Leadwork and flashings are the weatherproofing details that seal the joints where a roof meets something else — a wall, a chimney, a valley, or another roof slope. They are usually formed from sheet lead, shaped and dressed to cover these junctions so that rainwater is guided away rather than allowed to soak into the structure beneath. Where two surfaces meet at an angle or change in level, a flashing bridges the gap; lead is the traditional material because it is malleable, durable, and tolerant of the small movements a building makes through the seasons.
What do leadwork and flashings do?
The covering on a roof — tiles, slates, or sheet — sheds water across an unbroken plane very effectively. Problems arise at the edges and interruptions, where that plane stops or meets a vertical surface. Flashings exist to seal those breaks. They overlap the roof covering on one side and are secured against or into the adjoining surface on the other, forming a continuous waterproof transition.
Good leadwork works by gravity and overlap rather than by sealant alone. Water runs onto the flashing, is carried down the slope, and discharges back onto the main roof covering below the junction. Because lead can be dressed — worked by hand to follow the contours of tiles, brick courses, or curved details — it forms a close, shaped fit that a rigid material could not achieve.
The term covers several distinct components, each suited to a particular situation:
- Step flashing — used where a sloping roof meets a wall, such as the side of a chimney or an abutment. The lead is cut and turned into the mortar joints in a stepped pattern that follows the brick courses, so each piece overlaps the one below.
- Soakers — small individual pieces of lead laid beneath each course of slate or tile at a wall junction, hidden from view, working with a cover flashing above them.
- Cover flashings, apron flashings and back gutters — used at the front and rear of chimneys and at the heads of abutments, directing water away from the most exposed faces.
- Valley and saddle leadwork — channelling water where two roof slopes meet at an internal angle.
The vulnerable junctions they protect
Leadwork and flashings are the weatherproofing details that seal the joints where a roof meets something else — a wall, a chimney, a valley, or another roof slope.
Most roof leaks begin not on the open expanse of tiling but at a junction. These are the points where the simple logic of overlapping courses breaks down and water needs deliberate management. Understanding where they are explains why leadwork is concentrated in particular places.
The common vulnerable junctions include:
- Chimney stacks — a chimney interrupts the roof on all four sides. The front needs an apron, the sides need step flashing and often soakers, and the rear needs a back gutter to carry water around the obstruction. A failure at any one of these allows water into the roof space.
- Wall abutments — wherever a roof slope runs up to or alongside a wall, such as a lean-to against a house or an extension roof. Step flashing into the wall, combined with soakers, keeps this line watertight.
- Valleys — the internal angle where two slopes meet carries a large volume of concentrated run-off, making it one of the most demanding details on a roof.
- Parapet walls and gutters — where the roof finishes behind a raised wall, lead lines the gutter and dresses over the wall.
- Dormers and roof windows — projections through the roof create their own set of junctions, each needing flashing on the sides and head.
- Pipe and vent penetrations — anything passing through the covering needs a collar or slate to seal the opening.
Because these junctions are where weatherproofing is most at risk, they are also where workmanship matters most. A flashing that is too short, poorly lapped, or not properly secured into the masonry will let water track behind it. The mortar joint into which a step flashing is turned should be raked out cleanly and the lead held with lead wedges before being repointed, rather than simply smeared over with sealant.
Why lead is graded by code and thickness
Sheet lead used in roofing is graded by a system of codes, which describe its thickness. The code number rises with thickness: common roofing codes run from around Code 3 (thinner) up to Code 8 (thicker). Each code has a recognised colour marking on the sheet for identification, and each has an associated weight per square metre, since lead is sold and specified by mass.
The reason thickness is graded rather than left to choice is that different details demand different performance. A thin gauge dresses easily into intricate shapes but is more prone to fatigue and tearing where it is exposed or where it must accommodate movement. A thicker gauge resists wear and lasts longer in demanding positions but is heavier and harder to work. Matching the code to the job is part of forming durable leadwork:
- Lighter codes are typically used for soakers and small concealed pieces.
- Medium codes suit step flashings, cover flashings and aprons.
- Heavier codes are favoured for valleys, gutters and larger exposed areas that carry concentrated water and see more thermal movement.
Thickness alone does not prevent the main weakness of lead: thermal movement. Lead expands and contracts noticeably as it heats and cools, and if a sheet is too large or rigidly fixed at both ends, repeated movement causes it to crack along stress lines. For this reason, leadwork is laid in limited lengths with movement joints, and individual pieces are fixed at one point so they can move freely elsewhere.
This is where welts come in. A welt is a folded seam used to join two pieces of lead or to stiffen and finish a free edge. By folding the metal over on itself, a welt both creates a weatherproof joint that resists water being driven up under it and allows adjacent sheets to move independently. Along with rolls, drips and laps, welts are part of the vocabulary of traditional lead detailing that keeps a roof watertight without relying on adhesives. The recommended sheet sizes, fixing methods and joint spacings are set out in established codes of practice, and well-formed leadwork following those principles can last for many decades.
Reviewed: June 2026