Chimney Repairs: Repointing, Flaunching, Lead Flashings & Cowls

Summary

Chimneys are one of the highest-maintenance elements on any older building. They project above the roofline, are exposed on all four sides to weathering, experience thermal cycling from flue gases and outdoor temperatures, and are frequently neglected until water is getting in. In the UK, the vast majority of chimney problems are moisture-related: failed flaunching lets water into the chimney pot and stack; cracked repointing allows water to track down the face; and failed lead flashings allow water to enter at the junction with the roof covering.

Understanding which mortar to use is critical. Many chimneys — particularly on pre-1919 buildings — were built with lime mortar. Using ordinary Portland cement (OPC) for repairs creates a much harder, more rigid mortar than the surrounding brickwork, which concentrates movement stresses in the brickwork itself rather than the mortar joints, leading to spalling and cracking. Always match mortar type and strength to the substrate.

Chimney work almost always requires safe access to the stack itself. Two-storey ridge-height is typically 7–9 metres above ground — scaffolding is normally required. MEWP (cherry pickers) are sometimes used for single items but scaffolding provides a safe working platform for anything beyond a quick inspection.

Key Facts

• Flaunching mortar mix: Traditionally 3:1 sharp sand:cement with a weathered finish; best practice now favours a 1:2.5 NHL 3.5:sharp sand hydraulic lime mix for older stacks
• Repointing mortar for Victorian/Edwardian brickwork: NHL 2 or NHL 3.5 hydraulic lime:sand (1:2.5), never stronger than the brick
• Repointing mortar for post-1945 brickwork: 1:1:6 cement:lime:sand or 1:6 OPC:sharp sand acceptable
• Lead code for soakers: Code 3 (1.32mm) minimum; Code 4 (1.8mm) recommended for longevity
• Lead code for stepped and cover flashings: Code 4 (1.8mm) minimum; Code 5 (2.24mm) for exposed locations
• Lead soaker length: Tile gauge + 75mm tail minimum; width = 175mm minimum
• Step flashing lap: Minimum 65mm horizontal lap; dress minimum 25mm into the masonry joint
• Back gutter (chimney back): Code 5 or 6 lead; falls minimum 1:60 toward outlet
• Chimney height above ridge: For minimum 600mm above the ridge within 2.3m horizontal distance, or higher per BS EN 15287 [verify specific clause]
• Relevant standard for lead: BS EN 12588 (Rolled lead sheet) and the LSA Installation Guide
• DPC to chimney stack: A lead or bituminous felt DPC is required within the chimney at roof level — often missing on older stacks
• Pot types: Terracotta or concrete pots are standard; half-round, cannon, and octagonal profiles are common
• Cowl requirement: Not mandatory but recommended where chimney is capped or where birds are an issue
• Mortar joint profile: Weatherstruck joint is the standard profile for external chimney repointing (not flush, not recessed)

Quick Reference Table

Detailed Guidance

Repointing a Chimney Stack

Preparation:

1. Rake out all failed mortar to a minimum 15mm depth using an angle grinder or plugging chisel — never just skim over existing mortar
2. Brush out dust and damp the joint thoroughly before applying new mortar
3. On lime mortar work, ensure the background is wet but not saturated

Mixing and application:

• Use pre-bagged hydraulic lime mortars (e.g. Tarmac Limelite, St Astier NHL) for consistent quality — site-mixed lime can vary
• Apply in lifts no greater than 10mm; work in the shade if possible — direct sunlight will dry lime mortar too fast
• Strike the joints to a weatherstruck profile: top edge back, bottom edge projecting, so rainwater is shed away from the brick face
• Keep the work damp with a light mist spray for the first 48–72 hours in dry weather

Colour matching: Lime mortars lighten significantly as they cure — colour matching is difficult in the short term. Advise customers that new pointing will stand out initially and weather in over 12–24 months.

Flaunching

Flaunching is the mortar collar that holds the chimney pot in place at the top of the stack. It fails due to frost action, UV degradation, and differential movement.

When to replace: If the flaunching shows cracks, is loose (tap with a hammer — a hollow sound indicates separation from the stack), or if the pot rocks, it must be replaced. Partial repairs rarely last.

Full replacement procedure:

1. Chip back all existing flaunching to the masonry crown
2. Remove the pot(s) and set aside if reusable
3. Make good any cracked brickwork at the top of the stack
4. Set pot(s) in position — ensure they are centred and vertical
5. Haunch up around the pot with a 1:3 hydraulic lime mortar or class B engineering-quality sand:cement mix, feathering to drain water away from the pot and off the stack
6. The flaunching should cover at least 50mm of the pot base and slope at minimum 45° to the outer edge of the stack

For heritage properties, flaunching in NHL mortar blends better with existing lime masonry and is more sympathetic to movement.

