Faulty lead flashing is a common cause of roof leaks, and catching the early signs can save you a significant repair bill.
Lead is the material used at every point on a roof where two surfaces meet or where a roof meets a wall. Flashings seal the joint between a chimney stack and the roof slope. Valleys channel rainwater down between two roof pitches. Soakers sit beneath each course of slates or tiles along a junction. Flat roof coverings use lead sheet to provide a durable, weathertight surface over bays, dormers and outbuildings.
Every one of those applications relies on lead being cut, dressed and fixed by hand. It cannot be rushed. Lead that has been forced into shape rather than carefully worked will crack, lift or pull away from the mortar pointing faster than it should, and a gap of even a few millimetres is enough for driven rain to find its way into the roof structure beneath.
When choosing someone for lead work, look for a roofer who works to traditional standards and uses code-appropriate lead sheet throughout. The code number refers to the thickness of the sheet: a higher code means thicker, heavier lead, and the right code for each location is determined by the size of the area being covered and the exposure the roof faces.
Lead problems rarely announce themselves with an obvious stream of water. More often, the first signs are subtle and easy to dismiss as something else entirely. Knowing what to look for is the most useful thing any homeowner can have.
Damp patches on a chimney breast wall or ceiling. If you notice a damp stain appearing on an internal wall near a chimney, or on the ceiling of a room that sits below a chimney stack, the flashing is the first place to investigate. The damp may only appear after heavy or prolonged rain, which is why it can take months before the pattern becomes obvious. A stain that grows slowly and dries between wet spells is a classic sign of flashing failure, not a plumbing leak.
Staining on the chimney breast that moves down the wall over time. Tide marks that shift position or grow slightly each winter are consistent with water tracking behind loose or open-jointed lead rather than running down a fixed crack.
Lifting or buckling lead visible from the ground. If you can see lead flashing that appears to have lifted away from the roof surface, or that has developed a visible ripple or buckle, it has either been installed in lengths that are too long (lead needs to be installed in shorter sections to allow for thermal movement) or it has oxidised and hardened to the point where it can no longer flex. Either way, the seal has been compromised.
Failed or crumbling mortar pointing along the top edge of a flashing. The top edge of a step or cover flashing is usually tucked into a mortar joint in the brickwork and pointed over. When that pointing shrinks and cracks, water runs behind the lead rather than over it. From the ground this can look like a general chimney maintenance issue, but the consequence is water getting behind the flashing.
Missing lead sections. In older properties, short sections of lead can be lost entirely during storms, or removed during a previous repair and not replaced correctly. A gap in a valley or a missing soaker is not always visible without getting onto the roof, but a damp patch directly below where two roof slopes meet is a strong indicator.
Rust staining on external render or brickwork below a valley or parapet. Water that has been tracking behind failed lead picks up tannins from timber and iron from fixings as it goes. The resulting rust or dark staining on an external face below a roof junction is a reliable sign that water has been getting in somewhere above it.
A musty smell in a roof space or upper room. If the leak has been slow and low-level for some time, the first thing you might notice is not visible damp but a smell. Timber in a roof structure that has been repeatedly wetted and dried will begin to support mould growth even if there is no standing water or visible saturation.
Understanding why lead fails helps you ask the right questions when a roofer comes to look at your property. Not all lead problems are the result of age or wear. A significant number are caused by poor installation in the first place, sometimes on a relatively new roof.
Lengths that are too long. Lead expands and contracts with temperature changes. In the UK climate, a roof in full sun on a summer day can see surface temperatures well above ambient air temperature, while the same surface can be close to freezing in winter. Lead sheet must be installed in relatively short sections, typically no more than 1.5 metres for most applications, with separate lapped sections above. If a single unbroken length of lead is dressed across a long chimney or down a long valley, it will eventually fatigue and crack at points of stress, or pull itself out of the mortar joint at the top edge.
Incorrect code for the application. Using lead that is too thin for the exposure or the size of the area means the material will not last as long as it should. Code 3 lead, for example, is suitable for soakers but not for large cover flashings or flat roof bays where a heavier gauge is needed.
Mortar pointing that was not raked out properly before re-pointing. When a flashing is refixed, the old mortar must be raked out of the joint to sufficient depth and the new mortar packed firmly behind the lead. If the pointing is applied only superficially over crumbling old mortar, it will fail quickly regardless of the quality of the lead itself.
