How to replace a broken window pane

Inspired by a helpful UK guide. This walk-through is based on "DIY — How to replace a broken window pane with putty" from Philippa Tuttiett, a UK DIY presenter who has appeared on Channel 4 and Channel 5. The video covers the full process clearly, from removing the old glass to finishing the putty bead, and is well worth watching before you open the tube.

1. Protect yourself before touching anything

Thick gloves and safety glasses on before you go near broken glass. No exceptions. Wrap a cloth around your hand when pressing out shards, and work slowly. Sharp edges are unpredictable and a cut at this stage delays the whole job.

Lay a dust sheet or cardboard on the floor below the window to catch falling glass. Have a stiff brush and cardboard box ready for clearing up the pieces.

2. Remove the broken glass

Wearing thick gloves, gently work any loose shards free. If the pane is still mostly intact, score around the putty line with a hacking knife or old chisel to free it, then ease the glass out carefully.

There are often small metal glazing sprigs (tiny headless nails) holding the glass in place. Pull these out with pliers before removing the glass. Do not force anything or more will break.

3. Clear out all the old putty

Use a hacking knife or cold chisel to remove every bit of old putty from the rebate. Old putty gets very hard — warm dry days make this easier, cold damp ones make it miserable. Work patiently around all four sides.

Once the putty is out, sand the rebate lightly and give bare timber a coat of raw linseed oil or primer. This stops the wood sucking the oil out of the new putty too quickly and causing it to crack.

4. Measure the opening and order the glass

Measure the opening in at least two places in each direction — windows are rarely perfectly square. Take the smallest measurement and deduct 3 mm from each dimension to give the glass a little room to expand and sit without cracking.

Most glass merchants and builders’ merchants can cut glass to size. Ask for 4 mm float glass for a standard window. Take your measurements written down rather than relying on memory.

5. Apply the bed of putty

Knead the putty until it is soft and pliable. Press it firmly into the rebate all the way round to form a continuous bed, roughly 3–4 mm deep. This cushions the glass and seals the back edge.

Linseed oil putty is the traditional material for timber frames — it stays workable for a good while and keys well to wood. Do not use it on metal frames, which need a different compound.

6. Press the glass into place and fit the sprigs

Press the glass firmly but evenly into the putty bed, pushing from the edges rather than the centre. It should sink down to leave around 2 mm of putty visible from the front.

Tap in new glazing sprigs every 300 mm or so around the perimeter to hold the glass while the putty cures. Slide the shaft of a screwdriver along the glass surface to drive them in — this avoids knocking the pane.

7. Apply the front putty bead and neaten up

Roll a sausage of putty and press it firmly into the angle between the glass and the frame on all four sides. Draw a putty knife at roughly 45 degrees along the bead to form a neat chamfer. One smooth pass is better than several nervous ones.

Trim away any surplus putty from the glass with the knife. Leave it to harden for two to three weeks before painting over it. Paint the putty slightly onto the glass to seal the joint and keep water out.

When to call a handyman

This guide is for single-glazed timber or steel frames with putty. Double-glazed units are a different job entirely — the sealed unit needs replacing as a whole, often with specialist equipment. Call Richard if you are unsure about the frame type, if the glass is large or high up, or if the timber rebate is rotten and needs repairing before the new glass goes in.