Although the end products look very similar, there are three very distinct methods of producing the lead sheet that is sold and distributed in today’s UK marketplace.

Lead sheet is not only the UK’s oldest and most trusted roofing metal, it’s also one of the most environmentally “green” of all the building materials in use today, with its low melting point, total recyclability and established national scrap reclamation system.

Its legendary long term performance has been proven over centuries of use but there are elements of its production, specification and installation that are critical to that maintenance-free longevity.

Lead sheet is soft enough to be dressed and shaped by hand in situ to fit any detail, particularly when in the hands of a craftsman from the Lead Contractors Association.

As a soft metal however, it reacts to thermal changes, expanding in warm weather and contracting as it cools. It is the awareness of – and allowance for – this thermal movement that is absolutely critical during installation and which is at the root of the essential references to sizing and fixing in the comprehensive technical literature issued by the Lead Sheet Association.

The extent to which lead expands and contracts depends on the degree of exposure to direct sunlight / prevailing wind / rain / ice / snow and on the size of the sheet, which must be kept within recommended limits and fixed in such a way as to properly allow for this movement.

The rate (speed) of expansion is determined by the frictional resistance of the lead to the substrate, in which the vital role of using the correct underlay also becomes apparent.

If thermal change is accepted as the “driver” in the sheet moving, then frictional resistance is the “brake” and it is the combined effect of these two principles that determines how much the sheet will move.

However, if a sheet has variations in thickness, the thinner parts of the sheet will react more quickly to temperature change and have less friction resistance than the thicker parts and will therefore move more. It is these variations within the sheet that eventually can cause ripples in the surface, ultimately leading to cracking and failure.

A further performance factor is that lead sheet used in traditional roofing and cladding is not 100% pure lead. During manufacture copper is added and the amount of existing impurities such as antimony and tin are restricted in order to enhance durability. Strictly controlling these elements, in terms of quantity and distribution, are critical to the eventual performance of the sheet.

It is however the consistency of thickness that is fundamental in allowing for thermal movement and the consequential friction resistance, when fixing lead sheet into place. It is this principle, observed over many decades that forms the basis of the advice contained in the LSA’s “Lead Sheet Manual” and BS6915.

Consistency in the thickness of the sheet enables the range and rate of both thermal movement and frictional resistance to be more predictable. It is on this basic principle – that the sheet will move in a consistent manner – that the specific sizing and fixing recommendations of the Lead Sheet Association and the detailing contained in BS6915 (Code of Practice for Design and Construction of Fully Supported Lead Sheet Roof and Wall Coverings) are based.

For this reason the 25 Year Guarantee Scheme offered by members of the Lead Contractors Association is only available for projects using rolled lead sheet to BSEN12588 or traditional sand cast sheet from an approved manufacturer.

The method of manufacture will determine the consistency of thickness in lead sheet and specifiers need to remember when using the National Building Specification Clauses that under H71 of the NBS the type of lead sheet must be inserted if they wish to ensure rolled sheet to BSEN 12588 or traditional sand cast sheet is used.

Rolled Lead Sheet to BSEN 12588 (formerly BS1178)

The most widely used lead sheet in today’s construction industry is also the most consistent and is produced by the simple process of reducing the thickness of a warm, soft slab of lead by passing it backwards and forwards between two rollers.

Originally a manual operation, over time this centuries-old process has become increasingly mechanised, to the point that today’s computer controlled production can achieve precise thicknesses right across the sheet.

Rolled lead sheet is now produced to the European Standard BSEN12588, which also dictates the amount of copper to be added and the amount of antimony, tin and other elements that are permitted to remain during the refining production process, in order to provide the ideal mix, distributed evenly throughout the sheet to maximise performance.

The European Standard also states that the thickness tolerance of the finished product shall not vary by more than +/- 5% AT ANY POINT on the sheet. This applies throughout the standard product range, from Code 3 (1.32mm thick) to Code 8 (3.55mm) and beyond, right up to Code 15 (6.35mm).

The controlled degree of consistency in thickness means that when fixed into place, the sheet will tend to expand and contract at a uniform rate, with the same friction resistance right across the width and length.

