A leak that keeps returning around a chimney or skylight almost always traces back to one part of the roof: the flashing. Lead flashing is the thin metal seal that joins a flat surface (a chimney stack, a skylight frame, a roof valley) to the surrounding tiles or slates. When it cracks, lifts, or corrodes, rainwater finds the smallest gap and works its way into the roof structure, often long before any damage is visible from inside the house.

This guide explains exactly why these leaks happen, the warning signs to look for, and how they're properly repaired.

Why Chimneys and Skylights Are the Most Common Leak Points on a Roof

Most roofs are made of overlapping tiles or slates, which shed water naturally. Chimneys and skylights break that continuous surface, so a flexible material is needed to bridge the join and keep it watertight. That's the job of lead flashing.

Because these junctions rely on a single strip of jointed metal rather than layered tiles, they're inherently more vulnerable than the rest of the roof. A few millimetres of movement, corrosion, or bad original installation is enough to let water in.

The Main Causes of Leaks Around Chimneys and Skylights

1. Cracked or split lead flashing: Lead expands and contracts with temperature changes. Over the years, this constant movement causes hairline cracks, especially at bends and joints. Once a crack opens, water gets behind the flashing and runs down inside the chimney breast or roof timbers rather than off the roof.

2. Flashing that has lifted or come away from the wall: Flashing is dressed (pressed) into a mortar joint or chase in the brickwork. If the mortar erodes, or the flashing wasn't installed deep enough originally, it can pull away from the wall, leaving a direct gap for rain to enter.

3. Corrosion from age or poor-quality material: Not all flashing is equal. Thin or low-grade lead corrodes faster, particularly in exposed or coastal locations. Once lead corrodes, it becomes brittle and porous, and patch repairs rarely hold for long.

4. Poor original installation: Flashing that wasn't cut, dressed, or sealed correctly during a previous roofing job is one of the most common causes of "new roof, old leak." This is especially common where a roof was tiled by a general contractor rather than a specialist roofer.

5. Skylight seals failing: On skylights specifically, the leak often isn't the glass unit itself but the flashing kit and seal around its frame. UV exposure, weathering, and movement in the timber upstand can all break this seal over time.

6. Blocked or damaged guttering forcing water back up: Water that can't drain properly can back up against the base of a chimney or skylight upstand, increasing the chance it finds a way under the flashing. Keeping fascia, soffits, and guttering clear and in good repair reduces this risk considerably.

Warning Signs You Have a Flashing Leak

A damp patch on the ceiling directly below a chimney breast or skylight, especially after heavy rain
Discoloured or peeling plaster near a chimney stack
Visible daylight around a skylight frame from inside the loft
Green or white staining on brickwork beneath flashing (a sign water is tracking down the wall)
Musty smells in a loft space that weren't there before
Missing or visibly lifted strips of grey metal at roof junctions when viewed from ground level

If any of these sound familiar, it's worth getting the roof looked at before the next spell of heavy rain, since a small flashing crack left unaddressed can lead to damp, rotten timber, and much costlier repairs.

How Lead Flashing Leaks Are Properly Repaired

Inspection first. A proper repair starts with identifying exactly where water is entering, not just where the stain appears inside, since water can travel along timbers before showing up on a ceiling.

Like-for-like lead replacement. Code 4 or Code 5 lead (the standard weights used in UK roofing) is cut and dressed to the specific chimney or skylight shape, then correctly chased into the brickwork mortar joint rather than simply sealed on top with mastic.

Full re-dressing at joints and steps. On chimneys, this includes the apron, soakers, and stepped flashing along the sides, not just the front section, since a leak can originate from any one of these.

Skylight flashing kits. For skylights, this usually means fitting a manufacturer-approved flashing kit designed for the specific roof pitch and tile profile, rather than a generic seal.

Avoiding the mastic trap. Sealant and mastic are sometimes used as a quick fix, but they degrade within a year or two and rarely address the underlying gap. A correctly dressed lead repair lasts significantly longer.

This kind of work sits alongside broader roofing projects such as roof installation and repair, and is often picked up during a routine inspection even when it wasn't the original reason for the callout.

Is It a DIY Fix or a Job for a Professional?

Flashing repairs involve working at height, cutting and dressing metal precisely, and chasing into brickwork correctly, so this isn't a job for sealant from a hardware shop. A flashing repair that isn't dressed properly often fails again within months, and in the meantime water can be causing hidden damage to timbers, insulation, and ceilings. A specialist roofer can identify the actual entry point (which is often not where the stain appears) and fit a repair that's built to last.

Conclusion

Leaks around chimneys and skylights are rarely random. In almost every case, they trace back to lead flashing that has cracked, lifted, corroded, or was never dressed correctly in the first place. Spotting the early signs- damp patches under a chimney breast, staining on brickwork, or daylight around a skylight frame- and acting before the next heavy rainfall can be the difference between a straightforward flashing repair and a much larger job involving rotten timber or internal damage.

Because flashing work involves precise cutting, dressing, and correct integration into the brickwork, it's not something to leave to sealant or a temporary patch. Persistent leaks around a chimney or skylight are worth investigating properly rather than patching repeatedly. Hibbs Roofing and Building Ltd offer free roof inspections across Ashford and the surrounding Kent area, identifying the real cause of a leak and fitting a lasting lead flashing repair rather than a short-term fix.

FAQs

Can a small crack in lead flashing really cause a big leak?

Yes. Even a hairline crack allows water in during heavy or prolonged rain, and because it sits at a roof junction, that water often runs directly into the loft or wall cavity rather than off the roof.

How long does lead flashing usually last?

Good-quality lead flashing, correctly installed, typically lasts 25 years or more. Leaks usually point to either age, poor original installation, or physical damage rather than lead simply "wearing out" early.

Does a leak around a skylight mean the skylight itself is faulty?

Not usually. In most cases, the glazing unit is fine, and the issue is the flashing or seal around the frame, which can be repaired or replaced without removing the skylight.

Will home insurance cover a flashing repair?

This depends on the policy and the cause of the damage. Sudden storm damage is often covered, while gradual wear from age typically isn't, so it's worth checking your specific policy wording.