Flashings seal the junction between a roof covering and a vertical surface — a chimney stack, parapet wall, dormer cheek, or abutment with a higher wall. They are critical weatherproofing details, and their failure is one of the most common causes of roof leaks. Lead has been the material of choice for centuries because of its formability, durability and self-sealing capability, but theft and cost have driven significant uptake of alternatives in recent years.
Lead Codes and Thickness
Lead sheet for roofing is specified by Code number, which corresponds to thickness and mass per square metre:
| Code | Thickness (mm) | Mass (kg/m²) | Common Application |
|---|---|---|---|
| Code 3 | 1.32 | 14.97 | Soakers, underlays at slates |
| Code 4 | 1.80 | 20.41 | Step flashings, aprons, upstands (most common) |
| Code 5 | 2.24 | 25.40 | Valley gutters, parapet gutters, flat roof coverings |
| Code 6 | 2.65 | 30.05 | Large gutter linings, flat roof areas |
| Code 7 | 3.15 | 35.72 | Parapet copings, heavy-duty gutter linings |
| Code 8 | 3.55 | 40.26 | Structural gutter linings, pool surrounds |
For the majority of residential flashings — step flashings at chimney abutments, apron flashings over bay windows, soakers under slates — Code 4 is standard. Code 3 is correct for soakers (which are hidden between slates and not exposed to UV). Code 5 is recommended for valley gutters and any flashing with standing water risk.
The Lead Sheet Association (LSA) Technical Manual (5th edition) is the authoritative reference for all lead roofing details.
Typical Flashing Details
Step and cover flashings (chimney abutments): the most common lead detail. Step flashings weave between slate/tile courses; the cover flashing overlaps them and is dressed into a raked mortar chase (25 mm deep minimum) or mechanically fixed with lead wedges. Maximum length per section: 1.5 m to prevent thermal movement cracking. Minimum upstand: 75 mm above tile surface.
Soakers: L-shaped pieces of Code 3 lead placed under each slate at an abutment, with the vertical leg tucked behind the cover flashing. Width = (tile gauge + headlap), leg height minimum 75 mm.
Valley gutters: Code 5 lead formed into a valley channel between intersecting roof slopes. Minimum width 400 mm for pitches 30–45°; more at shallower pitches. Joints made by welting or lapping a minimum of 150 mm, with the upper sheet over the lower.
Apron flashings: a single-piece detail at the front face of a chimney stack. Code 4; minimum 150 mm upstand behind the stack and minimum 150 mm overlap onto the roof covering.
Lead Theft
Lead theft from roofs remains a significant problem across England and Wales, particularly on churches, public buildings and properties in urban areas. The Scrap Metal Dealers Act 2013 requires cash-free transactions at scrap merchants, which has helped but not eliminated the problem.
For properties with repeated theft incidents or high-risk locations, specifying an alternative material is practical rather than a downgrade.
Alternatives to Lead Flashing
Several products are established as lead substitutes in the UK:
Ubiflex (and similar flexible non-lead strips): a multilayer composite of polymers and bitumen, typically 1.5–2 mm thick and supplied in 5 m rolls in various widths. It is self-adhesive or uses mortar-bed fixing. Extremely flexible in cold weather; no thermal expansion cracking; no theft value. Not recommended for valley gutters where standing water may occur for extended periods. Typical installed cost: £10–£18/m run for standard abutment widths.
GRP (glass-reinforced polyester) flashings: moulded to specific profiles for standard chimney sections and valley applications. Very rigid; excellent chemical resistance; no thermal creep. Colour-matched products available. Less suitable for non-standard shapes that require on-site forming. Typical cost: £15–£25/m for standard chimney flashings, more for valley liners.
Self-adhesive flashing tape (flashband type): butyl or bitumen-based self-adhesive strips with a foil or mineral facing. Used for low-risk repairs, sealing pipe penetrations and temporary weather protection. Not a structural replacement for proper soakers and cover flashings on long-term work. Typically £4–£8/m for 150 mm wide product.
Zinc: a traditional alternative with similar forming properties to lead. Code 6 zinc (0.65 mm) is the approximate equivalent of Code 4 lead in terms of weathering life. More expensive than lead by weight in the current market; typically used on contemporary architecture where a grey metallic aesthetic is desired. Lifespan 60–80 years.
Cost Comparison (Installed, per Linear Metre)
For a standard chimney abutment flashing (step + cover, approximately 400 mm effective width):
| Material | Installed Cost (£/m) | Lifespan | Theft Risk |
|---|---|---|---|
| Lead Code 4 | £25–£45 | 100+ years | High |
| Ubiflex / non-lead composite | £10–£18 | 25–40 years | None |
| GRP flashing system | £15–£25 | 30–50 years | None |
| Zinc | £30–£55 | 60–80 years | Low |
| Self-adhesive flashband (repair) | £5–£10 | 5–15 years | None |
UK Standards and Building Regs
- BS EN 12588: European standard for milled lead sheet for building use — thicknesses, tolerances, chemical composition.
- LSA Technical Manual (5th ed.): detailed industry guidance on all lead roofing and flashing details.
- BS 5534:2014: covers fixing of slates and tiles but references flashing and soaker requirements at abutments.
- Building Regulations Part C: flashings are part of the moisture-resistance strategy; they must be detailed to prevent penetration at junctions for the life of the roof.
On listed buildings and in conservation areas, lead is almost universally required by planning conditions. Where an alternative is proposed, written approval from the conservation officer is advisable before commencing work.
