If you’ve got a leak and a chimney, I’m going to save you some time.
It’s probably the flashing.
I’d estimate 40-50% of all leak callouts I attend trace back to the chimney flashing. Not the tiles. Not the felt. Not mysterious “roof failure.” The flashing.
Yet most homeowners have never heard of it, don’t know what it does, and can’t spot a problem from ground level.
Let me fix that.
What Is Chimney Flashing?
Your chimney is a brick box sticking through your roof. The roof is supposed to be waterproof. So where the roof meets the chimney, there’s a vulnerable joint—a gap between two different materials that needs to be sealed.
That seal is your flashing.
Traditional flashing consists of:
- Step flashing: Individual pieces of lead that “step” up alongside the chimney as the roof slopes
- Back gutter: A lead channel at the high side of the chimney that directs water around it
- Apron flashing: A piece that runs across the front (low side) of the chimney
- Counter flashing: Lead tucked into the mortar joints of the chimney itself, overlapping the other flashing
When it’s done right, water hits your roof, runs down, hits the flashing, and is directed away from the chimney joint, back onto the roof surface, into your gutters.
When it fails, water runs down the chimney stack and into your house.
Why Chimney Flashing Fails
Lead Fatigue
Lead flashing should last 80-100 years. But that assumes proper installation.
Lead expands and contracts with temperature. If it’s installed too tight—dressed too firmly against the brickwork with no room to move—it will eventually crack from fatigue.
I see this constantly. The roofer fitted it beautifully, pinned down every edge, hammered it snug. And 20 years later, it’s cracked along every stress line.
Good lead installation has slight slack, allowing thermal movement without cracking.
Mortar Failure
The counter flashing is tucked into a mortar joint (a “raggle”). The mortar should hold it firmly for decades.
But mortar deteriorates. British weather—wet, then freezing, then wet again—destroys mortar faster than most climates. Once the mortar crumbles, the flashing lifts, and water gets behind it.
This is especially common on chimneys where the mortar was already old or where cement repointing was done badly.
Bodge Repairs
Here’s what bad roofers do when flashing fails: they cover it with sealant.
Grey mastic. Silicone. Roofing “repair” tape. Whatever’s in the van.
This stops the leak temporarily. It also hides the underlying failure while damage continues underneath.
I’ve peeled back mastic “repairs” to find rotted timbers, saturated brickwork, and damage that would have been a £500 fix had it been done properly years ago.
If someone has previously “sealed” your chimney with sealant, you almost certainly have a hidden problem.
Incorrect Installation
Lead flashing is a skilled trade. Not every roofer can do it properly.
Common installation errors:
- Flashing too short (doesn’t extend far enough under tiles)
- Steps too small (water bypasses them)
- No back gutter (water pools against chimney)
- Counter flashing set too shallow (pulls out over time)
- Wrong grade lead (too thin, deteriorates faster)
Bad installation can fail within 10-15 years, compared to 80+ for proper work.
How to Spot Flashing Problems
From Inside Your House
The classic signs of chimney flashing failure:
- Damp patches on the ceiling near (not always directly under) the chimney
- Damp patches on walls in the room where the chimney is
- Water stains in the attic on or near the chimney stack
- Musty smell in upstairs rooms, worse after rain
The leak won’t necessarily appear directly below the chimney. Water tracking down timbers can emerge metres away from where it entered.
From Ground Level
You probably can’t see the flashing itself from the ground, but you can spot warning signs:
- Visible grey sealant around the chimney base (suggests previous bodge repair)
- Chimney mortar visibly crumbling (same mortar holds the flashing)
- Lead that’s lifted away from the brickwork (visible gap)
- Chimney leaning (movement will have cracked all flashing joints)
From On the Roof
This is what I look for during inspections:
- Cracks in the lead (visible splits, especially at corners)
- Lifted counter flashing (no longer bedded in mortar)
- Sealant covering joints (previous repair, probable ongoing leak)
- Lead that’s thin or pitted (end of life)
- Poor overlap with tiles (water getting under the lead)
- Missing or inadequate back gutter
If your roof is accessible and you’re comfortable with heights (and safety equipment), you can check some of this yourself. But honestly, a professional inspection is worth it—we know what to look for.
