Lead Roofing

Lead has been used on roofs since antiquity because it is easy to cast, roll and dress into complicated shapes without cracking. In the UK and Ireland it remains a standard material for flashings, valleys, dormer cheeks and roofs, bay windows, canopies, small flat roofs and the roofs of historic churches and public buildings. In North America lead roofs are rarer and mostly found on historic buildings; one notable eighteenth-century example was the Rosewell mansion in Virginia.

Material

Rolled lead sheet in the UK and much of Europe is made to BS EN 12588 and specified by "code" numbers from 3 to 8, which indicate thickness. Codes 5 to 8 (2.24 to 3.55 mm) are typical for roofs and gutters; codes 3 and 4 are used for soakers and flashings.

Key design principles

Guidance from the Lead Sheet Association (now the Lead Sheet Training Academy) sets out how to size and fix lead:

• Bay size: pieces must not be too long or large for their thickness, otherwise thermal movement causes fatigue cracking and the soft metal creeps downhill.
• Joints: long runs are broken up with drips on flat roofs and with wood-cored or hollow rolls along the slope; welded joints are used in some details.
• Falls: flat lead roofs need a minimum fall of 1:80 to avoid ponding.
• Flashing lengths: limited to about 1.5 m (5 ft), or 1 m (3.3 ft) where bonded to bitumen or single-ply membranes.
• Laps: depend on pitch, from about 75 mm (3 in) on vertical work to around 395 mm (15.5 in) on pitched roofs.
• Ventilation: lead on decks is laid over a through-ventilated void of at least 50 mm (2 in), or 25 mm (1 in) on vertical work, to prevent underside corrosion from condensation.
• Edges and contact with mortar: free edges are clipped; lead built into masonry is coated with bitumen paint to protect it from alkali.

Durability

Well-detailed lead roofs last many decades. Failures are usually linked to oversize bays, poor fixing, inadequate ventilation or the wrong code.