CPD: Design and specification of lead sheet cladding
Applications to traditional and preformed vertical cladding, weatherings, dormer windows, and door and bay hoods sponsored by the Lead Sheet Association (LSA)
This is the second lead sheet CPD. The first CPD, published in AJ Specification in August 2012, was a general introduction to the use of lead sheet in buildings.
As a trade association for manufacturers of rolled lead compliant with BSEN 12588:2006, the LSA provides technical advice on the application of rolled lead sheet to the construction industry and a wide range of leadwork installation courses including QCF (Qualification and Credit Framework) Levels 2 and 3. Visit www.leadsheet.co.uk
Part 1: Traditional cladding
Vertical walls and mansards steeper than 80° can be clad in situ with lead sheet in a similar manner to laying traditional lead roofing, though using slightly lighter codes. Dimensions of cladding panels are normally limited to 600mm x 2m (see chart). Larger panels are feasible but require elaborate intermediate fixings that are prone to cracking as the lead expands and contracts under fluctuating ambient temperatures.
For vertical joints, wood-cored rolls or welts are favoured, while hollow rolls are used occasionally and standing seams only where there are no angles or risk of damage by maintenance equipment. Horizontal joints in moderate exposure are simple overlaps 75mm high. The lower edges of panels are restrained against wind lift by copper or stainless steel clips, with the most secure, invisible option being to fold the lower edge up behind a continuous horizontal clip.
The whole system operates as rainscreen cladding and relies on adequate ventilation behind panels to prevent condensation and corrosion.
Part 2: Cladding details
Lead cladding panels in codes 4, 5 and 6 are traditionally secured at the head by two rows of brass or stainless nails or screws with washers, depending on substrate.
Where lead cladding panels abut parapets, corners, overhangs and window openings, junctions often call for intricate on-site cutting, welding and/or beating into shape. At window openings, vertical joints should be positioned on either side. Then lead flashings are dressed underneath the sill and up the jambs and folded up underneath the head to form a drip.
To prevent unsightly white carbonate deposits and subsequent staining, a patination coat should be pplied, preferably directly after the lead is laid.
Part 3: Preformed cladding
As an alternative to in-situ fixing, lead sheet can be preformed into rigid cladding panels. This modern method of construction can reduce time on site and produces crisp, flush facades without projecting welts, rolls or standing seams. Maximum panel sizes are identical to in situ applications.
Backing panels are normally exterior quality plywood 20-25mm thick, though occasionally building regulations may require fire-resistant board. Backing panels are prepared by bolting on four galvanised or stainless steel brackets. Then the panel is faced in lead sheet, which is folded 75mm round all four edges and fixed at the top with copper or stainless steel clout nails. Finally, a lead upstand 125mm high is then welded to the top of each panel to serve as soaker.
Part 4: Preformed erection
Where a structural frame forms part of the external wall, the preformed lead-faced panels are bolted directly to it. Where the external wall is solid, it is overlaid with a lattice of vertical and horizontal timber grounds at the same spacing as the joints between panels. Then horizontal bars of galvanised or stainless steel, strong enough to support the panels, are fixed using wall anchors through the timber grounds to the solid wall. After that, the timber grounds are faced with lead sheet.
Panels are then erected by hooking their brackets over the steel bars, leaving a ventilation space behind. To accommodate thermal movement of the panels, 25mm gaps are normally left around all edges.
Part 5: Preformed junctions
Where preformed cladding panels on solid walls abut parapets, corners, overhangs and window openings, leadfaced timber grounds are adapted in situ to orm weather-tight junctions.
At window openings, the preformed panels are brought to finish flush with the jambs and head, and 50mm below the sill. Before installing the panels, several lead flashings are dressed in situ beneath the sill, up the jambs and folded to form a drip below the head. The sill flashing should include 50-75mm upstands on either side that are later overlapped by the jamb flashings. The sill flashing should overlap the soaker upstand attached to the preformed panel below. At the top of either jamb, a gusset should be welded in to block driving rain.
Part 6: Dormer windows
In dormer windows, the most basic lead sheet components comprise weatherings to sill and jambs and flashings along both sides or ‘cheeks’. Sill weatherings require welded lead gussets that negotiate jamb upstands and both corners. Jamb weatherings should cover sides and front, with an extra 50mm to be tucked under cheek facings.
Dormer windows can also be completely covered in lead sheet across top and cheeks. Cheeks can be clad in lead sheet, as discussed in Parts 1 and 2. The top can then be covered in lead sheet that is turned down over the sides by at least 75mm.
Other variants of dormer windows include double-pitched tops, barrel-vaulted tops, and dormers that are partly or fully inset into the roof slope. The tops can also be tiled or slated to match the surrounding roofing.
Part 7: Weatherings
Being highly resistant to atmospheric pollution and acid rain, lead sheet offers ideal, durable protection for cornices and parapets. An underlay of building paper or polyester felt protects the lead from uneven substrates.
For narrow sloping projections, such as string courses, lead sheet is simply lapped at end-to-end joints and its back edge wedged into a masonry joint at 300mm centres. For cornices wider than 200mm, joints are normally welted and incorporate copper or stainless steel fixing clips. The back edge of wider weathering is turned up against the wall by 75mm and covered with a flashing. For cornices that slope at an angle of more than 10°, the weathering should be nailed or screwed along the top, and the nail or screw heads sealed in hot lead and covered with a flashing.
Part 8: Door and bay hoods
Door hoods and bay window tops lend themselves to a variety of eye-catching designs in lead sheet. For pitched door hoods smaller 1.5m x 500mm for code 4 and 2m x 600mm for code 5, hoods and weathering can be formed in one piece. Larger hoods and tops require separate flashings to cover wall upstands.
Inadequate fixings cause the most failures in pitched lead coverings. Where a triangular sheet tapers towards the top, additional fixings can be screwed through the undercloak of the wood-cored rolls and the wall upstand.