Patent glazing is a roof glazing system in which the glass is not bedded in putty but clamped between the upstands of a metal-clad bar. The bar itself — traditionally a steel inverted-T section — carries the load, while a folded zinc or lead sheath around it forms the watertight channel the panes sit in. The Dutch term is *patentroede*.
How the system works
The arrangement is as simple as it is clever:
- The steel bar takes the loads: self-weight, snow and wind.
- The folded zinc or lead wraps the steel and forms an upstand on each side with a small channel beneath it.
- The glass panes rest on those upstands along their long edges, held by clips or a capping, with no continuous sealant joint.
- Condensation and any water that gets past the glass run down the channels to the eaves.
The advantage is that the glass is free to move and a broken pane can be replaced without hacking out bedding. Modern versions replace steel with extruded aluminium and gaskets, but the principle of clamping rather than bedding is unchanged.
Applications
Patent glazing is found above all in:
- Nineteenth and early twentieth century rooflights over arcades, station concourses, studios and stairwells
- Greenhouses and orangeries
- Factory roofs with sawtooth glazing
- Contemporary aluminium systems for large glazed roofs and canopies
Points to watch in restoration
- Corrosion in the core — the steel under the zinc sheath rusts unseen. On inspection always look for swelling and split zinc, which indicate rust jacking within.
- Bimetallic corrosion — zinc and lead in contact with other metals, and with some timbers and cementitious materials, corrode faster. Keep materials separated.
- Pitch — a patent glazed roof needs enough fall for the channels to drain; laid too flat it leaks and traps dirt.
- Safety — old single glazing in a rooflight is not non-fragile. Replacement work today uses laminated safety glass.
Related terms
- Glazing bar
- Rooflight
- Glazed roof
- Zinc
- Lead
- Orangery
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.