Written by Andrew, Lead Roofer
CORC Certified
Public Liability Insured
Emergency Roof Repairs 24/7
CORC Certified
Public Liability Insured
Emergency Roof Repairs 24/7
Why chimney flashing fails, how much repairs cost, and which materials last longest. Expert guide for London homeowners from a CORC certified roofer.
If you’ve noticed damp patches on your ceiling near the chimney, there’s a good chance the flashing is to blame. According to the Federation of Master Builders, most chimney leaks can be traced back to just two problems: worn mortar joints or flashing that has started to fail.
Chimney flashing is the weatherproof seal between your chimney stack and the roof tiles. When it works, you never think about it. When it fails, water gets in and the damage starts.
This guide explains what chimney flashing is, why it fails, how much repairs cost, and what materials actually last. If you own a Victorian or Edwardian property in London, this is especially relevant. These older homes were built with chimneys as standard, and after 100+ years, the original flashing has often had enough.
Chimney flashing is a system of metal components that create a watertight barrier where your chimney meets the roof. It typically consists of several parts working together:
Step flashing runs along the sides of the chimney in a stair-step pattern, with each piece overlapping the one below. This directs water away from the junction and onto the roof tiles.
Counter flashing (also called cover flashing) sits above the step flashing and is tucked into the mortar joints of the chimney. It covers the top edge of the step flashing to prevent water getting behind it.
Back gutter (or chimney cricket) sits at the back of the chimney on the uphill side. This is where debris and water naturally collect, so the back gutter channels it around the chimney and back onto the roof.
Apron flashing covers the front of the chimney where it meets the lower roof slope.
When all these components are properly installed and sealed, water flows around the chimney rather than into your home. The problem is that flashing doesn’t last forever, and poor installation can cause it to fail much sooner than it should.
There are several reasons chimney flashing deteriorates over time. Understanding these helps you spot problems early and choose the right repair approach.
Flashing is constantly exposed to rain, frost, wind, and UV radiation. Over decades, this takes its toll. Metal can corrode, sealants dry out and crack, and mortar joints crumble. The freeze-thaw cycle during winter is particularly damaging. Water gets into small cracks, freezes, expands, and widens the gap. Repeat this hundreds of times and you have a leak waiting to happen.
Lead and other metals expand and contract with temperature changes. On a sunny day, your roof can reach 60°C or more. At night it drops significantly. This constant movement puts stress on the flashing, particularly at fixed points. If the flashing hasn’t been installed with enough allowance for thermal movement, it will eventually crack or pull away from the chimney.
This is more common than you might expect. The Federation of Master Builders notes that chimney flashing is one of the toughest parts of a roofing job, requiring specialist skill. Not all roofers have the experience to do it properly. Some will even skip replacing the flashing during a re-roof, relying on the old material to keep the new roof dry. This rarely ends well.
Common installation errors include:
Buildings move over time. Settlement, subsidence, and even normal seasonal movement can cause the chimney to shift slightly relative to the roof. When this happens, gaps can open up between the flashing and the chimney or roof surface. Older properties in areas like Hampstead and Chelsea often have this issue, particularly where clay soils cause seasonal ground movement.
The mortar that holds counter flashing in place doesn’t last forever. Once it starts to crumble, the flashing can work loose and allow water in. Chimney stacks also suffer from mortar deterioration in the joints between bricks, which compounds the problem. If the chimney itself is porous, water can bypass even good flashing by soaking through the masonry.
Catching flashing problems early can save you significant money on repairs. Here’s what to look for:
Inside your home:
On your roof:
On your chimney:
If you can’t easily see your roof from the ground, a drone roof survey can identify flashing problems without anyone having to climb up there.
When it comes to chimney flashing, you have options. But not all materials are equal. Here’s how they compare:
| Material | Typical Lifespan | Pros | Cons |
|---|---|---|---|
| Rolled lead sheet | 80-100+ years | Extremely durable, malleable, traditional appearance, 100% recyclable | Higher upfront cost, requires skilled installation |
| Aluminium | 20-40 years | Lightweight, cheaper initial cost | Surface degradation, fatigue cracking over time |
| Zinc | 20-40 years | Moderate durability, lower cost than lead | Corrodes in polluted or marine environments |
| Flashband/butyl tape | 5-15 years | Very cheap, easy DIY application | Short lifespan, adhesive degrades under UV, looks poor |
| Lead-free alternatives | Varies (check manufacturer) | No scrap value (deters theft), lighter weight | Less proven track record, varies by product |
The Lead Sheet Training Academy (LSTA) technical manuals repeatedly reference lead’s proven multi-generation service life. According to industry data, rolled lead sheet installed correctly and maintained periodically routinely achieves 80 to 100 years of performance. Some lead roofs have lasted over 400 years. Welsh slate removed from St Asaph Cathedral after 400 years in use was found to still be serviceable.
This is why lead remains the material of choice for quality leadwork, particularly on period properties where longevity matters.
The British Standard BS 6915:2001+A1:2014 is the code of practice for lead sheet installation. It specifies Code 4 lead (1.80mm thick) for chimney flashings, with Code 5 recommended for back gutters and more exposed positions. The standard also sets maximum lengths (1.5m per piece with 100mm laps) and requires a minimum 25mm chase depth into mortar joints.
When you factor in the cost of scaffolding, labour, and the disruption of having work done multiple times over your ownership of a property, lead is almost always the most economical choice over the long term.
