Chimney Repairs: Repointing, Flaunching, Lead Flashings & Cowls

Quick Answer: Chimney repairs — including repointing, flaunching, and reflashing — are exempt from Building Regulations notification when carried out as maintenance on existing chimneys, but must use appropriate lime or hydraulic mortar (not OPC) to avoid cracking. Lead flashings must comply with BS EN 12588 and the Lead Sheet Association (LSA) guidelines; lead code 4 (1.8mm) is the minimum for soakers and stepped flashings.

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

Quick Reference Table

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Lead Code Thickness (mm) Weight (kg/m²) Typical Application
Code 3 1.32 14.97 Soakers, small flashings
Code 4 1.80 20.41 Step flashings, cover flashings
Code 5 2.24 25.40 Back gutter, parapet copings
Code 6 2.65 30.05 Flat roof, large gutters
Code 7 3.15 35.72 Heavy duty flat roof
Code 8 3.55 40.26 Specialist applications
Mortar Type Mix Ratio Suitable For
NHL 2 lime mortar 1:3 (lime:sand) Soft brick, pre-1850 buildings
NHL 3.5 lime mortar 1:2.5 (lime:sand) Victorian/Edwardian brickwork
1:1:6 cement:lime:sand As stated Post-1945 standard brickwork
1:6 OPC:sharp sand As stated Modern hard brick, engineering brick
OPC neat or strong mix Avoid Will crack the brick — do not use

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:

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):

Stepped cover flashing (over the soakers):

Back gutter (behind a chimney where water collects):

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

Chimney Cowls

Cowls are fitted to:

Types:

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