Sand and Machine Cast Sheet
Sand Cast Sheet
The Romans first used sand cast sheet for water ducts and storage tanks. Its use was extended by the Normans to include roofing applications. Many church and cathedral roofs originally used sand cast sheet and records show the material has an outstanding lifespan when fitted correctly.
Today, the traditional method of manufacture persists, with molten lead being poured across a prepared bed of sand and then "skimmed" to the required thickness. The material is mostly hand-produced by a small number of specialist companies, providing sand cast sheet primarily for its rough grain aesthetic appearance or where there is a requirement on an historic property to replace “as existing”.
Its past performance record is generally due to the overall thickness and weight of the sheet used, with a 12” (305mm) square traditionally weighing 6 - 8lbs (2.7 - 3.6kg).
Sand cast sheet is not produced to any fixed chemical composition, it is not made to any British and European Standard, it does not have an Agrément Certificate.
Machine Cast Sheet
Machine cast or DM (Direct Method) lead sheet was developed as a building material in the mid 1950’s in Australia and introduced as such in the UK in the 1980’s.
It should not be confused with either rolled sheet or traditional sand cast sheet.
Produced by immersing a rotating water cooled metal drum into a bath of molten lead, the lead solidifies on the surface of the drum and is lifted from the molten bath and is peeled away as the drum rotates. The thickness of the sheet thus produced is determined by the speed of the drum rotation, the depth of the immersion and the temperature variation between drum and bath.
Machine cast sheet differs from rolled and sand cast sheet in both surface finish and grain structure. The surface of the finished sheet not in contact with the drum has a dimpled finish whilst the other side is comparatively smooth.
Machine cast sheet is not made to a British and European Standard and cannot be consistently produced in the UK to the thickness tolerances achieved by rolled sheet. Individual DM manufacturers in the UK have obtained their own British Board of Agrément Certificates but these only detail the chemical, physical and technical characteristics of each manufacturer’s own product.