How do I replace a broken pane in a timber window?

Removing the old pane and putty safely, measuring for the replacement, and bedding the new glass so it does not rattle or leak.

Difficulty: intermediate
Time: 2 hr
Read: 4 min
Safety: warning

Short answer

Tape the broken pane, remove it with the putty and the sprigs, measure the rebate and take 3mm off each dimension, then bed the new pane on a thin layer of putty, fix with sprigs and finish with a putty fillet.

Single glazing in a timber frame is one of the few glazing jobs still genuinely within reach at home. The technique has not changed in a century and the materials are inexpensive.

Safety

Broken glass causes serious cuts and glass under tension can spring without warning. Wear cut-resistant gloves, eye protection, long sleeves and stout shoes, and tape the cracked pane before disturbing it. Never carry a large pane alone, and dispose of broken glass wrapped and labelled. Where the pane is in a door, a side panel, or within 800mm of floor level, safety glass is a legal requirement — do not fit ordinary float glass there.

What you'll need

Cut-resistant gloves and eye protection
Linseed oil putty
Glazing sprigs and a pin hammer or a glazier's tool
An old chisel or a hacking knife
A putty knife
Masking tape
A tape measure

Step by step

Tape the broken pane.Criss-cross masking or duct tape over both faces. It holds the fragments together while you work and turns a dangerous job into a manageable one.
Wear the right protection.Cut-resistant gloves rather than gardening gloves, eye protection, sleeves and closed shoes. Most glazing injuries are to the forearm and the palm.
Remove the glass from the top down.Work the fragments out of the rebate. Gravity is your enemy here — take the upper pieces first so nothing falls onto your hands.
Chop out the old putty.A hacking knife or a sharp old chisel, working along the rebate. Softening it first with a heat gun on low helps, but keep the heat away from adjacent glass, which cracks from thermal shock.
Pull the sprigs.Small headless pins in the rebate held the old pane. Pincers get them out. Leaving one behind cracks the new pane as you press it in.
Clean and prime the rebate.Bare timber sucks the oil out of linseed putty, which then crumbles within a couple of years. A coat of primer or a thin oil wash on the rebate is essential.
Measure the rebate accurately.Width and height inside the rebate at several points, take the smallest of each, and subtract 3mm from both so the glass has room to move and to be bedded.
Specify safety glass where required.Toughened or laminated glass is required in doors, in panels beside doors, and generally below 800mm from floor level. A glass merchant will advise, and it costs surprisingly little more.
Bed the pane on back putty.Run a thin continuous layer of putty around the rebate, press the glass into it from the edges rather than the centre, and squeeze it down to a 1–2mm bed.
Fix with sprigs.Two per side minimum, more on a large pane, tapped in flat against the glass by sliding the hammer along the surface rather than swinging it.
Run the front fillet.Press putty into the rebate with your thumb, then strike it off at an angle with a putty knife so the line sits just below the sight line of the glazing bar on the inside.
Leave it to skin before painting.Linseed putty needs one to four weeks to form a skin. Paint it then, and take the paint 2mm onto the glass to seal the joint.

Tips

Warm the putty in your hands and knead it until it is like plasticine. Cold, unkneaded putty is unworkable and is why people give up on it.
Buy the glass cut to size. Merchants cut to the millimetre for very little money and it removes the hardest part of the job.
Where the frame takes beads rather than putty, the job is much simpler — remove the beads, replace the glass on packers, and pin the beads back.

Common mistakes

Fitting ordinary float glass in a door or a low-level panel — The building regulations across the UK require safety glass in critical locations because ordinary glass breaks into long, sharp shards. This is a genuine safety and legal issue, not a preference, and it affects insurance and future sale.
Not priming the rebate before puttying — Bare timber draws the linseed oil out of the putty, which then dries hard, shrinks and falls out in flakes within a year or two. The pane starts rattling and water gets into the rebate.

If it doesn't work

New pane rattles

Cause: Insufficient back putty or too few sprigs — Fix: Take the fillet off, re-bed and add sprigs.

Putty cracks and falls out after a year

Cause: Rebate not primed, or putty painted too soon or too late — Fix: Rake out, prime the rebate, re-putty and paint after skinning.

Glass cracked while fitting

Cause: Cut too tight, or a sprig left behind — Fix: Always allow 3mm clearance on both dimensions and check the rebate is clear.

Water getting past the putty line

Cause: Paint not carried onto the glass — Fix: Take the paint 2mm over the putty onto the glass to seal the joint.

Questions people ask

Can I use silicone instead of putty?

On a bead-glazed frame, yes, a glazing silicone works. On a traditional putty rebate, silicone does not hold the glass or shed water the same way, and it makes the next replacement much harder.

Where must I use safety glass?

In doors and in the panels immediately beside them up to 1500mm, and generally in any glazing below 800mm from floor level. If in doubt, use it — the extra cost is small.