What Causes Chimney Leaks?

Cause 1: Failed Lead Step Flashing

This is the single most common cause of chimney leaks on Victorian properties in Great Malvern. The lead step flashing runs up the sides of the chimney stack, with individual lead steps tucked into the mortar joints of the brickwork and dressed down over the adjacent slates. Where the lead has been in place for 60 to 100 years, crystallisation, mortar erosion and thermal movement have typically caused the top edge to lift away from the brickwork, admitting water behind the flashing and directing it down the inside of the chimney junction.

The diagnostic sign is a damp patch on the plaster of the chimney breast at or just below ceiling level, often more pronounced after wind-driven rain on the affected face of the chimney. The repair is full stripping of the affected step flashing, raking out of the mortar joints, and installation of new Code 5 lead step flashing dressed into raked joints and pointed over.

Cause 2: Failed Flaunching Around the Pots

Approved Document J of the Building Regulations sets requirements for the construction and weathering of chimneys, including the provision of flashings and weatherings to prevent moisture penetration into the building at roof junctions.

Source: Approved Document J — Combustion Appliances and Fuel Storage Systems

The flaunching is the mortar collar formed around the base of each chimney pot where it sits in the stack. It is the most exposed mortar on the entire roof, subject to maximum freeze-thaw cycling and UV degradation, and on Victorian Great Malvern properties that have not had chimney maintenance in 20 or more years, failed flaunching is almost universal. Cracked and missing flaunching allows water to run down the outside of the pot into the flue and directly into the chimney void, producing a dripping or damp patch that appears to come from inside the stack.

Cause 3: Eroded Pointing on the Stack

The brickwork of a Victorian chimney stack is pointed with mortar that, on the exposed upper sections above the roofline, deteriorates significantly faster than the equivalent mortar at ground level. A stack that was last repointed 20 to 30 years ago in the standard sand-and-cement mix will typically show significant erosion on its weather faces. Eroded pointing allows capillary absorption of rainwater directly into the masonry, which then migrates down through the stack and emerges as a damp patch on the internal chimney breast.

Cause 4: Failed Back Gutter

The back gutter is the lead channel formed between the back face of the chimney stack and the upslope roof surface. It collects water running down the roof above the chimney and directs it around the sides of the stack to the step flashings. On Victorian properties where the original back gutter lead has reached the end of its life, water overflows the deteriorated channel and runs straight down the back of the stack into the roof void. This type of failure can be difficult to diagnose as the water entry point is concealed behind the stack from both above and below.

Cause 5: Mortar Used Instead of Lead

We encounter this regularly in Great Malvern: a previous maintenance campaign has filled the gap between chimney stack and roof surface with mortar rather than re-leading correctly. Mortar is not a flexible material, and as the chimney and roof structure move independently under thermal load, the mortar cracks within a few years. Each crack is a water entry point. A chimney that was “repaired” with mortar five years ago and is now leaking has not been repaired — it has been temporarily concealed.

Last reviewed: 22 July 2026 by Malvern Roofing Experts. Call 01684 439601 for current pricing.

Stopping a Chimney Leak for Good in Great Malvern

Fixing a chimney leak properly means treating the cause, not the stain on the ceiling. On Great Malvern’s tall Victorian villas, the stacks are among the most exposed features on any roof in the district, taking wind-driven rain off the hills from every direction. The durable repair depends on getting three things right: the lead, the pointing and the flaunching. Stepped and soaker lead flashings must be dressed to the correct code and lapped properly into the brickwork, because a lead detail that merely looks tidy but is under-gauged will lift again in the next gale. The pointing needs to be the right mortar for the age of the stack — hydraulic lime on pre-1920 brickwork, not a hard cement that cracks the soft old bricks it is meant to protect. And the flaunching, the mortar collar around the pots, has to be sound or rain simply runs down inside the flue.

The mistake we see most often on Great Malvern chimneys is a patch repair to one visible symptom while the real fault sits a foot away, unaddressed. A smear of mastic over a cracked flashing buys a season; the water finds the next weak point and the homeowner is back where they started. That is why we survey all four faces of a stack, the pots, the flaunching and every lead junction before quoting — a three-storey villa here often has several separate leak paths, and a quote that prices only the one you can see from the ground is not solving the problem. Done properly, in the right materials, a chimney repair on these villas lasts decades rather than seasons.

How We Diagnose a Chimney Leak

Diagnosis is where a chimney repair is won or lost. A stain on a bedroom ceiling rarely sits directly beneath the fault that caused it, because water entering at a stack runs along timbers and felt before it drops, sometimes several feet from where it got in. We start at the top and work down: checking the pots and flaunching, then each face of the stack for cracked or missing pointing, then every lead junction — the front apron, the stepped side flashings, the back gutter behind the stack that is so often the real culprit on Great Malvern’s larger villas. Only once we know exactly where the water enters do we quote, because pricing a repair to the stain rather than the source is guesswork that leaves you leaking.

On a redundant chimney — one no longer serving a fire — there is a further option worth weighing: removing the stack to below roof level and tiling over, which permanently ends the leak risk and the future maintenance a tall Victorian stack demands. It is not always the right call, especially where the stack is part of a symmetrical conservation-area frontage, but for a hidden rear stack that has leaked repeatedly it can be the honest long-term answer rather than repairing the same failure every few years.

One last point specific to Great Malvern: many of these villas share party walls, and a shared chimney stack straddling the boundary between two houses is common. Where a leak comes from a shared stack, responsibility for the repair is usually shared too, which can complicate a job that is otherwise straightforward. We are happy to survey a shared stack and provide a clear written report that both households can work from, so the repair gets done properly rather than stalling over who pays for what.