Lead Soakers and Cover Flashings: Code 3 Soakers With Code 4 Covers, Lap Lengths and Fixing Details

Summary

At any abutment between a pitched tiled roof and a vertical surface — a chimney breast, a parapet, a dormer cheek, or a projecting wall — water needs to be directed from the junction into the tile field below without allowing it to track behind the tiles or behind the plaster/render of the wall. The traditional UK solution is the soaker-and-cover system: a soaker hidden at each course that hooks over the tile and up the wall, and a cover flashing over the top that sheds water away from the joint.

The two-piece system has been the standard in UK domestic roofing for well over a century because it solves a fundamental problem: the roof and the wall move at different rates as they expand, contract, and settle. A one-piece flashing fixed to both the wall and the tiles would crack or tear in service. The soaker sits against the wall but is fixed only to the tile gauge; the cover flashing is fixed only to the wall joint. The lap between them provides the weathering line while allowing each piece to move independently. This is not over-engineering — it is the reason properly installed leadwork lasts 60 years and poorly installed flashings need replacing in ten.

Common failures in soaker-and-cover installations include: soakers sized too small (insufficient height to cover the gap between tiles), cover flashings that are too short in the wall joint (blown out by frost or vibration), laps between soaker and cover that are inadequate (allowing wind-driven rain to track through), and cover flashings longer than 1,500 mm that have cracked from thermal stress. Every one of these failures is preventable with correct sizing from the outset.

Key Facts

• Soaker definition — a short piece of lead, one per tile course, that fits between adjacent tiles, hooks over the tile batten, and turns up the abutment wall; completely hidden when tiles are in place
• Cover flashing definition — a continuous strip of lead tucked into the mortar joint of the abutment wall, dressed down over the face of the tiles; visible from outside
• Soaker minimum code — Code 3 (1.32 mm) per BS 6915 and the Lead Sheet Manual; Code 4 preferred for durability and on exposed or south-facing roofs
• Cover flashing minimum code — Code 4 (1.80 mm); Code 5 for wide flashings or heavily exposed locations
• Soaker height formula — height = tile gauge + tile lap + 25 mm (the 25 mm is the hook-over allowance at the tile batten)
• Soaker width formula — width = tile width + 25 mm (the 25 mm is the upstand against the wall; some guides use + 40 mm for wider upstand on exposed roofs)
• Cover flashing depth into joint — minimum 65 mm (LCA guidance); 75 mm preferred for security against blow-out
• Cover flashing lap over soakers — minimum 75 mm; 100 mm preferred; this is the only weather barrier at the junction
• Maximum cover flashing bay length — 1,500 mm (LCA Lead Sheet Manual); this limit exists because longer bays will crack under thermal cycling
• Minimum distance between cover flashing joints — no minimum in the standard, but joints should be staggered from soaker laps; a 150 mm lap at cover flashing joints is standard
• Step flashings — the cover flashing can be stepped (cut to follow the slope) or continuous; stepped is more common in coursed masonry; continuous is simpler but requires careful dressing
• Pointing at top of cover flashing — should be lead wool or non-setting mastic; NOT conventional sand-cement mortar, which will crack and allow water behind the flashing
• Wedging — cover flashings are typically secured with lead wedges driven into the raked-out mortar joint before pointing; stainless steel clip fixings are an alternative
• Tile hook detail — the soaker should hook over the tile batten or clips; the hook is typically 25 mm; without a hook, the soaker will slide down under thermal movement
• Soaker installation sequence — soakers go in as tiling proceeds; each soaker is placed before the tile above it is fixed; the cover flashing is installed after tiling is complete

Quick Reference Table

Note: Always measure actual tile gauge and lap on the specific roof rather than relying on nominal dimensions.

Detailed Guidance

Sizing Soakers: Working the Formula

The soaker height formula — gauge + lap + 25 mm — is straightforward but is often misunderstood in practice.

• Gauge is the exposed face of each tile or slate (the distance from the tail of one tile to the tail of the tile above it). This is the dimension you would measure from the roof.
• Lap is the overlap between the tile above and the tile two courses below (the double coverage dimension). For plain tiles this is typically 65 mm; for slates it varies with pitch and exposure.
• 25 mm is the hook allowance. The soaker is folded over the tile batten at the top, and this 25 mm forms the hook that prevents it sliding down.

