Common Lead Roof Failures (And How to Prevent Them)
Introduction
Common lead roof failures are typically caused by incorrect detailing, poor installation, or a lack of understanding of how lead behaves over time.
In heritage and commercial roofing, these failures often result in splitting, water ingress, and premature deterioration — often leading to costly remedial works or full replacement.
At Silk Leadwork, we regularly assess and rectify failed leadwork across London and the South East, identifying root causes and delivering long-term solutions in line with BS EN 12588.
The Most Common Lead Roof Failures
Thermal Movement & Splitting
Lead expands and contracts significantly with temperature changes. If this movement is not properly allowed for, the material will fatigue over time and eventually split.
Typical causes:
- Oversized bays
- Incorrect detailing
- Restricted movement at fixings
Result:
Cracking along rolls, joints, or fixing points, often leading to water ingress.
Solution:
Correct bay sizing, appropriate detailing, and installation methods that allow controlled thermal movement.
Poor Fixing Methods
Lead needs to be fixed securely, but in a way that still allows for movement.
Failures occur when:
- Fixings are too rigid
- Incorrect clips or materials are used
- Sheets are over-fixed
This creates stress within the material, leading to tearing around fixings, distortion, and eventual failure.
Proper fixing methods strike a balance between stability and flexibility.
Incorrect Lap Detailing
Incorrect lap detailing is one of the most common causes of leaks in lead roofing.
Typical issues include:
- Insufficient overlap
- Poorly formed or positioned laps
- Inadequate consideration of roof pitch
When laps are not formed correctly, water can track beneath the lead covering — often going unnoticed until internal damage becomes visible.
Correct lap detailing is essential to ensure the roof performs as intended over the long term.
Lack of Ventilation & Underside Corrosion
One of the most overlooked causes of lead roof failure is what happens beneath the sheet, not on top of it.
Lead is often assumed to fail due to external exposure, however in many cases deterioration begins on the underside, driven by trapped moisture and a lack of ventilation within the roof build-up.
What this looks like in practice
Failures of this type typically present as:
- Saturated or stained timber decking
- White or grey powdery corrosion on the underside of the lead
- Persistent damp conditions with no clear drainage path
This indicates a roof structure that is unable to dry out, allowing moisture to remain trapped beneath the lead over long periods.
Why it happens
Lack of ventilation
Without adequate airflow:
- Warm, moisture-laden air becomes trapped beneath the lead
- Condensation forms on the underside of the sheet
- Moisture has no escape route
Over time, this leads to timber decay, corrosion of fixings, and deterioration of the lead itself.
Incorrect underlay selection
The choice of underlay plays a critical role in how the roof performs.
Where inappropriate materials are used:
- Moisture can be retained against the underside of the lead
- Separation between lead and substrate is reduced
- Drying of the structure is restricted
Rather than allowing the lead to breathe, the build-up effectively traps moisture within the system.
Combined build-up failure
In most cases, the issue is not caused by a single factor, but by a combination of:
- Lack of ventilation
- Incorrect or poorly performing underlay
- No allowance for airflow or drying
This creates a sealed environment where moisture is continuously present.
Why this type of failure is often missed
Unlike visible defects such as splits or laps, underside corrosion develops gradually and is often hidden from view.
By the time it becomes apparent:
- The timber substrate is already compromised
- Corrosion is well established
- Localised repairs are no longer effective
In many cases, full strip and replacement is required to resolve the issue properly.
How to prevent underside corrosion
Preventing this type of failure comes down to ensuring the roof build-up allows for both separation and airflow.
This includes:
- Incorporating adequate ventilation into the design such as a nicholson airtrack system
- Using appropriate underlay materials that do not retain moisture
- Allowing the structure to dry out naturally
- Avoiding sealed build-ups beneath the lead
When these principles are followed, the underside of the lead remains stable and long-term performance is maintained.
Identifying & Resolving Lead Roof Failures
Lead roof failures are rarely isolated to a single issue. In most cases, visible defects such as splits, leaks, or corrosion are symptoms of underlying problems within the design or installation.
A proper assessment should focus on:
- Identifying the root cause of failure, not just the visible issue
- Understanding how the existing build-up is performing
- Determining whether repair or full replacement is the most effective solution
At Silk Leadwork, we approach each project with a focus on long-term performance — ensuring any remedial works address the underlying problem, rather than providing a temporary fix.
When Repair Is Not Enough
While some leadwork can be repaired, there are situations where repairs are no longer viable.
This typically includes:
- Widespread splitting caused by thermal movement
- Ongoing water ingress due to poor detailing
- Underside corrosion and substrate failure
- Incorrect original design or material specification
In these cases, targeted repairs often lead to repeated issues. A full replacement, carried out with correct detailing and build-up, is usually the most cost-effective long-term solution.
Speak to a Leadwork Specialist
If you are dealing with failed leadwork or require an informed opinion on an existing roof, we can review your project and provide clear, practical advice.
We regularly work with:
- Main contractors
- Surveyors
- Property managers
- Heritage consultants
Whether you have drawings, specifications, or an existing issue on site, we can assess the situation and advise on the most appropriate course of action.
further information on correct detailing for leadworks can be found at the lead sheet training academy or the lead contractors association.