Roof leadwork is the use of sheet lead to seal the joints where a pitched roof meets a vertical surface or changes direction — around chimneys, against walls, in valleys and over abutments. Flashing is the general term for the weatherproofing detail at these junctions, and lead remains the most common material for it in the UK because it is malleable, durable and tolerant of thermal movement. Where the roof covering itself does the bulk of the work shedding water, leadwork manages the awkward edges and intersections that tiles or slates cannot close on their own.
What roof leadwork actually does
The function of leadwork is to bridge gaps that the main roof covering leaves open. A pitched roof sheds rain by overlap, but at any point where the slope meets brickwork, a chimney stack or another roof plane, that overlapping system breaks down. Lead seals those breaks by being dressed — shaped by hand — to follow the profile of both surfaces and direct water back onto the tiles or slates below.
Lead is chosen for this because it bends and stretches without cracking and lasts for decades when correctly installed. It also accommodates the constant expansion and contraction that temperature change causes; a detail that is too rigid, or a single piece of lead made too long, will eventually split. Good leadwork is therefore as much about allowing controlled movement as about forming a watertight seal.
Where lead is used on a pitched roof
Where the roof covering itself does the bulk of the work shedding water, leadwork manages the awkward edges and intersections that tiles or slates cannot close on their own.
Lead appears wherever the roof plane is interrupted or terminated. The most frequent locations include:
- Chimney stacks — lead seals all four sides of a stack where it passes through the roof slope.
- Valleys — where two roof slopes meet at an internal angle, a lead-lined channel (a lead valley) carries the combined run-off down to the gutter.
- Abutments — where a roof slope meets a wall, such as a lean-to against the side of a house.
- Parapet and gutter lines — behind low walls at roof edges, and in internal box gutters.
- Around roof penetrations — soil pipes, vents and similar features pushing through the covering.
The detail used at each point depends on the orientation of the joint. A junction running across the slope is handled differently from one running up it, and the layout of the surrounding tiles determines how the lead is cut and overlapped.
Chimney flashings, soakers and step flashing
A chimney is the most demanding piece of leadwork on a typical house because it presents joints on all four faces. The detail varies by face. At the front, lowest edge of the stack a single piece called an apron flashing turns down over the tiles. At the back, a lead gutter — sometimes called a back gutter — collects water shed by the stack and channels it around the sides.
The two sides running up the slope are weatherproofed with soakers and step flashing working together. A soaker is a small L-shaped piece of lead tucked under each course of tiles or slates and turned up against the brickwork; one sits at each course, hidden from view. Over the top of the soakers, step flashing is dressed down the wall in a stepped line that follows the mortar joints, with each step tucked into a brickwork joint and wedged in place. The soakers carry water away beneath the tiles while the step flashing covers the upstand and is sealed into the wall. This two-part system is more robust than relying on a single piece of lead, particularly on steeper pitches.
Where step flashing meets the masonry, the lead is set into a raked-out mortar joint, wedged with lead clips, and then the joint is repointed. The quality of this pointing and the depth of the chase into the brickwork both affect how long the detail stays watertight.
Lead codes: why thickness is specified
Sheet lead is graded by a system of codes that indicate its thickness, and the code is colour-coded for identification. The codes commonly used in roofing run from Code 3 (the thinnest) upward through Code 4, Code 5, Code 6, Code 7 and Code 8, with each higher number being thicker and heavier. The figure relates to the historic weight of lead per square foot, but in practice it is used simply as a thickness specification.
Thickness is specified rather than left to chance because different details place different demands on the lead. Soakers, which are small and protected beneath tiles, can use a lighter code such as Code 3 or Code 4. Larger, more exposed details — valleys, back gutters and the wider flashings on a chimney — generally call for Code 4 or Code 5, and the most exposed or heavily worked areas may use thicker grades still. Using lead that is too thin for the location invites fatigue cracking; using lead that is excessively long in a single piece causes the same problem through thermal movement, which is why bays and joints are limited in size.
Common leadwork faults that let water in
Poor leadwork is one of the most frequent causes of roof leaks, and the failures tend to be predictable. A surveyor or roofer inspecting a problem will often look for the same recurring issues.
- Splitting from oversized pieces — a single length of lead made too long expands and contracts until it cracks. Proper practice keeps each piece within a limited length and uses welted or lapped joints between bays.
- Lead that is too thin — an under-specified code fatigues and tears far sooner than the correct grade.
- Failed or shallow pointing — step flashings set into a chase that is too shallow, or pointed with weak mortar, work loose and lift in the wind.
- Mortar fillets instead of lead — cement fillets used in place of proper flashing crack and shrink, opening a direct path for water; they are a common sign of a botched repair.
- Missing or displaced soakers — soakers that were never fitted, or have slipped, leave the side junctions of a chimney unprotected even when the visible step flashing looks intact.
- Inadequate laps and upstands — flashings that do not overlap the tiles enough, or do not turn up high enough against the wall, allow wind-driven rain to track behind them.
Because much leadwork sits where it cannot easily be seen — under tiles, behind chimneys, within valleys — a fault can let water in for some time before it shows as a stain on a ceiling. When investigating a leak on a pitched roof, the junctions and flashings are usually examined before the main covering, since the detailing is where water most often finds its way through.