On a plain tile roof with a 100 mm gauge and 65 mm lap, soaker height = 100 + 65 + 25 = 190 mm. This means a 190 mm tall piece of lead will fully cover the gap between adjacent tiles at each course and provide the batten hook.

The soaker width formula (tile width + 25 mm for the upstand) gives the total developed width of the lead. Most of this sits in the tile lap; 25 mm turns up the wall. On very exposed roofs, a 40 mm upstand is preferable and the formula becomes tile width + 40 mm.

Cut the soakers before tiling begins: you need the actual tile dimensions and the actual gauge set by the tiler. Cutting soakers to the wrong gauge is a common error when the order is placed before gauge is set.

Installing Soakers

Soakers are installed one at a time as tiling proceeds up the slope. The sequence is:

1. Fix the tile to the left of the abutment (or the tile at the abutment in the case of a verge detail).
2. Slide the soaker into position: it sits flat on the tile face, with the hook catching over the batten, the body of the soaker between the tiles, and the 25 mm upstand against the wall face.
3. Fix the next tile to the right, which traps the soaker in place laterally.
4. Continue up the slope: each new course gets a new soaker, overlapping the soaker below.

The soakers do not need to be nailed or clipped — they are mechanically trapped by the tiles on each side and above. The hook over the batten prevents downward movement. The tiler and leadworker should work in close coordination: if soakers are dropped in after a large area of tiling has been completed, individual tiles need to be lifted or re-fixed.

At the ridge, the topmost soaker must be cut to fit behind the ridge tile. At the eaves, the bottom soaker must be positioned so it discharges clear of any fascia or gutter; ensure the tail of the soaker reaches the full width of the tile and directs water into the gutter rather than behind it.

Installing Cover Flashings

Cover flashings are installed after the tiling is complete. The mortar joint that will receive the flashing must be raked out to a minimum depth of 25 mm (preferably 30 mm) before installation. A pressed-in joint is insufficient: the lead needs physical depth to be retained.

Cut the cover flashing to the required bay length (maximum 1,500 mm). Dress the top edge (the portion going into the joint) square and straight using a dresser. Dress the lower edge so it lies flat and smooth against the face of the tiles.

Insert the flashing into the raked-out joint. A minimum of 65 mm must go into the joint — measure from the face of the wall. Drive lead wedges into the joint at approximately 450 mm centres to hold the flashing in place before pointing. If using proprietary stainless steel spring clips, follow the manufacturer's fixing instructions.

Once wedged, point the joint with lead wool or a non-setting mastic sealant. Non-setting mastic (such as lead flashing sealant) is preferred over lead wool for most applications because it remains flexible throughout the thermal cycle and is less susceptible to vibration cracking. Never use sand-cement mortar to point a cover flashing joint: mortar cracks as the lead moves thermally, then falls out and the joint opens.

Dress the lower edge of the cover flashing down onto the tile face. The lap over the soakers must be a minimum of 75 mm; this is the only weather barrier. Dress the flashing firmly against the tile surface without cutting through the tile; a wooden dresser or a bossing mallet with a soft face is appropriate.

Step Flashings

Where the abutment wall runs alongside the slope of the roof (rather than across it), the cover flashing is stepped to follow the tile courses. A stepped flashing is a single piece of lead (or a series of short interlocking pieces) that rises in steps, with each step matching the tile gauge.

Stepped cover flashings are typically used alongside chimney stacks and at parapet abutments on sloped roofs. The step profile is cut out of the flat lead before installation. The step dimension (rise) equals the tile gauge.

On a brick wall, each step of the flashing should ideally sit in its own mortar joint. If the step dimension does not correspond to the brick course height, it is acceptable for a step to run through part of a brick. However, the flashing must still be fixed into a proper raked joint; it cannot be fixed with adhesive to the face of brickwork.

Thermal Movement and Why Bay Length Matters

Code 4 lead has a coefficient of thermal expansion of approximately 29 mm per 10 m per 100°C temperature change. In UK conditions, a south-facing cover flashing can experience a temperature range of approximately 70°C (from -5°C in winter to 65°C+ on a sunny summer day). For a 1,500 mm bay this means:

1.5 m × 29 mm / 10 m × 70/100°C = 3.1 mm of thermal movement per bay

The flashing is constrained at the top (in the mortar joint) and at the lower edge (against the tile). The 3.1 mm of thermal movement must be accommodated by the material's inherent flexibility. At 1,500 mm this is manageable; beyond 1,500 mm the cumulative movement approaches the point where the lead will try to buckle or will develop fatigue cracks at the fixed points. The 1,500 mm maximum bay length is therefore not arbitrary: it is derived from the thermal movement calculation.

