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Crumbling Window Putty
The angled fillet of putty holding the glass into a timber window has cracked, gone chalky, or fallen out in pieces.
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What's happening?
In a traditional timber window the glass is not held by beads. It is bedded into the rebate on a thin layer of putty, held with small glazing sprigs, and then finished with the sloping front fillet you can see from outside. Linseed oil putty cures by oxidising, and its whole working life depends on a paint film sealing it — paint holds the oils in and keeps the water out. Once the paint cracks, the putty dries, shrinks, loses grip on the glass and starts shedding. The fillet is not decorative: it is the weather seal for the glazing rebate and part of what holds the pane. When it goes, water gets into the rebate and into the end grain of the sash, which is where timber windows begin to rot.
What usually causes it?
- Paint film cracked or worn through, so the putty has dried out and shrunk away from the glass
- Putty applied over bare, unprimed timber, which drew the oils out and left it brittle from the start
- Overpainting that never sealed the putty-to-glass line, letting water run in behind the fillet
- Thermal and structural movement between glass and timber breaking the bond
- Original putty simply at the end of its life on a window that has not been painted for decades
- An earlier repair made with a non-flexible filler or silicone, which does not bond and traps water behind it
- Glazing sprigs missing or corroded, so the pane has moved slightly and broken the fillet
Is it dangerous?
The immediate risk is the glass. A pane whose putty has largely gone, with corroded or missing sprigs, is held mainly by friction — panes do fall out of neglected sashes, and a falling pane over a doorway or a path is a real hazard. The slower risk is rot: an open glazing rebate feeds water directly into the sash timber, and a sash that has rotted at the bottom rail becomes a repair or a replacement rather than a re-putty.
Can I fix it myself?
Raking out loose putty and re-puttying a small pane is a traditional and achievable job for a patient DIYer, and it repays doing well. The parts people get wrong are the parts that determine how long it lasts: the rebate must be dry, clean and primed before new putty goes in, the pane needs its sprigs, and the new fillet must be left to skin over before it is painted — then the paint must be carried a whisker onto the glass to seal the line. Do not putty over damp or soft timber, and do not use silicone in place of putty on a window meant to be puttied.
When should you call a glazier?
Call a glazier when a pane is loose in its rebate, when putty has gone from a large or high pane, or when the timber under the putty is soft. It is also worth calling where the window is a period or leaded one you would rather not learn on — matching the original putty line is part of how a sash looks right.
What will the glazier actually do?
- Rake out failed putty back to sound material and remove or renew the glazing sprigs
- Check the rebate timber for decay and report where a splice repair is needed before glazing
- Prime bare timber in the rebate so the new putty is not starved of its oils
- Re-bed the pane on fresh back putty and fix it with new sprigs
- Form the front fillet to match the existing line and finish the corners cleanly
- Return or advise on painting once the putty has skinned, sealing the paint onto the glass edge
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