CPD: Design and specification of rolled lead sheet
This CPD is sponsored by the Lead Sheet Association (LSA), a trade association for manufacturers of BS EN: 12588 2006-compliant rolled lead
The association provides technical advice to the construction industry on the use and application of rolled lead sheet and offers a wide range of lead- work installation training courses, including Qualification and Credit Framework (QCF) at levels 2 and 3. QCF Level 3 Heritage Skills Leadworkers cards have been issued to experienced leadworkers working on historic and listed buildings.
Part 1: Specification
Architects should specify lead sheet to BS EN 12588:2006, fitted in accordance with BS6915 and LSA recommendations, with all work to be carried out ideally by Lead Contractors Association members. It is essential to identify the type of sheet required. Rolls should be stored in dry conditions to avoid orange oxide staining; this can be unsightly and take a long time to weather away but does not affect long-term performance. The industry uses a colour-coding system to specify the thickness of lead sheet so it is easy to identify on site.
Lead sheet used in the building industry poses no risk to the environment or people and more than 95% of lead used in its manufacture comes from recycling. Lead is 100% recyclable; it can be recycled many times without its performance being affected. Its relatively low melting point of 327°C minimises its carbon footprint and environmental impact. Lead sheet has a proven life of hundreds of years.
Part 2: Manufacture
There are three methods of lead-sheet manufacture in Britain. Sand casting, used by the Romans, relies on the caster’s skill and the blend of reprocessed old sheet and new ingots. Sand casting table sizes limit sheet lengths. The side in contact with the sand bed is smoother and thicknesses vary significantly. It is usually manufactured to Codes 7 and 8.
Rolled lead sheet, introduced in the 18th century, is now manufactured with thickness variations of only ±5%, improving resistance to thermal movement and enabling performance to be predicted.
Continuous cast or DM (direct method) processes, available since the 1980s, produce sheets whose air-cooled sides are usually more dimpled and uneven. DM has no common lead manufacturing standard and is not BS EN 12588 compliant.
Part 3: Flashings
Lead sheet is used for a wide range of flashing applications. In the case of step flashings in Code 4 or 5 lead sheet, lengths are a maximum of 1.5m. Simple lead soakers, used at abutments, are usually Code 3 but often Code 4 for historic and listed buildings. Step and cover flashings are used with bold roll tiles, usually Code 4 or 5 with a maximum length of 1.5m. Simple saddles provide protection under ridge tiles. With lean-to flashings, the amount of lap over tiles or slates depends on the roof pitch and should be at least 150mm. One or two- part polysulfide sealants are suitable with lead sheet. Silicon sealants, if used, must be non-acid curing and low modular to avoid staining the lead sheet. Open widths of pitched valley gutters depend on roof pitches, rainfall rates and roof area.
Part 4: Weatherings
Lead sheet can be used to protect cornices and parapets, using very simple single-lock welted joints. Copper clips are inserted in the welts to secure the lead sheet against wind lift and suction. It is essential to clip to the free edges, using continuous clippings in exposed locations. A suitable drip-off point of about 25mm is recommended to avoid staining to the underside. The spacing of welted joints
on cornices and parapets depends on the lead sheet’s width and code. Where fixed points penetrate, joints should be positioned either side to accommodate thermal movement.
Part 5: Gutters
Lead-sheet gutter linings can be divided into four main types: box gutters, tapered gutters, parapet gutters, and wall-head/eaves gutters. Sheet sizes depend on thickness. The different joint types, steps across the fall, and wood- cored rolls, accommodate thermal movement. Typically, Code 4 lead sheet requires steps at 1.5m intervals but Code 8, which has more mechanical strength, requires them at intervals of 3m.
Catch pits should be at least 150mm deep and their areas determine gutter flow rates. Permanent connections are required at rainwater pipes to avoid leakage at joints if they are blocked. If neoprene joints are specified where it is impractical to form steps across the fall, they should be at 2m intervals (maximum) and, for Code 5, no more than 1m from fixed points or changes in direction.
Part 6: Roofing
The minimum fall for leadwork flat roofs is 1:80. Joints across the fall require drips or steps. Drip heights vary according to lead-sheet code, with a minimum of 55mm for Codes 4-6 and 60mm for Codes 7 and 8. For falls of 3-10°, lead must be welded over fixings to make them watertight, as capillary attraction holds water under extended splash laps.
Joint spacings must be suitable for the lead-sheet code. Underlays should be Class A Building Paper to BS1521 for lead sheet laid on plywood, cementitious products or oak, and geotextile for softwood boarding. Joints for pitched roofs, inclined at 10-80° from the horizontal can be wood-cored or hollow rolls. Sheets are secured at the top and clipped at their free edges to prevent wind lift and suction.
Part 7: Vertical cladding
Vertical lead-sheet cladding systems can be divided into two types: traditionally fixed and pre-formed panels. There are recommended panel sizes for the different codes of lead sheet. Joints for leadwork cladding at angles between 800 and 900 from the horizontal involve either wood-cored roll welts, which have a flatter appearance, or standing seams.
Standing seams should be left standing upright at the bottom of the cladding, as flattened down seams at the ends can restrict thermal movement. One way to avoid visible clips and achieve cleaner lines involves using continuous clipping. Typical pre-formed panel module sizes are 500 x 1500mm for Code 4 and 600 x 2000mm for Code 5. Lead-sheet clad rainscreen systems have been used for many years.
Part 8: Dormers
It is best to detail dormer cheeks in smaller sections, with vertical welted joints and clips in the welts to prevent wind suction and welted joints at jamb corners so the lead on the cheek can move separately. Dormer tops can be set out in different configurations and it is advisable to do this to prevent water from roofs higher up draining over windows. Some traditional fixing techniques, which affect the long-term performance of lead sheet, should be avoided; as an example, soldered dots and timber facings to dormers can restrict thermal movement. PDF, DXF and DWG format flashing, gutter, roofing and vertical cladding details, together with design guidelines, can be downloaded from the LSA website.