What lead flashing actually does on an older roof

Lead flashing is the quiet workhorse of any period roof. It is the sheet metal that seals the junctions where your roof covering meets a vertical surface — a chimney stack, a dormer cheek, a parapet wall, or the upstand of a valley. On older properties, lead has been the material of choice for centuries because it is soft, malleable, and can be dressed by hand to follow the contours of uneven stone or brickwork. It also expands and contracts with temperature changes, which matters on a roof that moves through the seasons.

When it is installed well, lead flashing can last for many decades. Code 4, 5, or 6 lead is still specified today for different exposures. But no flashing lasts forever. And because lead fails slowly, the first sign of trouble is often a damp patch on a ceiling or a stain on an inside wall — long after the damage has begun.

The quiet ways lead flashing fails

Unlike a slipped slate or a cracked tile, lead rarely gives you a dramatic warning. It fails quietly, often from the underside or along a fold. The most common problems are:

• Thermal fatigue: repeated expansion and contraction can cause hairline splits along creases, especially where lead has been folded too tightly or trapped by mortar.
• Corrosion: lead can react with mortar, especially cement-based mortar, or with other metals. You may see white, pink, or grey deposits on the surface.
• Lifting and springing: the lead can pull away from the wall or chimney, leaving a gap for wind-driven rain.
• Eroded mortar chase: the mortar that holds the lead into the wall can crumble, particularly after freeze-thaw cycles.
• Blocked or damaged valleys: lead-lined valleys can become blocked with debris, or the lead can split where it has been walked on.

None of these are instantly obvious from ground level. That is why a periodic inspection is so valuable.

Junctions that cause the most trouble

Some parts of a period roof are more vulnerable than others. Chimneys are the biggest culprit. The back gutter behind a chimney, the stepped flashing up its side, and the apron at the front all rely on lead. If the chimney has been repointed with cement, the lead may have been trapped and unable to move, leading to splits.

Valleys are another weak point. A lead-lined valley takes water from two roof slopes, so any split or blockage can send water straight into the roof space. Dormers are tricky too. The cheeks, the front apron, and the small soakers where the dormer roof meets the main slope all need careful lead work. Parapet walls and soil vent pipes are also common leak points.

Why older properties need a different approach

You cannot treat a 150-year-old roof like a new build. Older roofs move more, and they were often built with lime mortar, which is soft and breathable. If you point around lead flashing with hard cement, it can crack and trap water against the lead, causing corrosion. The lead itself may be thicker than modern equivalents, and it may have been repaired over the years with soldered patches or lead burning.

A specialist who understands period properties will know how to dress lead without over-working it, and how to use lime mortar where appropriate. They will also check that any replacement lead is the right code and thickness for the job. Replacing a small section of lead is often better than trying to patch it with bitumen or a sealant, which rarely lasts.

Practical maintenance checks you can do

You do not need to climb on the roof to keep an eye on things. From the ground, use binoculars to look at the chimney, dormers, and valleys. After a storm, check for any lifted or missing lead. Inside the loft, look for damp patches, stains, or daylight around junctions. After heavy rain, check for drips.

Keep gutters and valleys clear of leaves and moss. Do not walk on lead flashing — it is soft and easily damaged. Do not let anyone paint over lead, as this can trap moisture. If you see white powder on the lead, that is lead carbonate, which is a natural patina, but it can also signal that the lead is being attacked. A light touch is best: if you are not sure, call someone who is.

When to call in a specialist

If you spot a split, a lifted section, or a damp patch that keeps returning, it is time to call a roofer with experience of lead work and period properties. Lead flashing is not a DIY material. It needs to be dressed, folded, and secured correctly, and the surrounding mortar needs to be right. A good specialist will tell you whether a repair is possible or whether a section needs replacing.

Regular inspections every few years — and after any severe weather — will help you catch small problems before they become expensive. Lead flashing may be out of sight, but it is one of the most important defences your old roof has. Give it the attention it deserves, and it will keep the rain out for decades to come.