This consistency in movement has enabled the Lead Sheet Association to detail the best methods of sizing and fixing when installing lead sheet and which they have comprehensively described in the “Lead Sheet Manual”

Sand Cast Lead Sheet

Although rolled sheet is the most commonly used type of lead today, it is lead sheet produced by casting molten metal onto a bed of sand that is the oldest method of manufacture, pre dating the rolling mills of the early 17th Century.

With evidence of use in Roman times, traditional sand cast sheet adorns many historic buildings and was widely used in the 16th and 17th Century to roof the prominent buildings of the time, particularly the properties owned by the Church, which is why so many ecclesiastical buildings today still have lead sheet as their roof covering.

Sand cast sheet is often specified today as a direct like-for-like replacement in historic properties, particularly as a heritage requirement for listed buildings, or for the aesthetic attraction of its slightly grainy surface on one side, caused by contact with the sand bed during manufacture.

It is possible for the lead sheet to be taken from a roof, melted and then re-cast to be put back on the same roof, providing the opportunity to maintain the original material on a historic property.

Creating sand cast sheet involves molten lead being poured over a prepared smooth bed of sand along the length of a special trough or table. Whilst the moisture content within the sand and temperature of the metal when poured are contributory factors, the thickness of the sheet is principally determined by using a pre-set “strike” (skimmer) drawn down the length of the table before the lead solidifies.

The excess removed is simply returned to the pot and re heated for the next pour, minimising wastage.

Although originally a manual operation, some manufactures have now partly mechanised the process, introducing a machine operated strike to skim the molten lead and improve thickness consistency. Whilst targeting a thickness tolerance of +/-5%, the manufacturers of traditional sand cast sheet cannot match the “at any given point” thickness accuracy and consistency of a computer controlled rolling mill.

Historically, traditional sand cast sheet has been produced in the thicker Code range of thicknesses, which enables larger sized sheets to be used on site. Today this reduces the number of expansion joints required and consequently achieves a small saving in labour costs for installation.

A further commercial advantage of sand cast sheet is that it is regularly cut to the sizes required by the client, rather than produced in standard 3m and 6m roll lengths, as is the case with the bulk production of rolled sheet. Sand cast off-cuts are put straight back in the pot, again minimising waste.

Although traditional sand cast sheet is not produced to any fixed chemical composition and is not made to any British or European Standard, it has huge historical and architectural significance, particularly in the UK where it adorns many of our older and more prominent buildings.

Machine Cast Lead Sheet

Machine cast or DM (Direct Method) lead sheet was developed in Australia in the mid 1950’s. It was originally introduced to the UK in the 1970’s used in very thin sheets (less than 1mm) as an internal sound insulation material, but in the 1980’s began to be produced in thicker sheets and adopted for external roofing and weathering applications.

It should not be confused with either rolled lead sheet or traditional sand cast sheet.

The sheet is 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, then peeled away as the drum rotates. The final thickness of the sheet is determined by the speed of the drum rotation, the depth at which it is immersed in the molten bath and the temperature variation between drum and bath.

Machine cast (DM) sheet differs in appearance from both rolled sheet and traditional sand cast in that it has a dimpled surface on the side that has not been in contact with the drum and is relatively smooth on the other.

Machine cast (DM) lead is not produced to either a British or European Standard and cannot be consistently produced to the thickness tolerances achieved by rolled lead sheet.

Individual manufacturers in the UK have obtained their own British Board of Agrément certificate, but these only detail the chemical, physical and technical characteristics of each manufacturer’s own product.

In metallurgical terms, a uniformly distributed grain structure within the sheet improves the resistance to fatigue fractures. Machine cast sheet does not have the uniformly distributed grain structure of rolled lead.

Metal fatigue fractures caused by thermal movement stress most commonly occur at the thinnest point of a sheet and the thickness variations which occur in machine cast sheet make it more vulnerable to this problem. This was independently confirmed by a long term comparative study carried out by Cambridge University.

Machine cast (DM) lead sheet is not covered by the 25 Year Guarantee Scheme offered by members of the Lead Contractors Association.