Why are my leaded windows bowing or bulging outward?

If your leaded glass window is bulging outward or curving away from the frame, the lead has deteriorated to the point where it can't do its job anymore. This isn't cosmetic. It's structural, and it doesn't get better on its own.

Why it happens

Lead fatigues over time. Decades of temperature cycling expansion in summer, contraction in winter slowly work the lead at every joint. After 80 or 100 years that cumulative stress shows up as structural weakness, and the panel starts to bow under its own weight.

Windows that have lost their steel reinforcement bars are especially prone to this. Those bars are what give the panel rigidity beyond what the lead alone can provide. When they detach or corrode, the panel has nothing keeping it flat.

Why it won't stop

Once a panel starts bowing, it puts more stress on the weakest lead lines, which accelerates the deterioration further. A window that's slightly bowed today will be more bowed in two years. At some point it separates from the frame.

What I do about it

If the bowing is mild and caught early, I can sometimes add new reinforcement bars without doing a full re-lead. That stabilises the panel and stops it progressing. More significant bowing usually means the lead itself needs replacing. The panel comes out, old lead gets stripped, the glass pieces get cleaned and assessed, and I re-lead it with new came and new bars.

If your window is bowing, get it looked at. The earlier I see it, the simpler and less expensive the fix.