Why Does My Chimney Keep Leaking? | Reading Roofing Experts

Your chimney keeps leaking because mortar has been applied over failed lead flashings — and mortar cannot flex with lead's thermal movement, so it cracks every one to two winters. The permanent fix is stripping the mortar and replacing the lead, wedged correctly into the brick joint.

Of all the roofing problems we encounter across Reading Victorian and Edwardian stock, the persistently leaking chimney is the most frustrating - and the most consistently misrepaired. Properties in Newtown, Katesgrove, Caversham and along the Oxford Road terraces regularly come to us after two or three previous repairs have failed to solve the problem. The reason is almost always the same.

What Chimney Flashings Actually Are

Lead flashings are the metal seals that close the gap between a chimney stack and the surrounding roof slope. They have to seal a joint between two materials - brick and tile - that move independently as temperature changes. The brick stack and the roof structure expand and contract at different rates, which is why the joint must be flexible.

The correct installation involves: step flashings that interleave with each tile course up the side of the chimney; an apron flashing that covers the front junction between chimney and lower roof slope; and soakers - small individual lead pieces that sit behind each step flashing to provide a secondary seal. At the back of the chimney, a back gutter channels water away from the junction. Each element requires the lead to be cut, dressed and fixed into the brick course above, with the edge of the lead wedged into a raked-out mortar joint and pointed over - not sealed over.

Why Mortar Repairs Always Fail

Lead sheet for flashings is specified by code under BS EN 12588 — code 4 (1.8mm) for step and cover flashings, code 5 (2.24mm) for back gutters — and correctly dressed lead has a service life of 50-80 years against one to two winters for a mortar patch.

Lead moves. A lead flashing on a south-facing chimney in Reading experiences temperature variation from approximately -5C in winter to 70C-plus in summer when the sun heats the dark lead surface directly. Lead has a thermal expansion coefficient of 0.0000291 per degree Celsius - meaning a 1-metre length of lead changes by approximately 2mm across a 70-degree temperature range. Multiply that across a chimney stack with 3-4 metres of flashing and you have significant seasonal movement.

Mortar is rigid. It cannot accommodate this movement. Applied as a seal over lead flashings, it cracks within one to two thermal cycles - typically within a year. The crack admits water, which then tracks behind the remaining mortar, and the problem returns - often worse than before because water has been tracking in a wider path during the failed repair period.

⚠ If a roofer has applied mortar to your chimney flashing area and the leak has returned - that repair has failed. Mortar over lead is not a repair, it is a delay. Call us for a permanent fix: 0118 327 2383

The Repair That Actually Holds

Correct lead flashing repair involves stripping the existing mortar and any failed lead, preparing the brick course to accept new lead, cutting and dressing new code 4 or code 5 lead sheet to the correct profile, wedging the top edge into the raked-out brick joint and securing with lead wedges, and then pointing over the top of the wedged joint - not over the face of the lead. A correctly installed lead flashing requires no maintenance for 40-50 years.

Reading Properties Most Affected

The highest frequency of chimney flashing failures we see is on Victorian and Edwardian terraces with shared chimney stacks - particularly in Newtown and Katesgrove. These shared stacks serve two properties and have typically been repaired by two different roofers over their history, sometimes with incompatible approaches. The south-facing apron flashing on these stacks sees the greatest thermal cycling and is the most frequent failure point.

Caversham Heights properties have a secondary problem: the exposed south-westerly position means wind-driven rain is applied more directly against chimney-to-roof junctions than on sheltered valley-floor properties.

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Last reviewed: 25 April 2026 by Reading Roofing Experts.

Chimney Leaks on Reading Victorian Properties

Reading's inner Victorian terraces — Newtown, Katesgrove, Caversham and the surrounding streets — generate more chimney leak call-outs per street than any other property type in the area. The combination of high chimney stack density, 100-to-120-year-old lead flashings, and the Thames Valley humidity creates a specific failure pattern that we address routinely.

The fundamental issue is that mortar is routinely applied over lead flashings as a repair, and mortar cannot accommodate lead's thermal movement. In summer, lead expands by several millimetres per metre. In winter it contracts. Mortar — which is rigid once cured — cracks at these movement points within one to two thermal cycles. Water enters through the cracks, tracks behind the lead, and appears as a damp patch on the chimney breast inside. The homeowner calls a roofer, mortar is applied over the cracked mortar, and the cycle repeats.

The permanent fix requires stripping all mortar from the flashing line, assessing the lead for serviceable life, and either redressing sound lead correctly back into the brick joint or replacing with new code 4 or code 5 lead. The wedged top edge is pointed with a small bead of flexible sealant or lime pointing — never with a rigid cement or mortar over the face of the lead. This is what permanently eliminates the leak. It costs more than a mortar patch and takes longer. It does not fail on the next winter.

On Victorian party chimney stacks shared between terraced properties in Newtown and Katesgrove, flashing failure on one property often affects the adjacent property — water tracking across the party wall or behind shared flashing details. We are experienced in multi-property chimney repairs and can advise on the most practical approach where the repair involves flashing shared between two or more owners.

Common Questions

The most common reason is mortar applied over original lead flashing rather than lead replacement. Mortar cannot accommodate the seasonal movement of lead and fails within 1-3 years. If your chimney has been repaired more than once by applying mortar to the flashing area, the lead itself needs replacing - not sealing over again.

A full chimney flashing replacement on a standard Reading property costs 380-700 pounds depending on chimney size and access. Larger chimneys on Caversham Heights properties requiring scaffold access cost more. Free quotes provided with no call-out fee.

On Victorian and Edwardian properties in Reading - Newtown, Katesgrove, Caversham - lime mortar is the correct specification. Portland cement is harder than the original brick, prevents normal moisture movement and causes brick face spalling over time. On post-1950 brick chimneys, a hydraulic lime or flexible mortar compound is still preferable to pure Portland cement on exposed faces.

The main indicator from inside the property is a damp patch on the chimney breast wall or ceiling adjacent to the chimney, particularly one that appears or worsens after rain and dries in dry weather. From outside, visible gaps between lead and brick, or mortar applied directly over what should be a lead flashing joint, are reliable indicators. We inspect for free.

They address different problems. Repointing addresses brick joint mortar failure. Lead flashing replacement addresses the junction between stack and roof slope. Many chimneys need both, and it is most cost-effective to do both while the scaffold is up. We assess both elements and recommend only what is actually needed.

Yes. Disused chimney stacks can be capped to prevent water entry while retaining ventilation. A chimney cap and ventilated cowl is the standard approach. On period properties in Newtown or Caversham, capping is done in a manner that maintains the original appearance.

A party stack serves two properties but both owners share responsibility for the structural elements. When one owner repairs correctly and the other does not, the problem recurs. We are experienced in coordinating repairs across both properties.

Why Does My Chimney Keep Leaking in Reading?