Cutting a circular hole in glass for a ventilation fitting is something a careful DIYer can attempt on standard annealed (float) glass — but it requires the right tools, patience, and a clear understanding of what can go wrong. If the glass is toughened (tempered), laminated, or already installed in a window frame, the job almost certainly needs a professional glazier.
Understanding Your Glass Type First
Before you pick up any tool, you must identify what type of glass you're working with. This is the single most important step.
- Annealed (float) glass — plain, unstrengthened glass. This is the only type you can realistically cut yourself.
- Toughened (tempered) glass — cannot be cut after toughening. Any attempt will shatter it into hundreds of small pieces. Toughened glass is required by the NZ Building Code in safety-critical locations (beside doors, low-level panels, bathrooms, etc.).
- Laminated glass — two or more layers bonded with an interlayer. Specialist cutting is required.
- Double-glazed units (IGUs) — cutting into a sealed unit will destroy the seal and cause fogging. These must be replaced or modified by a glazier.
If you're unsure what type of glass you have, tap it gently near a corner — toughened glass often has a faint ring, and most toughened panes carry a small etched or printed mark in a corner. When in doubt, ask a glazier.
Tools You'll Need
To cut a circular hole in annealed glass, you'll need:
- Circle cutter (beam compass glass cutter) — available from Bunnings or Mitre 10. These have a suction-cup centre pivot and an adjustable scoring arm.
- Glass cutting oil or kerosene — to lubricate the scoring wheel.
- Glass pliers or tapping tool — to run and open the score line.
- Safety glasses and cut-resistant gloves — non-negotiable.
- A flat, stable work surface — a sheet of carpet or foam underlay works well.
- Masking tape — to mark your centre point and protect the surface.
Step-by-Step: Cutting the Hole
1. Prepare your workspace
Lay the glass flat on a padded, level surface. Clean the glass thoroughly — dust and grit will deflect your cutter. Mark the centre of your hole with a felt-tip pen on masking tape.
2. Set up the circle cutter
Attach the suction cup to the centre mark. Set the arm to the correct radius for your ventilation fitting (measure the fitting's spigot diameter carefully — most domestic ventilation fittings are 100 mm or 150 mm in diameter).
3. Score the circle
Apply a small amount of cutting oil to the wheel. In one smooth, continuous motion, score a complete circle. Apply firm, even pressure — you should hear a consistent hissing sound. Score once only; going over the line again weakens the cut.
4. Score relief lines
From inside the circle, score several straight lines from the centre outward to the circular score, like spokes on a wheel. This helps you remove the glass in segments without cracking the surrounding pane.
5. Tap and remove
Using the ball end of your glass cutter or a small rubber mallet, gently tap beneath each scored segment until you see the score "run" (a faint white line appears). Carefully break out each segment with glass pliers, working from the centre outward.
6. Smooth the edge
Use a diamond hand pad or carborundum stone (wet) to smooth the inside edge of the hole. This reduces the risk of chipping and makes fitting the ventilation collar easier.
Safety Considerations
Glass cutting produces sharp shards and fine glass dust. Always:
- Wear safety glasses and cut-resistant gloves throughout.
- Work in a well-ventilated area and avoid inhaling glass dust.
- Dispose of glass offcuts safely — wrap in newspaper and place in a cardboard box before putting in the bin. Check your local council's guidelines for glass disposal.
- Keep children and pets well away from your work area.
For health and safety guidance, WorkSafe NZ has useful information on working safely with glass and hazardous materials.
NZ Building Code and Consent Considerations
Adding a ventilation fitting to a window or wall may interact with NZ Building Code clauses E2 (external moisture), G4 (ventilation), and E3 (internal moisture). In most cases, fitting a small ventilation grille is a minor repair or maintenance task that doesn't require a building consent under Schedule 1 exemptions — but this depends on the scope of work and your specific council. Always confirm with your local council before starting, particularly if the work involves modifying a weathertight element.
If the ventilation fitting is part of a mechanical system (e.g., a rangehood or bathroom fan ducted through glass), the electrical connections must be carried out by a registered electrician — this is restricted electrical work under the Electricity Act. Check EWRB for what homeowners can and cannot do themselves.
When to Call a Professional
Call a qualified glazier if:
- The glass is toughened, laminated, or part of a double-glazed unit.
- The glass is already installed in a frame (cutting in situ is very difficult and risky).
- The hole required is large (over 150 mm) or very close to the edge of the pane.
- You're not confident in your ability to score cleanly and consistently.
- The glass is in a safety-critical location as defined by the NZ Building Code.
A professional glazier has specialist hole-saw equipment that produces clean, accurate results with far less risk of cracking the pane. You can find qualified local glaziers on tradedirectory.co.nz.
Window & Glass Association