What Proper Flashing Repair Costs
Let me break this down with real numbers from 2026 quotes:
Minor Repairs
If the lead is fundamentally sound but has developed a small crack or lifted section:
- Re-seat counter flashing in mortar: £150-300
- Repair single crack with lead welding: £150-250
- Replace mortar pointing around flashing: £200-400
Partial Replacement
If one section has failed but the rest is fine:
- Replace apron flashing only: £300-500
- Replace step flashing on one side: £400-700
- Replace back gutter: £400-700
Full Replacement
If the flashing is end-of-life or poorly installed:
- Full chimney re-flash (standard chimney): £800-1,500
- Full re-flash (large or complex chimney): £1,200-2,000
- Re-flash plus chimney repointing: £1,500-3,000
Variables That Affect Cost
Access: A simple two-storey with scaffold from the garden is cheaper than a four-storey terraced house with traffic management on a busy street.
Chimney size: A small soil pipe stack takes an hour. A massive Victorian chimney breast with multiple pots takes a day.
Roof type: Working around slate requires more care than concrete tiles.
What’s underneath: If we find rot when we lift the old flashing, the scope (and price) increases.
The Bodge vs. The Proper Job
I’m going to be direct: there’s a lot of bad chimney work out there.
The £150 “Repair”
Roofer climbs up, spends 30 minutes slathering sealant around the chimney, climbs down, hands you a bill.
This will stop the leak. For a while. Maybe a year. Maybe two if you’re lucky.
Meanwhile, the underlying problem continues. The failed lead. The crumbling mortar. Whatever caused the leak is still there, still letting water in, just now covered with a layer of grey goo.
When the sealant eventually fails (and it will—sealant is not a permanent solution), the damage underneath has had years to worsen.
The £1,200 Proper Job
Roofer removes old lead and sealant bodges. Assesses the mortar and repoints if necessary. Cuts and installs new lead flashing—proper code 4 lead, properly dressed, properly bedded. Tests for water tightness.
This will last 50-80 years if done correctly.
The sealant repair seems cheaper. But you’ll need it redone every few years, plus eventually the proper job anyway, plus the damage that accumulated while you were sealing over the problem.
Ask for proper lead work. Accept nothing less.
Can You DIY Chimney Flashing?
I’m not going to tell you it’s impossible. Some competent DIYers have done it.
But I’ll tell you this:
- Lead flashing is skilled work—there’s a reason it’s a separate trade specialty
- Mistakes cause leaks that can cost thousands in damage
- Working at height on a roof requires proper safety equipment
- The cost of materials (code 4 lead, proper mortar) isn’t far off the labour cost
If you’ve got experience with lead work, good access, and proper equipment, go for it.
If you’re thinking “how hard can it be?”—that’s the thought that keeps me busy fixing DIY disasters.
How Often Should Flashing Be Inspected?
Annual visual check: You or your roofer should eyeball the chimney once a year. Look for obvious lifting, cracking, or sealant suggesting previous issues.
After storms: High winds can lift lead that’s already weakened. Check after any significant weather event.
Every 5 years: Detailed inspection, ideally by someone on the roof, not just binoculars from the garden.
At purchase: If you’re buying a house, make sure the surveyor specifically examines the chimney flashing—many don’t, or only glance at it from ground level.
Prevention Is Everything
The reality is this: chimney flashing, properly installed and maintained, should outlast you.
But “properly installed” is doing a lot of work in that sentence. Many British homes have flashing that was:
- Done cheaply during original construction
- Bodged during previous repairs
- Damaged by building movement
- Deteriorated from decades of weather
If you’ve got an older house, assume the flashing needs checking.
If you’ve got a damp patch anywhere near the chimney, assume it’s flashing until proven otherwise.
And if someone offers to “seal it” for a couple of hundred quid—ask them to price the proper repair instead. Future you will thank present you.
Summary: What You Need to Know
- Chimney flashing is the #1 leak cause in British homes with chimneys
- You can’t see problems from the ground—get it inspected properly
- Sealant repairs are bodges that hide ongoing damage
- Proper lead replacement costs £800-1,500 for typical chimneys
- The cost of ignoring it runs into thousands when rot sets in
Don’t wait for the ceiling stain to spread. By then, you’re paying for timber repairs too.
Suspect your chimney flashing is leaking?
We specialise in traditional lead flashing work across London.
Book Free Chimney Assessment →
Or call: +44 89 981 9675
Seamus O’Brien has been fixing British chimney leaks for over 20 years. The flashing is almost always the culprit.
Tags:
Need Professional Roofing Services?
Our expert team is ready to help with all your roofing needs across UK.
Get Your Free Quote