Leadwork on Roofs: Code Numbers, Flashings, Valleys and Step Flashing Specification

Quick Answer: UK leadwork follows BS EN 12588 for material and the Lead Sheet Training Academy (LSTA) manual for detailing. Lead is sold by Code (a thickness number from 3 to 8 corresponding to 1.32–3.55 mm). Code 4 is the minimum for chimney flashings, Code 5 for valleys and soakers, Code 6 for parapet gutters and bay tops. Maximum unbroken length is dictated by thermal movement: 1.5 m for Code 4, 2.0 m for Code 5, 2.5 m for Code 6 in standard exposure. Specifying the right Code and lap length is the difference between 60-year leadwork and a 5-year callback.

Summary

Lead remains the standard for small-area waterproofing on pitched roofs because nothing else matches its ability to be dressed to complex shapes, accept thermal movement and last 60+ years on a properly detailed installation. The skill is in detailing — not the material. A poorly fitted Code 5 valley fails in 3 winters; a properly fitted one outlasts the tiles around it.

The LSTA's Lead Sheet Manual is the authoritative practical reference for UK lead detailing. It pre-dates BS 5534's 2014 update but remains the technical bible for valleys, step flashings, soakers, aprons and bay-window tops. Builders and roofers should keep a copy on the van.

The two technical levers that drive every lead detail are thermal movement (lead expands roughly 0.030 mm per metre per °C — a 3 m sheet moves 7–8 mm between a winter night and a summer afternoon) and the wind-uplift profile of where the lead sits. Get the unbroken length right and clip pattern right, and lead behaves predictably. Run a 4 m unbroken sheet for a parapet gutter, and it will buckle, split or pump out within a year.

Key Facts

Quick Reference Table

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Detail Code Max length / size Lap Notes
Chimney apron (front/back) 4 1.5 m 100 mm Wedged into mortar joint, dressed over tile
Chimney soaker 4 (or 3) 175 × 175 mm per tile gauge Interlocks with each tile course
Step flashing (lapped to soaker) 4 1.5 m sections 100 mm step lap 65 mm into joint, 25 mm onto tile
Cover flashing (single piece) 4 or 5 1.5–2.0 m 100 mm Dressed over tiles min 75 mm
Valley lining 5 1.5 m sections 150 mm Rolled edge or copper clip retention
Parapet gutter sole 6 2.5 m 150 mm Solid timber sub-deck, sloped to outlet
Bay top (small <1.5 m²) 4 one piece n/a Welted edges, drips at front
Bay top (large 1.5–2.5 m²) 5 or 6 one piece per fall Stepped with welted joints if longer
Parapet capping 6 or 7 2.0 m 150 mm Welted at joints, copper clip retention
Dormer cheek 5 per panel 100 mm Welted edges, drip-line bottom

Detailed Guidance

Code Selection in Plain English

Lead Codes are essentially thickness in 1/64-inch steps — a legacy of pre-metric shipbuilding. The numbers don't intuitively map to performance, so rules of thumb help:

Going up one Code adds roughly 25% to the cost per m². The temptation to "spec heavy and hope" is wasteful — Code 6 in a chimney apron position is overkill, Code 5 in a soaker position is wasted material that's also harder to dress neatly.

Thermal Movement: The Core Constraint

Lead expands and contracts more than almost any other roofing material. Designed correctly, this movement is absorbed at welted joints, lapped sections and free-floating dressings. Designed wrong, it tears the lead at fixing points, unzips dressings out of mortar joints, or pumps water under cover flashings.

The empirical rules from LSTA:

Buckled lead in a parapet gutter is almost always a thermal movement failure — too long an unbroken run, or solid-fixed at both ends.

