Lead Chimney Flashings: Back Gutter, Side Step-and-Covers, Front Apron and Soakers — Step-by-Step Details

Summary

Chimney flashings are among the most technically demanding and failure-prone details in domestic leadwork. A chimney penetrating a pitched roof is exposed to rain from every angle, driven by wind that is often redirected and accelerated by the chimney stack itself. A properly detailed chimney flashing is not a single piece of lead — it is a system of four interrelated components, each performing a distinct waterproofing function.

The back gutter collects rainwater running down the rear face of the chimney and the roof slope behind it, discharging it to the sides. The side step-and-cover flashings, working in conjunction with individual soakers at each tile course, seal the junction between the roof slope and each face of the chimney. The front apron prevents water tracking behind the bottom edge of the chimney and under the tiles below. Each element must be correctly sized, correctly fixed, and correctly coordinated with the others to create a watertight assembly.

Failures most commonly occur when only one or two elements are replaced on maintenance work, leaving the others in their original deteriorated state; when the back gutter upstand is too shallow, allowing wind-driven rain to breach it; or when soakers are omitted entirely and the step flashing is relied upon alone to turn the water — a detail that invariably fails within a few years. All domestic chimney flashings should be Code 4 as a minimum, with Code 5 used for chimneys in high rainfall areas (notably Wales, the Lake District, and much of Scotland) or where the chimney is wide.

Key Facts

• Code 4 lead — standard specification for all domestic chimney flashing elements; 1.80mm thick, 20 kg/m²
• Code 5 lead — required for wide chimneys (over 1,500mm across the slope), high-rainfall regions, and for back gutters with large catchment areas
• Back gutter width — minimum 75mm clear gutter width measured from chimney face to upstand; 100mm recommended
• Back gutter upstand — minimum 150mm up the chimney face; increases to 200mm on very steep pitches or exposed sites
• Back gutter onto roof — minimum 100mm onto the roof surface beyond the last tile course
• Back gutter outlet — discharges to a continuous lead gutter, a hopper, or soakaway; never left to run loose over tiles
• Step flashing height — each step must be cut to follow the tile gauge; minimum 65mm turned into the mortar joint
• Step flashing horizontal overlap — each step overlaps the one below by a minimum of 65mm
• Soaker width — 175mm minimum (100mm up the wall + 75mm onto the tile)
• Soaker length — tile gauge + 25mm minimum (gauge being the exposed face of the tile)
• Cover flashing over soakers — turns 75mm minimum into the mortar joint; covers the top of each soaker by at least 65mm
• Front apron upstand — minimum 150mm up the chimney face
• Front apron onto tiles — minimum 150mm onto the tile surface below the chimney
• Lead wedges — 25mm × 25mm soft lead strips hammered into mortar joint to retain flashings; at 450mm centres maximum
• Repointing — all lead-wedged joints repointed with cement mortar (1:3 mix) or flexible pointing compound after fixing
• BS 6915:2001 — the British Standard for design and installation of slating and tiling including flashings
• LCA Manual — the Lead Contractor's Association (Lead Sheet Association) Manual is the primary technical reference

Quick Reference Table

Detailed Guidance

Back Gutter Construction

The back gutter runs horizontally across the full width of the chimney at its rear (upslope) face. It is the most critical element of the assembly because it must collect and safely discharge all water running down the chimney's back face plus rainfall landing on the roof slope trapped behind the chimney.

The gutter is formed from a single sheet of lead (or two sheets lapped if the chimney is wide) dressed over a timber tilting fillet and back board. The gutter pan itself must be a minimum 75mm wide measured at its base; on chimneys wider than 600mm a width of 100–150mm is preferable to handle the increased catchment. The lead is turned up the back face of the chimney by a minimum 150mm to form the upstand. This upstand must be dressed tight against the masonry and fixed with lead wedges at 450mm centres, then repointed.

The front edge of the back gutter (the edge nearest the chimney top) is turned under the first course of tiles or slates on the roof slope. The lead must extend a minimum 100mm onto the roof under these tiles. A lead clip or tack at the edge prevents the lead from rolling back under wind uplift.

On chimneys over 600mm wide, the back gutter should be in Code 5 to provide adequate weight and stiffness. The gutter must be laid with a positive fall (minimum 1:80) towards one or both sides to discharge to the side gutters or an outlet. A flat back gutter that ponds water will corrode prematurely at standing joints.

Side Step-and-Cover Flashing System

The sides of the chimney are sealed with a two-part system: soakers placed at each tile course during tiling, and a continuous stepped cover flashing laid over them and turned into the mortar joints of the chimney brickwork.

Soakers are small individual pieces of Code 4 lead, one per tile course per side. Each soaker sits at right angles to the run of the tiles, with one leg lying flat on the tile and the other turning up the face of the chimney. Dimensions: a minimum of 175mm total width (75mm on tile + 100mm up the wall); length equal to the tile gauge plus 25mm so the top of the soaker is concealed by the overlying tile. Soakers are not fixed; they are held in place by the weight of the tiles.

Cover flashings (also called step flashings) are cut with a series of steps, each step corresponding to one tile course. The depth of each step equals the tile gauge. The flashing is turned a minimum 65mm into the horizontal mortar joint at each step, fixed with lead wedges, and repointed. The cover flashing laps down over the top of each soaker by at least 65mm to prevent capillary action drawing water behind the step. The vertical face of the cover flashing should be a minimum 75mm in height measured down from the mortar joint.

