Each code is a defined thickness and weight per square metre of rolled lead sheet, identified by a colour marking on the roll. Roofing specifications call up a code per application - lighter codes for soakers and simple flashings, heavier for valleys, gutter linings and full coverings - so material is ordered by code rather than by thickness in millimetres.

Because lead moves substantially with temperature, and movement that cannot be taken up by sliding is absorbed as repeated flexing along a fixed line, which fatigue-cracks the sheet. Limiting piece size and using the correct fixings and laps lets each piece expand and contract freely. Oversized pieces are the most common cause of failed leadwork.

Milled sheet is rolled to a consistent, precise thickness and is the standard product for most construction work. Cast lead is produced by pouring molten lead onto a sand bed, giving a thicker, slightly irregular sheet with a characteristic textured surface, and is used on heritage and conservation work where the original material and appearance must be matched.

By mechanical means that allow movement: copper or stainless clips, welts, laps, and dressing into masonry chases secured with wedges and pointed. Lead welding, traditionally called lead burning, is used for watertight joints on gutter linings and complex details, and is a specialist skill distinct from soldering.

As lead equivalent thickness for a stated radiation energy, which is what a physicist or radiation protection adviser calculates for the room and equipment. Sheet is then ordered to that thickness. Joints, fixings and penetrations must maintain the equivalent thickness, since an unlapped seam is a direct path for radiation.

The exposure comes from what you do to the sheet rather than from handling it, so the controls target the hot and dusty operations: lead welding, cutting, grinding and stripping all need local extraction or respiratory protection. Dressing and bossing cold sheet is comparatively low risk, though hand-to-mouth transfer still applies. Offcuts are controlled waste for a licensed recycler.

No, and it should not be painted on the weather face - it forms its own protective carbonate patina. Patination oil is applied after installation to prevent the white carbonate staining that otherwise washes down onto masonry below during the first months. The underside is sometimes coated where it contacts damp or alkaline substrates.