High density and high atomic number together. Attenuation of gamma and X-radiation depends on how much mass and how many electrons the radiation must pass through, and lead delivers more of both per millimetre than any other metal at a practical cost. Tungsten performs better still but costs many times more, which restricts it to weight-critical uses.

A traditional sizing system in which a code number corresponds to a specific thickness and weight per unit area of rolled lead sheet, with each code colour-marked for identification on site. Roofing specifications call up a code for each application - flashings, soakers, valleys, hips - so the material is ordered by code rather than by grade.

For roofing, flashings, weatherings and heritage restoration it remains widely specified and legal. What is prohibited in most jurisdictions is lead in potable water pipework and in paint. Occupational exposure, hygiene, waste handling and disposal are separately regulated, so the controls apply to how it is worked rather than banning the material outright.

Controls against ingestion and inhalation above all: no eating, drinking or smoking while working with it, washing before breaks, and respiratory protection wherever lead is heated, cut, burned or abraded, since fume and dust are the principal exposure routes. Offcuts and waste are controlled waste and must be routed to a licensed recycler.

Considerably longer than any common substitute - well-detailed lead roofing and flashing routinely exceeds a hundred years, which is why it persists in heritage and high-value work despite its cost. Life is governed by detailing rather than by the metal: correct sheet sizes, adequate allowance for thermal movement and proper fixings determine whether it fatigues.

It resists sulphuric acid particularly well, forming an insoluble lead sulphate film that protects the metal beneath, which is why lead lining persists in some acid plant and battery manufacture. It is attacked by nitric and organic acids, and by soft, acidic water - the reason lead pipework is prohibited for drinking water.

Its density and high internal damping let a thin sheet block low-frequency airborne sound that much thicker lightweight barriers transmit, and its softness lets it absorb vibrational energy rather than resonate. Loaded vinyl barriers and lead-lined panels exploit this in acoustic enclosures and studio construction.