Lead Flashings

Lead is the gold standard for chimney flashings. Aluminium and proprietary flashings (e.g. Leadax, stepped rubber systems) are available but lead remains the most durable and easiest to dress on irregular masonry.

Soakers (under stepped flashing at the side of a chimney against a tile or slate roof):

• Cut to: gauge + 75mm long × 175mm wide minimum
• Bent to an L-shape — sits over each course of tiles/slates
• One per tile course going up the chimney
• No fixings required — the tiles hold them

Stepped cover flashing (over the soakers):

• Lead Code 4 or 5
• Chased into the mortar joint — rake joint out 25mm minimum
• Secure with lead wedges driven into the joint at minimum 450mm centres
• Point over with mortar or proprietary flashing sealant (Flashband is not a permanent fix)
• Step dimension: horizontal = tile gauge; vertical step = tile course height

Back gutter (behind a chimney where water collects):

• The most leak-prone area — often undersized or missing entirely on older properties
• Code 5 or 6 lead; size to match the volume of water draining off the roof slope above
• Should have a dressed lead soaker/cover to the rear of the chimney and extend minimum 150mm up the back face of the chimney
• Falls minimum 1:60 to the outlet; fit a lead-dressed outlet

Front apron flashing (front face of chimney over the roof):

• Code 4 lead; step down at minimum 150mm over the roof covering
• Turn up minimum 75mm against the chimney face

Chimney Cowls

Cowls are fitted to:

• Reduce downdraught (common in exposed coastal or hilltop locations)
• Prevent birds and vermin entering unused flues
• Cap off redundant flues
• Improve draw on difficult flues

Types:

• Rotating cowl: Spins in wind to create draw — good for low-pressure draught problems
• Anti-downdraught cowl: Fixed design that deflects wind upward regardless of direction — popular for boiler flues
• Bird guard: Mesh cage — never fit on an active wood-burning or solid fuel flue as the mesh will block with soot
• Capping cowl: For permanently redundant flues — must still allow ventilation to prevent condensation in the flue

Important: Any open flue on a property with a gas appliance that uses natural draught must maintain ventilation. Consult a Gas Safe registered engineer before capping a flue if there is any gas appliance in the room below.

Frequently Asked Questions

Can I use ordinary cement for repointing a Victorian chimney?

No — OPC mortar is approximately 3–4 times stronger than the brick in a typical Victorian wall. When the stack moves (and it will — thermal movement in a chimney is significant), the mortar does not absorb the movement, and the bricks crack and spall instead. Use an NHL 3.5 hydraulic lime mortar at a 1:2.5 ratio with sharp sand.

My customer's chimney is not used — do I still need to maintain the lead flashings?

Yes. A redundant chimney still needs to be weathertight to prevent water ingress. The internal temperature differentials in an unused flue can actually be worse for condensation. Either maintain the flashings fully or, if the customer agrees, cap the chimney with a weathertight cover and ensure there is still adequate ventilation to the flue interior.

How do I know if the flaunching is failing without getting on the roof?

Use a pair of binoculars from the ground — look for cracks running across the flaunching or separating from the pot. If the pot appears to lean or tilt, the flaunching has likely failed on one side. Staining on the brickwork below the pot is a strong indicator of water tracking through cracked flaunching.

What fixings should I use for lead flashings?

Use lead wedges cut from scrap Code 4 or 5 lead sheet, driven into the chase at 450mm centres maximum. Never use copper or steel fixings — electrolytic corrosion will cause the lead to fail at the fixing point. Point the chase over with a mortar compatible with the surrounding masonry (lime mortar for older walls).

Do I need planning permission to replace chimney pots or remove a chimney?

Replacing like-for-like chimney pots on a non-listed building is permitted development. Demolishing a chimney stack (even a redundant one) visible from a public highway may require planning permission on some property types — check with the LPA. On listed buildings, consent is required for any alteration to the chimney.

Regulations & Standards

• BS EN 12588 (Rolled lead sheet for building purposes) — specification for lead sheet thickness and properties

• BS 8000-6 (Workmanship on building sites — Part 6: Slating and tiling) — covers flashings installation

• Building Regulations Approved Document J (Combustion appliances and fuel storage) — chimney heights and flue requirements

• BS EN 15287 (Chimneys: Design, installation and commissioning) — structural and performance requirements

• Lead Sheet Association (LSA) Installation Guide — definitive industry reference for lead flashings

• Lead Sheet Association — Technical Guidance — Comprehensive lead flashing installation guidance

• The Historic England Practical Building Conservation Series — Lime mortar guidance for historic masonry

• NHBC Standards Chapter 7.2 — Chimney construction requirements for new build

• flat roofing — Flat roof to chimney junction details

• working at height — Scaffold and access requirements for chimney work

• listed buildings — Consent requirements for listed building chimney repairs

• building control — When building control sign-off is needed