Choosing the right glass thickness for a window replacement isn't just about picking what looks similar to what you already have. In New Zealand, the correct thickness depends on the size of the pane, your home's wind zone, the window's location, and whether safety glazing is required under the NZ Building Code. Getting it wrong can be a safety risk — and may not comply with code.
Why Glass Thickness Matters
Glass thickness affects structural strength, wind load resistance, thermal performance, and safety. A pane that's too thin for its size or exposure can flex excessively, crack under wind pressure, or shatter dangerously. In New Zealand, our coastal and high-wind locations make this especially important.
Common glass thicknesses available in NZ are 3 mm, 4 mm, 5 mm, 6 mm, 8 mm, 10 mm, and 12 mm, with thicker options available for special applications. For most standard residential windows, 4 mm to 6 mm is typical, but this is a starting point — not a rule.
Key Factors That Determine the Right Thickness
1. Pane Size (Area and Aspect Ratio)
The larger the pane, the thicker the glass needs to be. A small bathroom louvre and a large picture window have very different requirements. As a general guide:
- Small panes (under roughly 0.5 m²): 3–4 mm may be sufficient
- Medium panes (0.5–1.5 m²): 4–6 mm is common
- Large panes (over 1.5 m²): 6 mm or more is often required
Always confirm with your glazier, as the aspect ratio (height vs. width) also affects the calculation.
2. Wind Zone
New Zealand is divided into wind zones under NZS 3604 (Timber-framed buildings) — Low, Medium, High, Very High, and Extra High. Homes in exposed coastal or elevated areas face much greater wind loads, requiring thicker or toughened glass. Check your local council or ask a glazier to confirm your property's wind zone.
3. Safety Glazing Requirements
Under NZ Building Code clause F2 (Hazardous Building Materials) and related clauses, certain locations require safety glass — either toughened (tempered) or laminated glass — regardless of thickness. These locations typically include:
- Doors and sidelights (glazing within 300 mm of a door handle or within 1500 mm of the floor in a door panel)
- Low-level glazing (panes with the bottom edge within 800 mm of the floor)
- Wet areas such as shower screens and bathrooms
- Stairwells and balustrades
Safety glass is available in toughened (which shatters into small, less harmful pieces) and laminated (which holds together when broken) forms. Your glazier will advise which type suits your situation. You can read more about the Building Code requirements at MBIE Building Performance.
4. Double Glazing and IGUs
If you're upgrading to double glazing (an Insulated Glass Unit or IGU), the overall unit thickness increases significantly — typically 20–28 mm total, made up of two glass panes with a spacer and air or gas gap between them. Each pane within an IGU is commonly 4–6 mm. Double glazing improves thermal performance and is encouraged under Energywise guidance for warmer, drier homes. If your window frame was designed for single glazing, it may need modification to accept a thicker IGU.
5. Acoustic and Special Performance Glass
For windows facing busy roads or noisy neighbours, acoustic laminated glass (typically 6.38 mm or thicker) can significantly reduce noise. This is a specialist product — discuss your needs with a glazier.
Does Replacing Glass Require a Building Consent?
Like-for-like glass replacement in an existing window frame is generally exempt from building consent under Schedule 1 of the Building Act 2004. However, if you're changing the window size, frame, or upgrading to a significantly different glazing system, consent may be required. Always check with your local council before starting work, as exemptions have conditions. See MBIE Building Performance for Schedule 1 guidance.
A Handy Reference Table
| Situation | Typical Glass Type | Typical Thickness |
|---|---|---|
| Small standard window (low wind zone) | Float glass | 3–4 mm |
| Medium residential window | Float or toughened | 4–6 mm |
| Large picture window or high wind zone | Toughened | 6–10 mm |
| Door panel or low-level glazing | Toughened or laminated | 6 mm+ (safety rated) |
| Shower screen | Toughened | 6–10 mm |
| Double glazing unit (IGU) | Float/toughened (per pane) | 4–6 mm per pane |
| Acoustic glazing | Laminated | 6.38 mm+ |
This table is a general guide only. Always confirm requirements with a qualified glazier.
How to Measure Your Existing Glass
If you're trying to match existing glass, use a vernier calliper or glass thickness gauge for an accurate reading. Don't rely on visual estimation — 4 mm and 5 mm look almost identical. Your glazier will measure on-site before ordering.
When to Call a Professional
Glass selection and installation involves safety-critical decisions that are best made by a qualified glazier. While a homeowner can remove and refit glass in some situations, incorrect glass selection, improper cutting, or failure to use safety glass where required can result in serious injury and non-compliant work. For anything beyond a straightforward small pane replacement — especially safety glazing locations, large panes, double glazing, or high wind zone properties — engage a qualified glazier. You can find experienced local glaziers on tradedirectory.co.nz.
Also note: if your window replacement involves any changes to the building structure, waterproofing (e.g. around flashings), or electrical elements (such as heated glass), additional licensed tradespeople may be required. Electrical work must be carried out by a registered electrician — see the Electrical Workers Registration Board for more information.
Window & Glass Association
