Lead Roofing Work: Code Numbers, Bay Widths, Welted Drips & Soakers

Summary

Lead is one of the oldest roofing and weatherproofing materials, valued for its exceptional weather resistance, longevity (60–100+ years), and workability. When correctly installed, a well-executed lead roof will outlast most other parts of the building. When incorrectly installed — wrong code, wrong bay width, poor drip details — lead will buckle, crack, and fail within years.

The Lead Sheet Association (LSA) is the UK trade body and produces the definitive technical manual: the Lead Sheet Manual Volume 1 (Rolled Lead Sheet — The Complete Manual). All lead roofing work should follow LSA guidance.

For roofers, plumbers, and general builders, lead work appears across a range of situations: flat roof sections (bays over bay walls), chimney flashings and back gutters, parapet details, valley linings, soakers between roof tiles and abutments, and decorative copings.

Key Facts

• BS EN 12588 — British Standard for rolled lead sheet for building use; specifies codes and thickness
• Code 3 — 1.32mm thick, 14.97 kg/m²; soakers, vertical and step flashings, valley linings in sheltered situations
• Code 4 — 1.80mm thick, 20.41 kg/m²; parapet gutters, chimney back gutters, some flat roofs
• Code 5 — 2.24mm thick, 25.40 kg/m²; flat roofs (dormer tops, bay tops); most common for small flat sections
• Code 6 — 2.65mm thick, 30.05 kg/m²; flat roofs and larger details; more resistant to damage
• Code 7 — 3.15mm thick, 35.72 kg/m²; very exposed or heavily trafficked areas
• Code 8 — 3.55mm thick, 40.26 kg/m²; specialist applications
• Maximum bay width — the maximum width of a lead bay before expansion causes buckling: 500mm (Code 5), 600mm (Code 6); measured parallel to the slope
• Maximum bay length — maximum length (measured down the slope): 1.5m for Code 5; 2.25m for Code 6; limited by thermal movement
• Welted drip — the joint at the lower edge of each bay in flat leadwork; lead is folded double (welted) and dressed over a timber drip batten (minimum 40mm high); prevents rainwater tracking under the join
• Soaker — small piece of lead slipped under slates or tiles at abutments (wall meets roof); works with step flashings above; typically Code 3
• Step flashing — lead cut in steps and built into the mortar courses of a masonry wall; covers the soakers; typically Code 3
• Back gutter — lead gutter behind a chimney stack; collects rainwater from the roof above the chimney and discharges around the sides; Code 4 minimum
• Thermal expansion — lead expands approximately 1mm per metre per 30°C temperature change; this is why bay width limits are critical
• Patination oil — applied to new lead to prevent white staining (lead carbonate) forming and running onto roof tiles or walls below; typically sprayed or wiped on

Quick Reference Table

Detailed Guidance

Flat Roof Bay Construction

Flat leadwork is formed in bays separated by drip battens. Each bay is a discrete piece of lead with its own joints.

Setting out:

1. Calculate bay size: max 500mm wide (Code 5) × 1.5m long (measured down the slope)
2. Mark out drip batten positions at every 1.5m down the slope
3. Fix 40mm drip battens across the full width of the roof at each bay junction

Laying lead:

1. Lead sheet should be laid with the grain running up the slope (to resist thermal cracking)
2. Cut each bay piece to size: bay width + 50mm (for welting at the drip) + 50mm (for upstands)
3. Nail the lead at the top of each bay using copper clout nails spaced 50mm apart — lead must be free at the bottom and sides (no nailing) to allow thermal movement

Welted drip joint:

1. Dress the lead over the top of the drip batten (the 40mm face)
2. Turn the lead under and welt it up (fold double) against the vertical face of the drip batten
3. The next bay's lead overlaps from above, covering the welted drip
4. The overlap at the drip should be a minimum of 50mm beyond the drip face

Side joints (bay wall to wall):

• At the low end of each side, form a separate piece to lap over the drip
• At abutment with a wall: turn up a minimum 75mm on the wall face; secure with lead wedges or clips into the mortar joint; cover with flashing

Splash lap:

• Where lead meets a vertical abutment at the top of the slope, form an upstand of minimum 75mm
• Finish with a separate flashing covering the upstand by minimum 75mm