Where the side flashing meets the back gutter at the top, the soakers should run continuously from the bottom of the chimney to where they tuck under the side upstand of the back gutter. The cover flashing at the top step must lap over the upstand of the back gutter.

Front Apron

The front apron covers the junction at the bottom (downslope) face of the chimney where it meets the roof surface. Unlike the side flashings, the front apron is a single continuous piece.

The apron is turned a minimum 150mm up the face of the chimney and fixed with lead wedges into a mortar joint, then repointed. The lower edge of the apron extends a minimum 150mm onto the tiles below — this measurement is taken from the top of the exposed face of the tile. The apron is cut at each side to tuck under the first soaker on each side, lapping behind them to prevent a capillary path.

The front apron is generally dressed over a small timber tilting piece at its base to throw water clear of the tile surface and prevent ponding at the junction. The bottom edge should be neatly turned or cut; a drip edge of 5–10mm is helpful on low-pitch roofs.

Fixing, Wedging and Repointing Details

All lead turned into mortar joints must be retained with lead wedges driven firmly into the joint. Wedges are cut from Code 3 or Code 4 scrap lead, typically 25mm × 25mm × 10mm. They are placed at a maximum of 450mm centres, with wedges at both ends of every flashing turn-in. After all wedges are driven, the joint is raked out to a depth of 25mm minimum and repointed with either:

• 1:3 cement:sand mortar (suitable for sheltered locations)
• Flexible polyurethane pointing compound (preferred for exposed locations and lime-pointed brickwork)

Mortar pointing must be tooled to a slightly recessed, weather-struck profile to shed water. Flush or raised pointing traps water at the flashing line.

Code Selection and Exposure Considerations

Code 4 (1.80mm, 20 kg/m²) is the standard minimum for all domestic chimney flashings. Code 5 (2.24mm, 25 kg/m²) should be used in the following circumstances:

• Chimney width (measured across the roof slope) exceeds 1,500mm
• Site is in a high rainfall area (annual rainfall over 1,000mm — broadly the western upland zones of the UK)
• Back gutter catchment area exceeds 1.5m²
• Chimney is on a building in an elevated or highly exposed position

Where the roof pitch is below 20°, both Code 4 and Code 5 flashings require particular attention to the front apron projection, which should be increased to 200mm onto the tile surface.

Frequently Asked Questions

Can I use Code 3 lead for soakers to save cost?

No. Code 3 (1.32mm) is too thin for soakers at chimney abutments. It will be easily punctured during tiling and will not provide adequate durability. The LCA Manual specifies Code 4 as the minimum for all chimney flashing elements, including soakers. Code 3 is only acceptable for soakers at simple pitched roof abutments away from chimneys where the exposure is significantly lower.

What if the chimney brickwork joints are very thin or raked back deeply?

If existing mortar joints are less than 10mm deep, the flashing cannot be wedged and repointed adequately. The joint should be cut back to a minimum 25mm depth using an angle grinder or plugging chisel before the flashing is installed. On historic lime-pointed chimneys, use a non-hydraulic lime mortar for repointing rather than OPC-based mixes.

The back gutter is discharging onto the tiles. Is that acceptable?

No. A back gutter outlet running loose onto tiles will erode the tiles over time and in high-flow events will cause water to be redirected in uncontrolled ways. The back gutter should discharge into a lead-lined valley gutter, a hopper connected to a downpipe, or into the side step flashing system where the catchment is small. Where the catchment is over 0.5m², a formal outlet into a downpipe or gutter is required.

How do I deal with a chimney that is wider than one sheet of lead?

For back gutters wider than approximately 600mm, use two sheets of Code 5 lead lapped a minimum 150mm at a welted or lapped seam. For side cover flashings on wide chimneys, use separate code 4 sheets for each section, lapping a minimum 150mm at any horizontal joints. Never attempt to stretch a single sheet across an oversized chimney — fatigue cracking at the stretched edges will follow within a few years.

Do the step flashings need to be welded or soldered at the corner where front apron meets side flashing?

No soldering or welding is required or recommended at these corners in domestic work. The front apron is tucked under the first soaker on each side. The cover flashing at the bottom step is cut and dressed down to overlay the top edge of the apron. If the junction is a concern on an exposed chimney, a small gusset piece of lead can be tucked in at the corner and dressed to both faces.

Regulations & Standards

• BS 6915:2001 — Design and installation of slating and tiling for pitched roofs and vertical cladding; includes requirements for flashings at chimney abutments and the sizing of soakers

• Lead Sheet Association (LSA) Manual — Primary technical reference for leadwork in the UK; specifies minimum dimensions for all chimney flashing elements, code selection, and bay sizing

• Building Regulations Approved Document C (Site preparation and resistance to contaminants and moisture) — requires that junctions between a roof covering and a wall are adequately weatherproofed

• BS EN 12588 — Specification for rolled lead sheet for building purposes; defines code thicknesses

• Lead Sheet Association — Rolled Lead Sheet: The Complete Manual — Primary industry reference; covers all chimney flashing dimensions and specifications

• BS 6915:2001 — British Standard for slating and tiling design and installation

• NHBC Standards Chapter 7.2 — Pitched roofs; references flashing requirements for chimneys

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• lead hip and ridge rolls — Hip and ridge lead details on the same roof

• lead parapet gutters — Gutter sizing and outlet design principles

• lead flat roof coverings — Code selection and bay sizing for flat lead