Replacing a cracked glass insert in an exterior door is something a confident DIYer can tackle in straightforward situations — but there are important rules around safety glazing, the type of door, and the glass specification that you must get right before you start. Get it wrong and you could face a safety hazard, a failed home inspection, or a void insurance claim.
Why the Type of Glass Matters Most
In New Zealand, exterior doors are considered a safety glazing location under NZS 4223: Glazing in Buildings. This means any glass in or immediately beside an exterior door must be a safety glazing product — typically toughened (tempered) glass or laminated safety glass. Standard float glass is not acceptable and is illegal in these locations.
Safety glass is required because a person could fall into or through a door panel. If you install the wrong glass and someone is injured, you could be personally liable. Always check the existing glass for an etched or printed safety mark (look in a corner of the pane) before ordering a replacement, and match the specification exactly.
For double-glazed units (IGUs — insulated glass units), the replacement must also meet the NZ Building Code clause H1 energy efficiency requirements if you are in a regulated climate zone. A like-for-like replacement of a failed IGU is generally acceptable, but check with your local council if you are unsure whether a building consent is needed.
Do You Need a Building Consent?
Replacing a broken pane with the same specification glass in the same opening is generally treated as maintenance and falls under Schedule 1 exemptions — meaning no building consent is required. However, this is not a blanket rule. If you are changing the size of the opening, upgrading to a different glass type, or the door is part of a heritage or listed building, consent may be needed. Always confirm with your local council before starting work. You can find general guidance at MBIE Building Performance.
Assessing Whether This Is a DIY Job
Before reaching for your tools, ask yourself:
- Is the glass a single pane set into a simple timber rebate, or is it a sealed double-glazed unit (IGU)?
- Is the door a standard hinged timber door, or is it an aluminium-framed, composite, or fire-rated door?
- Do you know the exact glass specification (thickness, type, any Low-E coating)?
- Are you comfortable working with sharp materials and using a glass cutter or ordering cut-to-size glass?
Good candidates for DIY
- Single-pane toughened glass in a timber-rebated door
- Glass held in place with timber beading (pinned or screwed) rather than structural silicone or glazing tape
- Straightforward rectangular panes with no special coatings
Better left to a professional glazier
- Double-glazed IGUs (these must be factory-sealed; you cannot reseal them at home)
- Aluminium-framed doors (require specialist tools and correct glazing tape/packers)
- Laminated or specialty glass (fire-rated, acoustic, decorative)
- Any glass with an unusual shape or size
- Situations where you cannot confirm the correct safety glass specification
Step-by-Step: Replacing a Single Pane in a Timber Door
- Make the area safe. Wear heavy-duty cut-resistant gloves and safety glasses. Lay a drop sheet to catch glass fragments. Keep children and pets well away.
- Remove the beading. Use a flat bar or chisel to carefully prise off the timber beading around the glass. Number each piece so you can refit them in order.
- Remove the broken glass. Work from the top down. Use masking tape across cracks before removal to hold fragments together. Dispose of glass safely — wrap in newspaper and place in a cardboard box labelled "broken glass" before putting in your general rubbish bin.
- Clean the rebate. Remove old putty, glazing tape, or silicone. Sand smooth if needed.
- Measure carefully. Measure the rebate opening (height and width) and deduct 3–4 mm from each dimension to allow for expansion. Double-check before ordering.
- Order the correct safety glass. Take your measurements and the existing glass specification to a glazing supplier or hardware store. Confirm the glass meets NZS 4223 requirements for safety locations. Bunnings and Mitre 10 can cut glass to size in-store, though for toughened glass you will need to order from a glazing specialist as it cannot be cut after tempering.
- Bed the new glass. Apply a thin bead of glazing compound or fit new glazing tape into the rebate. Set the glass on glazing packers (small plastic blocks) at the bottom corners to keep it centred and prevent direct contact with the timber.
- Refit the beading. Pin or screw the beading back into place, applying a small bead of paintable sealant where the beading meets the glass if required.
- Seal and finish. Apply a bead of exterior-grade silicone or glazing sealant around the perimeter if needed. Sika produce suitable exterior glazing sealants widely available in NZ.
When to Call a Professional
Call a licensed glazier if:
- The door has an aluminium frame or the glass is part of a sealed IGU
- You cannot identify the correct safety glass specification
- The door is fire-rated (fire doors require certified components and must not be modified by unqualified persons)
- The crack was caused by structural movement, which may indicate a deeper building issue
- You simply aren't confident — a professional glazier will complete the job quickly, correctly, and with the right materials
Glaziers are not a licensed trade in the same way as electricians or plumbers, but a qualified tradesperson will know NZS 4223 inside out and will guarantee their work. You can find local glaziers on tradedirectory.co.nz.
A Note on Safety
Broken glass is one of the most common causes of serious lacerations in home DIY. Never rush the removal process, always wear appropriate PPE, and never attempt to carry a large pane of glass alone.
Window & Glass Association