Soaker and Step Flashing System

Where a pitched roof meets a wall (abutment), soakers and step flashings work together as a two-part system:

Soakers:

• Width = tile/slate gauge (measured from tail to lap) + 25mm
• Length = tile/slate gauge + 75mm minimum
• Each soaker is slipped under the tile above it and against the wall face
• They alternate with each course of tiles — one soaker per tile course

Step flashings:

• Separate pieces of lead cut to follow the staggered profile of the wall courses
• Step width = course height; each step should cover the soaker beneath by minimum 65mm
• Turn into the horizontal mortar joint minimum 25mm; point or wedge with lead wedges
• At the top: finish with a cover flashing over the last step, min 150mm over the top soaker

Chimney Flashings and Back Gutter

Chimneys on pitched roofs require a complete set of lead details:

Apron (front of chimney):

• Code 4; lies over the tiles at the front face of the chimney
• Minimum 150mm up the face of the chimney; minimum 150mm over the tiles
• Dressed into mortar joint at the sides

Side (step) flashings:

• Code 3 soakers and Code 3/4 step flashings at each side of the chimney
• As per abutment detail above

Back gutter:

• Collects all water that comes down the roof slope above the chimney
• Code 4 minimum; the gutter must drain to each side of the chimney
• Minimum 75mm upstand against chimney face; 100mm over roof tiles at sides
• Outlet must direct water away from chimney base — often turns under step flashings to discharge at the sides

Cap flashing:

• Covers the top of all upstand flashings on the chimney; typically in one or two pieces
• Turned minimum 25mm into mortar joint; wedged with lead wedges and pointed with mortar

Common Failures and How to Avoid Them

Rippling/buckling of flat bay:

• Caused by exceeding maximum bay width or failing to leave free sides
• Prevention: strictly observe 500mm width limit; never nail sides of bays

Cracking at folds:

• Usually caused by bending lead when cold (below 5°C) without annealing (heating to restore workability)
• Prevention: store lead in a warm area before working; never work with cold lead

White staining on tiles below:

• Lead carbonate forming on new lead and washing onto tiles or render
• Prevention: apply patination oil to all new lead within 24 hours of installation; re-apply after any cleaning

Soaker migration:

• Soakers moving out of position if not weighted by the tile above
• Prevention: ensure each soaker is fully covered by the tile; use a small nail through the soaker into the batten if tiles are very light

Frequently Asked Questions

How do I join two sheets of lead on a flat roof?

All joins in flat leadwork should be welted seams — never soldered on site. For the side joint (parallel to the slope): form a 25mm welt on one edge, hook the next sheet's edge into it, and fold the welt over twice to lock. For the drip joint (across the slope): welted drip as described above. Soldering is acceptable only for decorative details and specialised applications with annealed, cleaned lead — not for structural waterproofing.

Can I use lead alternative flashings?

Lead alternatives (Code 3 equivalent in aluminium, zinc, or polymer-coated steel) are acceptable in some situations, particularly on buildings where lead theft is a concern. They must be installed per the manufacturer's instructions and typically cannot be worked to the same tight profiles as lead. Check planning requirements — Listed Buildings and Conservation Areas usually require traditional lead.

What's the difference between lead and lead sheet?

"Lead" in the roofing context always refers to rolled lead sheet — BS EN 12588 rolled lead. It is not the same as sheet lead used for radiation shielding or specialist applications. The codes (3–8) are the same system throughout the roofing and plumbing industries.

Regulations & Standards

• BS EN 12588:2006 — Rolled lead sheet for building purposes

• Lead Sheet Association Manual (Vol 1) — definitive technical guidance for all lead work

• Approved Document C — Site preparation and resistance to contaminants and moisture; lead weathering details

• BS 6915:2001 — Design and construction of fully supported lead sheet roof and wall coverings

• Lead Sheet Association Technical Manual — Volumes 1 and 2; definitive UK lead reference

• NFRC Lead Sheet Guidance — National Federation of Roofing Contractors

• LSA Training Courses — Recognized lead worker training

• chimney repairs — Chimney repointing and lead flashing work

• pitched roof repairs — Soakers and flashings as part of tile repairs

• flat roofing — Lead vs EPDM vs GRP for flat roofs

• pipe materials — Lead use in old plumbing pipes (historical context)