Chimney flashing repair costs vary depending on the extent of the work needed, the materials used, and access requirements. Here are typical price ranges in London:
Minor repairs (repointing mortar, resealing small sections): £150-£400
Partial replacement (replacing one or two sections of flashing): £400-£800
Full replacement (all flashing around chimney): £800-£1,500+
Full replacement with back gutter rebuild: £1,200-£2,000+
These prices assume straightforward access. If your property needs scaffolding, add £500-£1,500 depending on the height and complexity. Some chimney maintenance work can be done from a roof ladder, but full flashing replacement usually requires proper access equipment.
The main variables affecting cost are:
Getting a proper assessment before committing to work is important. What looks like a simple flashing repair can sometimes reveal underlying problems with the chimney stack itself.
Correct installation is the difference between flashing that lasts a century and flashing that fails in a decade. The LSTA Guidelines, aligned with BS 6915 and BS EN 12588, provide the framework for professional leadwork. Key requirements include:
Correct lead code: Using the right thickness for each application. Code 4 (1.80mm) for step flashings, Code 5 (2.24mm) for back gutters and exposed positions.
Appropriate lengths: Maximum 1.5m per piece to allow for thermal expansion. Longer pieces will buckle or crack.
Proper fixing: Lead wedges or clips to secure flashing into chases, never over-fixed which would prevent natural movement.
Adequate laps: Minimum 100mm overlap between pieces to prevent water tracking back.
Chase depth: At least 25mm into mortar joints, properly wedged and pointed with compatible sealant or mortar.
Ventilation: Where lead covers a flat area, proper through-ventilation beneath to prevent moisture build-up.
Patination oil: Applied after installation to prevent unsightly white carbonate staining and run-off marks.
Ecobat, one of the UK’s leading rolled lead sheet manufacturers, offers a 50-year warranty on their products when installed in accordance with LSTA guidelines. This gives you an idea of the confidence manufacturers have in properly installed lead.
Not every flashing problem requires full replacement. Here’s a rough guide:
Repair is usually sufficient when:
Replacement is usually needed when:
Full chimney work may be needed when:
A good roofer will give you an honest assessment. Sometimes repointing the mortar and resealing the flashings is all that’s needed. Other times, you’re better off replacing everything properly rather than throwing money at repeated repairs.
Around 21% of UK homes were built before 1919, according to the Centre for Ageing Better. In London, particularly in areas like Hampstead, Chelsea, Battersea, and Islington, the proportion is much higher. These Victorian and Edwardian properties were built with coal fires as the primary heating source, which meant every room needed a fireplace and every fireplace needed a chimney.
The result is a housing stock with an enormous number of chimneys, many now over 100 years old. The majority of these Victorian houses were roofed with Welsh slate, which can last 150 years or more. If you’re wondering about the condition of your slate as well as your flashing, see our guide on how long slate roofs last. The original lead flashing on these properties, if it was lead, may well have been installed when the house was built. Even the best lead has a finite lifespan, and many Victorian flashings are now approaching or past the end of their service life.
The English Housing Survey 2023-24 found that 29% of pre-1919 dwellings fail to meet the Decent Homes Standard. Roof and chimney issues are a significant contributor to this. If you own a period property in London, having your roof and chimneys surveyed is a sensible investment.
Lead flashing, properly installed to BS 6915 standards, typically lasts 80 to 100 years. Some lead installations have lasted over 400 years. Alternative materials like aluminium and zinc last 20 to 40 years. Self-adhesive flashband products rarely exceed 5 to 15 years before needing replacement.
The most common sign is damp patches or water stains on ceilings and walls near the chimney. You might also notice peeling paint, a musty smell, or visible water during heavy rain. From outside, look for lifted or corroded flashing, missing mortar around the counter flashing, or gaps between the flashing and chimney.
Minor repairs like applying sealant to small gaps can be done DIY, but proper flashing replacement requires specialist skills and safe roof access. Lead flashing in particular needs to be cut, dressed, and installed to specific standards. Poor DIY repairs often make the problem worse or create new issues. For anything beyond basic maintenance, professional installation is recommended.
For longevity and performance, yes. Lead remains the benchmark material for chimney flashing, especially on period properties. It’s specified in British Standards, preferred by conservation officers for listed buildings, and backed by manufacturer warranties of up to 50 years. The higher upfront cost is offset by its significantly longer lifespan compared to alternatives.
Full chimney flashing replacement in London typically costs £800 to £1,500 for a standard chimney, potentially more for complex stacks or where scaffolding is required. Minor repairs start from around £150. The main cost variables are chimney size, material choice, and access requirements.
Generally yes. If your roof is being replaced, it makes sense to replace the flashing at the same time while access is already arranged. Reusing old flashing on a new roof is a false economy. The flashing is often the first part to fail, and replacing it later means paying for access twice.
Chimney flashing might not be the most glamorous part of your home, but it’s one of the most important. A properly installed lead flashing system will protect your property for decades, while a poor one will cause problems within years.
If you’re concerned about your chimney flashing, or you’ve noticed signs of water ingress near your chimney, getting a professional assessment is the sensible first step. Look for a roofer with specific leadwork experience and credentials, who works to BS 6915 standards and LSTA guidelines.
For more information about chimney repairs and maintenance, see our chimney maintenance services or contact us for a roof survey.
Sources and Further Reading:
Welsh slate lasts 150-200 years, Spanish 75-150 years. Learn what affects slate roof lifespan, signs...
Why chimney flashing fails, how much repairs cost, and which materials last longest. Expert guide...