:
- Back-bed the glass by running a thin bead of linseed oil putty or glazing compound into the rebate before pressing the glass back in. This prevents water sitting under the glass.
- Secure the glass with glazing sprigs (small metal pins tapped into the timber) or replace the glazing bead.
- Apply fresh linseed oil putty to the front face, pressing it firmly into the joint and angling it so water runs away from the glass. Smooth with a wetted putty knife.
- Alternatively, use a neutral-cure silicone rated for timber and glass — check the product is paintable if you intend to paint over it.
5. Paint to Protect
Allow putty to skin over (typically 1–2 weeks for linseed putty) before painting. Apply a quality exterior topcoat that laps slightly onto the glass — this paint-to-glass overlap is your primary weatherseal. Wattyl and Resene both offer durable exterior gloss and semi-gloss options suited to NZ conditions.
Ongoing Prevention
- Inspect and repaint timber windows every 3–5 years, or sooner if paint is peeling.
- Keep the paint-to-glass seal intact — this is your first line of defence.
- Ensure window sills drain freely and aren't holding water.
- Consider a clear timber preservative on unpainted rebate surfaces during maintenance.
- Check that flashings and head trims above windows are directing water away from the frame.
For guidance on weathertightness requirements, BRANZ and MBIE Building Performance both publish useful resources on external joinery maintenance.
When to Call a Professional
Call a qualified glazier or joiner if:
- The rot is deep or the frame feels spongy over a large area
- The glass needs to be removed and re-cut to size
- The window frame needs to be rebuilt or replaced entirely
- You're dealing with double-glazed units — these must be replaced as sealed units and require specialist handling
- The window is large, high up, or difficult to access safely
Replacing a window frame may require a building consent depending on the scope of work — always check with your local council first. The Licensed Building Practitioners scheme covers restricted building work, and your council can advise whether your job falls under Schedule 1 exemptions.
You can find qualified local glaziers and joiners on tradedirectory.co.nz.
Window & Glass Association
