If your roof has a chimney and is over 20 years old, the chimney flashing is the most likely source of any leak you develop. More leaks start at the chimney-roof junction than anywhere else on a typical Sussex property. The reason: flashings have to seal a complex junction between three materials (the chimney brickwork, the roof covering, and the lead flashing itself) under constant weather exposure, and they age out before the roof covering does.
This guide covers what chimney flashing actually is, why it fails, how to spot flashing leaks before they cause significant internal damage, and what a proper repair involves.
What Chimney Flashing Does
The chimney passes through the roof covering at right angles to the slope. This creates four junctions – front, back, and two sides – where water running down the roof has to either go around the chimney, under it (via soakers), or over it (via flashings).
A complete chimney flashing set includes:
Front Apron
A continuous lead sheet dressed across the front (down-slope) face of the chimney. Water arriving from above the chimney is diverted around the chimney by this apron before running down the roof below.
Back Flashing (Back Gutter)
A lead section behind the chimney (up-slope side) that catches water running down the roof above the chimney and directs it to either side. Without this, water would pile up against the back of the chimney and track down behind the brickwork.
Side Flashings (Step-and-Cover)
Stepped lead sections running up each side of the chimney, matching the gradient of the roof. Each step covers the upper edge of a tile course, ensuring water runs over the lead and onto the tiles rather than behind the chimney brickwork.
Soakers
Small L-shaped lead pieces fitted under each tile course on the chimney sides. The vertical part of the L runs up the chimney brickwork; the horizontal part sits under the tile. The side flashings sit over the vertical parts.
All four elements work together. Failure of any one creates a leak path.
Why Chimney Flashings Leak
Poor Original Installation
The most common cause of persistent chimney leaks is installation that wasn’t right to start with:
- Insufficient chase depth – lead pushed into a mortar joint rather than cut into the brickwork properly
- No wedging – lead relying on mortar alone to stay in the chase
- Too-long continuous sections – lead over 1.5m unjointed cracks from thermal movement
- Inadequate upstand – lead not extending high enough up the brickwork
- Wrong lead code – code 3 where code 4 or 5 was needed
Bad installations fail within 5-10 years. Good installations can last 80+ years.
Mortar Failure
Where the flashing enters the chimney brickwork, mortar points the gap. Over time, freeze-thaw cracks the mortar, rainwater erosion widens the cracks, mortar eventually fails and falls out, and water penetrates directly into the chase behind the lead.
Re-pointing every 15-25 years keeps this at bay. Neglected pointing is the secondary cause of most flashing leaks.
Lead Fatigue
Lead expands and contracts with temperature – a 2m lead section varies by 4-6mm between summer and winter. Over decades, this thermal cycling fatigues the lead at fixing points and corners. Cracks eventually appear at corners where the apron meets the side flashing, at welted joints if they weren’t properly formed, or at points where lead is fixed rigidly and can’t accommodate movement.
Storm Damage
Severe storm winds can lift flashing edges that have become marginally loose. Once lifted even slightly, the wind gets under the flashing and progressively tears it free. Sussex’s named Atlantic storms cause flashing damage every winter.
Chimney Movement
The chimney stack is a tall narrow structure that moves slightly with wind, thermal expansion, and settlement. When the movement exceeds the flashing’s tolerance, the chase opens and the pointing fails. Chimney stack movement beyond normal tolerance (leaning, visible cracks in the brickwork) is a structural issue requiring attention beyond just re-flashing.
Signs of Chimney Flashing Leaks
Internal Signs
- Water staining on chimney breast walls – often appearing on the floor directly below the chimney stack
- Damp patches on ceilings near the chimney position
- Peeling wallpaper or paint around the chimney breast
- Musty smell in rooms with an integral chimney breast
- Efflorescence (white salt deposits) on internal chimney brickwork
- Mould spots developing on chimney breast plaster
External Signs
- Lifted lead edges – visible from roof level or even from the ground with binoculars
- Green staining running down chimney brickwork from failing pointing
- Visible cracks in the mortar around the flashing
- Missing chimney pots or displaced flashing sections after a storm
- Chimney stack leaning – a separate structural issue but often accompanies flashing failure
Loft Signs
- Dampness on rafters near the chimney stack
- Water staining on the underlay around the chimney
- Wet insulation in the area of the chimney breast
- Mould on timbers adjacent to the chimney
Detecting Where a Flashing Leak Starts
Flashing leaks are often tricky to trace because water travels – it enters at one point and shows up at another. Professional investigation involves:
External Inspection
Closely checking each flashing element – front apron, back gutter, side flashings, soakers – for lifted edges, visible cracks, split corners, failed pointing, missing lead wedges.