Step Flashings to a Wall

The standard pitched-roof-to-wall detail is soakers + step flashings:

  1. Soaker (Code 3 or 4) — interlocks with each tile course, turns 75 mm up the wall, 100 mm onto the tile
  2. Step flashing (Code 4) — sits over the soaker, dressed into the mortar joint at the wall, stepped to follow the tile gauge
  3. Bottom of step flashing — 65 mm minimum into wall joint, wedged with lead wedges, tucked back with mortar
  4. Top of step flashing — laps the next piece by 100 mm minimum

The step shape follows the brickwork courses, with the flashing cut at 25 mm × 65 mm steps — 25 mm on the horizontal turn, 65 mm into the vertical joint. Cutting these by hand is the test of a competent roofer.

                          ┌── Wall
              [Step flashing] (Code 4)
              ╱│
             ╱ │ ← 65 mm into joint
   ┌───────╱  │
   │Soaker╱   │
   │ ↑ 75│   │
   ├──75─┤   │
   │Tile │   │
─────────┴───┘

Valley Linings

Lead valleys are Code 5 minimum, in 1.5 m lengths with 150 mm overlap at joints. The substrate must be a continuous timber valley board (no gaps), with a separating layer of building paper or breathable membrane. Edges are typically rolled over a hidden batten or held with copper clips — never face-nailed. Tiles cut to the valley line by 175 mm minimum each side of centre.

GRP valley troughs are an acceptable cheaper alternative for new builds — but lead remains the only durable choice for heritage and long-term work where the rest of the roof is also long-life material.

Bay-Window Tops

A bay-window top is essentially a small flat roof. Up to 1.5 m² it can be a single piece of Code 4 with welted edges and a drip-line at the front. Larger bays use Code 5 or 6, often as two welted panels with a roll joint across the centre.

The detail risks:

Soakers and Interlocking with Tile

Soakers are individual lead pieces that interlock with each course of tile or slate against a wall or chimney. Standard size is the tile gauge + 100 mm tail + 75 mm upturn, cut from Code 3 or 4. Each soaker laps the next at the gauge dimension; the step flashing covers the upturn.

Interlocking pantiles need wider soakers (typically 250 × 250 mm) because the tile profile creates a deeper section at the abutment.

The "Why Doesn't Mortar Do It" Question

It doesn't. Mortar bedding alone fails — at the bond line between mortar and lead, water tracks under the flashing in driving rain and at thermal movement reversals. Mortar is only a cosmetic finish at the wall joint after the lead is wedged into the chase. The wedges (small folded lead pieces) are what hold the flashing in place; the mortar caps the chase.

This is why pure mortar bedding of ridge and hip tiles was abolished by BS 5534:2014. The same principle applies to step flashings: mortar without wedges is not a fixing.

Frequently Asked Questions

Can I use Code 4 for a valley to save money?

Code 5 minimum is the published spec for valleys. The 25% material saving on Code 4 disappears the first time the lead splits at a thermal movement point — and it will, because Code 4 hasn't got the body to absorb the cyclic stress. Specify Code 5 and price the job accordingly.

Why is the patination oil a separate step?

Untreated lead reacts with rainwater in its first weeks to form lead carbonate — a chalky white film that streaks down the wall or roof below. Patination oil (a light mineral oil) seals the surface long enough for a stable patina to form, preventing the streaks. Apply 24 hours after fitting, before the first rain.

What's the lifespan of lead vs alternatives?

Properly detailed lead lasts 60+ years on roofs and 100+ on parapets and cladding where wear is low. The best alternatives — proprietary lead-replacement flashings (Easylead, Wakaflex), GRP valley troughs, aluminium cover flashings — give 25–40 years. For any heritage job, conservation work or roof you want to outlive the rest of the building, lead is the answer.

Are there theft risks I need to flag in a quote?

Lead theft remains a real risk on church roofs, schools, public buildings and any unprotected high-value installation. Mitigations include SmartWater marking, alarmed cabling, and proprietary roof cladding alternatives where lead is too tempting. Quote for theft mitigation explicitly when working on vulnerable buildings.

Can I use stainless or galvanised clips with lead?

Use copper or brass only. Stainless is acceptable in non-contact applications, but ferrous metals (galvanised, plain steel) cause electrolytic corrosion of the lead at contact points. The lead pits at every nail or clip contact and fails prematurely.

Regulations & Standards