Is This a DIY Job?

Replacing a cracked or broken single-glazed pane is one of the more achievable DIY glazing tasks — provided the window frame is in reasonable condition and the pane is of a standard, non-toughened type. Do not attempt to DIY-replace double-glazed units, toughened glass, or heritage windows — these require specialist equipment or expertise.

With the right preparation and care, most homeowners can complete a single-pane replacement in a couple of hours.

What You'll Need

  • Replacement glass cut to size (order from a glazier or glass merchant)
  • Glazing putty (linseed oil putty for timber frames; acrylic putty for UPVC/metal)
  • Hacking knife or old chisel
  • Glazing sprigs or wire glazing clips (for timber frames)
  • Putty knife / glazing knife
  • Tape measure and pencil
  • Safety gloves and safety glasses
  • Primer (for bare timber rebates)

Step 1: Measure the Rebate Opening

Measure the internal dimensions of the frame rebate — the groove the glass sits in — both width and height. Measure in at least two places for each dimension, as frames are rarely perfectly square. Take the smallest measurement in each direction and subtract 3 mm from both to give a small clearance gap. This is the size to order your glass.

Step 2: Remove the Broken Pane Safely

Put on your safety gloves and glasses before touching anything. If the pane is cracked but still largely intact, score it across the break and use running pliers to remove it in sections. For a shattered pane, carefully remove loose shards working from the top of the frame downwards. Never reach around broken glass — always grip from the face.

Lay old newspaper on the ground beneath the window to catch falling fragments.

Step 3: Remove the Old Putty and Sprigs

Use a hacking knife or stiff old chisel to chop out all the old, hard putty from the rebate. Work steadily and keep the tool angled to avoid damaging the frame. Pull out any old glazing sprigs (small headless nails) with pliers.

Once the rebate is clear, brush away dust and debris. If the bare timber is exposed and unpainted, apply a thin coat of primer and allow it to dry — otherwise the frame will absorb oil from the new putty too quickly, causing it to crack.

Step 4: Bed the New Glass

Roll a length of putty in your hands until it's a smooth, even rope about 8 mm in diameter. Press it firmly into the rebate all the way around — this is called the back putty or bed. It cushions the glass and forms a weathertight seal at the back.

Carefully lift the new pane and press it into the bed putty, using gentle, even pressure around the edges. Never press the centre of the glass. Press until the pane is approximately 2 mm from the back of the rebate.

Step 5: Secure with Sprigs

Tap new glazing sprigs into the frame every 20–25 cm around the perimeter, using the edge of your chisel or a small hammer. They should sit flush against the glass face and penetrate the timber frame by about 10 mm. These mechanically hold the glass in place while the putty cures.

Step 6: Apply the Face Putty

Roll another rope of putty and press it into the angle between the glass face and the rebate frame all around. Use a putty knife held at roughly 45° to draw a smooth chamfer, cutting away excess putty and leaving a neat triangular bead. The putty edge should be level with or just below the sight line of the surrounding woodwork.

Dip your knife in water if the putty sticks. Remove any fingerprints with a clean, damp cloth.

Step 7: Cure and Paint

Leave the putty to skin over for at least two to three weeks before painting. Once dry to the touch, apply a coat of exterior wood primer and then a topcoat of gloss paint. Run the paint approximately 2 mm onto the glass to seal the putty edge against rain.

When to Call a Glazier

If the frame timber is rotten, if the window is above ground-floor level and you lack safe access, or if the broken pane is larger than around 60 × 90 cm, it's worth getting a professional glazier involved. Safety always comes first.