Roof Space Inspection
From inside the loft, looking at where the chimney comes through the roof, the underlay around the chimney base, any visible daylight (indicates displaced flashing), water tracks on timbers showing direction of travel.
Water Test (Only When Necessary)
On complex or persistent leaks, a controlled water test can identify the entry point. Water is applied in sections – first the front apron, then each side, then the back – with an observer watching for internal water appearance. This isolates the specific failure zone.
Proper Repair Process
Localised Repair
If the flashing has failed at a specific point (one corner, one section of pointing), localised repair works: scaffold erected for safe access, affected area inspected, failed mortar raked out, loose lead re-dressed and wedged, new pointing applied.
Typical duration: 1-2 days. Extends the life of the flashing by another 10-20 years if the rest is still sound.
Full Flashing Replacement
If most of the flashing has reached end-of-life or if multiple elements are failing: scaffold erected, existing flashing stripped, chimney brickwork assessed and repointed where needed, new lead cut/dressed/fitted (front apron, back gutter, side flashings, soakers), all chases properly cut with lead wedged and pointed, final inspection and clean-up.
Typical duration: 3-5 days. Result: 50-80 year flashing life.
Integrated Repair
Often flashing repair reveals other chimney issues – failing pointing on the stack itself, cracked pot flaunching, loose pots. Fixing these at the same time while scaffold is up is far cheaper than returning later. A proper chimney survey before pricing the flashing work identifies what else needs doing.
What Makes Good vs Bad Flashing Work
Good Flashing Work
- Code 4 or 5 lead minimum (code 5 on larger or exposed chimneys)
- Proper chase depth – 25mm minimum into the brickwork
- Lead wedges at 300-500mm intervals holding the lead in the chase
- Appropriate upstand – 150mm minimum on side flashings
- Welted joints where lead sections join
- Lime-based pointing on older properties (heritage-compatible)
- Section lengths under 1.5m unjointed to allow thermal movement
- Clean dressed appearance – lead shaped to fit the brickwork profile
Bad Flashing Work
- Lead pushed into a mortar joint without chasing
- Cement-based pointing alone holding the lead in place
- Continuous long lead sections that can’t move thermally
- Missing wedges
- Inadequate upstands
- Code 3 lead used where code 4 or 5 was required
- Flashing tape or liquid sealants used instead of proper lead
Liquid sealants on top of failing flashings are the classic bodge. They stop the leak for 6-18 months, then fail worse than the original problem.
DIY vs Professional
Some flashing issues look DIY-fixable but usually aren’t: working at height requires scaffold or proper fall protection; lead dressing requires specialist skills developed over years; BS 5534 and lead installation standards require compliance; insurance and warranty terms often require certified workmanship.
Budget-conscious homeowners sometimes attempt to seal lifted flashings with sealant or tape. This works briefly but compounds the problem – the next proper repair has to remove the failed sealant as well as fix the underlying issue.
Preventing Flashing Failures
- Annual inspection – ground-level check with binoculars once a year, particularly after autumn storms
- Repointing cycle – lead flashings should be re-pointed every 15-25 years on exposed Sussex chimneys
- Don’t walk on flashings – aerial engineers, solar installers, chimney sweeps, roofers – anyone accessing the roof should avoid stepping on or near flashings
- Chimney stack maintenance – a healthy chimney stack (sound pointing, stable pot flaunching, no leaning) puts less stress on the flashing
Sussex-Specific Considerations
Heritage Chimneys
Mid Sussex has many listed and conservation-area properties with heritage chimneys. Flashing work on these requires traditional lead specifications (code 5 or heavier), lime-based pointing (not cement), heritage-compatible methods, and listed building consent where applicable.
See heritage chimneys repairs.
Coastal Positions
Coastal Sussex properties need code 5 or 6 lead (not code 4) and more generous upstands to handle wind-driven rain. Salt air accelerates pointing failure; inspection cycles should be shorter.
Storm Pattern
Every major named Atlantic storm over Sussex stresses chimney flashings. Post-storm inspection catches lifted edges before they escalate.
Insurance and Storm Damage
Storm damage to chimney flashings is usually covered by home insurance. Age-related failure is not. Professional inspection documenting damage cause is usually required for a claim: photographs of damaged flashing with context, roofer’s written assessment, storm event date and reference, estimate for repair.
Book an Inspection
If you suspect a chimney flashing leak, or if your chimney hasn’t been inspected in the past 5 years, book a free visit. We’ll assess the flashing, trace any leak back to source, and give you a clear recommendation on whether localised repair or full replacement is needed.