Leaded Light Window Repair

What's happening?

A true leaded light is a panel of small panes — quarries — held in H-section lead cames that are soldered at every junction and weatherproofed with cement rubbed into the joints. It is a self-supporting lattice, and it fails as a lattice: the lead work-hardens and creeps over decades, the cement dries out of the joints, and the panel starts to bow under its own weight and wind pressure. Once it bows, the geometry gets worse on its own and individual quarries crack as the lattice distorts. This is quite different from the stick-on lead strip applied to a flat sheet of glass in modern windows, where nothing is structural and a crack is a straight pane replacement.

What usually causes it?

• Lead cames work-hardened and crept over decades, so the panel has lost its stiffness and bows
• Cement dried out of the came joints, leaving the lattice loose and the panel leaking air and water
• Failed or broken solder joints at came junctions, so the lattice can articulate and distort further
• Individual quarries cracked as the bowing panel forced them out of plane
• Panel fitted without saddle bars, or with corroded bars that no longer restrain it
• Failed putty or cement where the panel beds into the surrounding timber or stone rebate
• An earlier repair done in silicone or with mismatched came, which flexes differently from the rest

When should you call a glazier?

Call a glazier who works with leaded lights when a panel has visibly bowed, when quarries have cracked, or when the panel draughts and leaks. Sooner is much better here: re-cementing and re-supporting a slightly bowed panel keeps the original glass, while a panel left until it is badly distorted usually has to be taken apart and rebuilt. Where the building is listed or in a conservation area, check the consent position before anything is altered.

What will the glazier actually do?

• Assess whether the panel can be re-cemented and supported in place, or needs removal and rebuilding
• Remove the panel to the bench where it is too distorted to work on in situ
• Ease the lattice back towards flat, renew broken solder joints, and replace cracked quarries with matching glass
• Rub fresh cement into the came joints on both faces and clean the panel off
• Fit or renew saddle bars so the panel is restrained against wind load in future
• Re-bed the panel into its rebate and make good the surrounding putty or cement

Small panes and glass thickness

Leaded lights are treated as a special case in the impact guidance, because a lattice of small panes behaves differently from one large sheet. Where glazing sits in a critical location, small panes are one of the accepted routes to compliance, subject to limits on their size and thickness.

• A small pane means an isolated pane, or one of a number of panes held in glazing bars, traditional leaded lights or copper lights.
• A single small pane should not exceed 0.5m² in area, with a maximum width of 250mm.
• Annealed glass in a small pane should normally be at least 6mm thick.
• In traditional leaded or copper lights, where fire resistance is not a requirement, 4mm glass may be used.