Lead Flashing: What It Is, Why It Fails, and How It's Repaired

If you have ever noticed damp patches near a chimney breast, around a skylight, or where a flat roof extension meets the main house wall, there is a good chance the culprit is failed flashing. Flashing is one of the most critical waterproofing details on any roof — and one of the most common sources of leaks.

Here is everything you need to know.

What Is Flashing?

Flashing is the waterproofing material used to seal the joints and transitions on a roof — the places where two different surfaces meet and where water would otherwise find a way in.

The most common locations for flashing are:

• Around chimney stacks — where the chimney penetrates the roof surface
• Around skylights and roof windows — where the frame meets the roof covering
• At abutments — where the roof meets a vertical wall (common on extensions and lean-to roofs)
• In valleys — the internal angles where two roof slopes meet
• Around soil pipes and other roof penetrations

What Is Flashing Made From?

Lead is the traditional and most durable flashing material. It is highly malleable (it can be shaped to fit complex profiles), extremely long-lasting (a well-installed lead flashing can last 50–100 years), and self-sealing to a degree (it expands and contracts with temperature changes without cracking). Lead is the preferred material for most quality roofing work.

Aluminium is a more affordable alternative, lighter than lead and resistant to corrosion. It is less malleable than lead and has a shorter lifespan (typically 20–30 years), but is a perfectly acceptable choice for many applications.

Mortar flashing — sometimes called "cement flashing" — is a traditional method where mortar is used to seal the joint between the chimney and the roof. It is less durable than lead and prone to cracking as the building moves and settles. Many older properties have mortar flashing that has cracked and needs replacing with lead.

Proprietary self-adhesive flashings — products like Flashband are sometimes used for repairs. They can be effective as a temporary measure but are not a substitute for properly installed lead flashing.

Why Does Flashing Fail?

Thermal movement — Lead expands and contracts significantly with temperature changes. Over many years, this movement can cause the flashing to pull away from the wall or roof surface, opening gaps.

Mortar failure — Lead flashing is typically tucked into a mortar joint in the brickwork (called a "chase") and held in place with mortar. When this mortar cracks or crumbles, the flashing lifts away from the wall.

Corrosion — While lead is highly resistant to corrosion, it can be affected by certain types of mortar and by contact with some metals. Aluminium flashing is more susceptible to corrosion in coastal environments.

Poor installation — Flashing that was not installed correctly — insufficient overlap, inadequate fixing into the chase, or the wrong material for the application — will fail prematurely.

Age — Even well-installed flashing has a finite lifespan. Mortar flashing in particular deteriorates relatively quickly.

Signs Your Flashing Has Failed

• Damp patches on the wall or ceiling near a chimney, skylight, or abutment
• Visible gaps between the flashing and the wall or roof surface
• Flashing that has visibly lifted, buckled, or pulled away
• Rust staining below aluminium flashing (a sign of corrosion)
• Cracked or crumbling mortar at the base of a chimney stack

How Flashing Is Repaired

The repair approach depends on the extent of the failure:

Minor mortar failure — If the flashing itself is in good condition but the mortar holding it in place has crumbled, the mortar can be raked out and replaced. The flashing is re-dressed (reshaped to fit tightly) and re-pointed.

Flashing replacement — If the flashing itself has cracked, corroded, or is beyond repair, it needs to be replaced. The old flashing is removed, the chase is cleaned out, new lead is cut and shaped to fit, and the top edge is tucked into the chase and secured with mortar.

Mortar flashing replacement — Old mortar flashing is removed and replaced with properly installed lead flashing. This is a permanent solution that will outlast mortar by decades.

Can You Repair Flashing Yourself?

We would strongly advise against DIY flashing repairs for two reasons:

1. Working at height is dangerous without proper training and equipment.
2. Flashing requires skill — a poorly fitted repair can make the problem worse by directing water into the building rather than away from it.

Get a Free Quote

Our team carries out flashing repairs and replacements across Peterborough, Cambridgeshire, Lincolnshire, and Norfolk. Call us on 07878 827 758 or use our contact form for a free, no-obligation assessment.