At cover flashing joints, a lap of 150 mm accommodates the relative movement between adjacent bays while maintaining the weathering line. The lap should be arranged so that the upper bay overlaps the lower (i.e. water runs across the lap, not into it).

Pointing Materials: Lead Wool vs Non-Setting Mastic

Lead wool is a twisted or woven lead fibre product that is hammered into the joint with a caulking tool. It fills the space around the flashing and creates a mechanical seal. Lead wool is traditional and durable, but it does not truly seal against water entry — it relies on the mechanical fit of the flashing in the joint for weathering. It is appropriate for well-formed raked joints with consistent depth.

Non-setting mastic (lead flashing sealant/pointing compound) is a flexible, gun-applied or hand-applied sealant that fills the joint and the interface between lead and masonry. It remains permanently flexible, accommodates thermal movement without cracking, and seals against wind-driven rain. It is the preferred pointing material in most current guidance. Apply it after wedging, overfilling slightly, then smooth flush with the wall face.

Sand-cement mortar — do not use for pointing cover flashings. Cement-based mortar is rigid; the thermal movement of the lead will cause it to crack within a few seasons, leaving a gap at the top of the flashing that admits water. Mortar also makes future maintenance more difficult, as it bonds to the brickwork and is hard to rake out cleanly.

Frequently Asked Questions

Do soakers need to be fixed with nails or clips?

No. Correctly sized soakers are mechanically retained by the tiles either side and the hook over the batten. The tile fixing (nail or clip) on each side of the soaker prevents lateral movement; the batten hook prevents downward movement. Nailing soakers to the batten is not standard practice and can restrict thermal movement.

Can I use a one-piece flashing instead of soakers and cover?

A one-piece flashing dressed over the tiles and into the wall joint is faster to install but does not allow differential movement between the roof and the wall. In practice, one-piece flashings crack at the angle between the vertical and horizontal faces within a few years on any roof subject to thermal cycling. The soaker-and-cover system is the correct approach for any permanent installation.

How do I deal with a cover flashing at an internal corner (e.g. where a dormer cheek meets the main roof)?

An internal corner is one of the trickiest details in leadwork. The flashing must be bossed or welded to form a watertight angle. A bossed internal corner is the traditional approach: the lead is dressed into the corner using a bossing mallet, working the surplus metal into the angle rather than cutting it. Welded corners using lead-burning equipment are acceptable on new work. On heritage projects, bossed corners are preferred.

What is the minimum pitch for soaker-and-cover to be effective?

The soaker-and-cover system works on any pitch where tiles can be fixed — generally 22.5° or above for plain tiles, higher for some single-lap tiles. Below 22.5°, the tile manufacturer's minimum pitch should be checked, and below that pitch an alternative roof covering is required. The soaker depth should be increased on lower pitches to compensate for slower water run-off.

How far should the cover flashing lap below the tile surface?

The minimum lap of the cover flashing over the soaker is 75 mm, measured from the tail of the soaker. In practice, the lower edge of the cover flashing is dressed to sit neatly against the face of the tile without projecting excessively. On very exposed roofs, a 100 mm lap is preferable.

Regulations & Standards

• BS 6915:2001 (with amendments) — Design and construction of fully supported lead sheet roof and wall coverings; contains soaker and cover flashing sizing guidance and minimum specifications

• BS EN 12588:1999 — Rolled lead sheet standard; defines Code 3 and Code 4 thicknesses used for soakers and cover flashings

• Building Regulations Approved Document C — Resistance to moisture; abutment flashings are part of the building weathering envelope

• NHBC Standards Chapter 7.2 — Pitched roofs; minimum standards for flashings in new-build warranty work

• Lead Sheet Association Lead Sheet Manual — Definitive UK technical guide to lead sheet installation; soaker and cover flashing sizing tables

• Lead Contractor Association — LEADSKILLS training materials on soaker and cover installation

• BSI BS 6915 — Full standard text

• bs en 12588 lead sheet — Code numbers 3–8, thicknesses, and colour coding

• lca leadskills qualification — LEADSKILLS Level 1–3 qualification covering soaker and cover installation

• lead valley gutters — Valley gutters, open and closed, code selection and bay lengths

• lead dormer flashings — Dormer back gutters, cheek flashings, and step-and-cover at dormer junction