Lead that has oxidised beyond its useful life. Good quality lead can last many decades, but it does eventually oxidise and harden. Once it loses its malleability it can crack under thermal movement and no longer forms a watertight seal. Lead on Victorian and Edwardian properties that has never been touched may well be original and past its useful life, even if it looks intact from below.
Bitumen or sealant used as a temporary fix. A common problem seen on roofs that have had piecemeal repairs over the years is the use of bitumen sealant or self-adhesive flashing tape over failing lead. These products do not last and eventually allow water to track underneath them. When the underlying lead is eventually inspected, it is often in worse condition than it would have been if the bitumen had not been there, because moisture gets trapped beneath.
Movement in the chimney stack. Where a chimney stack has settled, been struck by scaffolding or is subject to ongoing movement, flashings will be pulled out of position over time regardless of how well they were originally fitted. In these cases, re-pointing the flashing alone is not sufficient and the underlying cause needs to be understood first.
A slow lead leak rarely resolves itself and almost always gets worse over winter. The cost of repairing water-damaged timber, replacing saturated insulation or treating mould in a roof space consistently exceeds the cost of the lead work that would have prevented it. As a general guide, any internal damp that can be traced to a roof junction or chimney should be investigated before the next significant wet period.
If you can see lifted or cracked lead from the ground, or if mortar pointing along a chimney flashing is visibly crumbling, there is no benefit in waiting. These are not slow-developing problems. A single period of heavy rain with wind driving against the affected face can result in significant water entry in one event.
If the damp is very slight and intermittent, it is still worth having the roof inspected to establish the cause. Sometimes what appears to be a lead problem turns out to be a slipped tile or a deteriorated ridge, and vice versa. Getting a clear diagnosis before committing to any work is the sensible step.
A good lead repair is not simply a matter of pressing the existing lead back into place and pointing over it. If the lead has moved, cracked or oxidised, it needs to be replaced, not patched. Here is what a thorough job looks like.
On a chimney with all four sides to flash, the work involves a back gutter, two sets of soakers and step flashings on the sides, and a cover flashing at the front. Each element needs to be cut and fitted individually. This is skilled manual work, not something that can be done satisfactorily with a roll of flashing tape and a torch.
Valleys are the internal angles where two roof slopes meet. On older properties these are often lined with lead sheet formed into a trough shape to direct water down to the gutter. A failed lead valley is a significant source of water entry because the volume of water passing through a valley is far greater than at a flashing joint.
Lead valleys should be replaced in sections with adequate laps between each section, and the edges should be dressed up behind the adjacent tiles or slates to prevent any possibility of water tracking back underneath. The width of the exposed lead in a valley determines how much water it can handle; too narrow and water will be forced under the tiles in heavy rain.
On flat roof areas such as bay window roofs, dormer cheeks and garage roofs, lead sheet provides a durable surface when installed correctly at the right code. For small to medium flat roof areas, code 5 or code 6 lead is typical. The lead is laid in bays with roll or drip joints between each bay to allow for thermal movement. A flat roof covered in a single unbroken sheet of lead will eventually fail at the edges as the material works and fatigues.
Lead work is priced by the job rather than by a simple per-metre rate, because the complexity of the junction, the access required and the condition of the surrounding materials all affect how long the work takes. As a general guide, reflashing a chimney on a standard pitched roof in Norfolk typically costs in the range of £300 to £700, depending on the size of the stack and how many faces need new lead. A lead valley replacement on a two-storey property with scaffold access required will cost more. Flat roof lead work on a bay window roof can range from £400 upwards depending on size.
These figures are general indications based on typical current UK pricing and will vary with the specific job. Getting a clear, itemised quote that describes what is to be done, what materials are being used and what code of lead is specified is the best way to compare any estimates you receive.
Before agreeing to any lead work, there are questions worth asking any roofer, including us.
A roofer who answers these questions clearly and without hesitation is a roofer who understands the work. One who is vague about material specifications or who proposes to seal over existing lead with sealant should prompt further questions.
If you have noticed any of the warning signs described above, call us to arrange a roof survey so we can give you a clear picture of what is needed before any work is agreed.
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