The short answer
Lead flashing is fixed into a wall by cutting a slot, called a chase, along a mortar joint. The turned-up edge of the lead goes into the slot and is held with lead wedges, then the slot is pointed with mortar or a suitable sealant. The chase has to be deep and straight enough to grip the lead, or water gets behind it.
The visible lead is only half the flashing. The hidden half is the slot in the wall, and its depth and finish decide whether the joint stays dry.
What a chase is
Where a roof meets a masonry wall, rain can run down the wall and onto the roof, or blow in sideways at the join. A flashing seals the line. The lower part lies over the roof covering. The upper part, called the upstand, runs up the wall and has to be fixed there.
Why cut into the wall at all?
Because glue and surface fixing do not last. A strip of lead stuck onto brick pulls away as the metal moves and the surface weathers. Instead, the roofer cuts a slot into the wall and tucks the lead into it, where it is held mechanically and protected from rain.
The slot follows a mortar joint, since mortar is softer than brick or stone and easier to cut without harm to the masonry. On stepped sections, such as a gable wall or a chimney side, the chase steps up with the courses, and each step has its own piece of lead.
A chase cut straight into a brick or stone face, rather than along a joint, damages the masonry, which is another reason skill matters. Repairing a damaged brick costs far more than cutting the slot in the right place at the start.
Cutting and wedging
The roofer marks the line, then cuts the chase with a disc cutter or a hand tool. It should run along the joint to a consistent depth, and the bottom of the slot should be clear of dust and loose mortar so the lead sits properly.
Why does the depth matter?
A shallow chase leaves the lead with almost nothing to grip. It lifts in wind, the pointing falls out, and water runs into the gap. A chase deep enough to hold the lead firmly keeps the upstand tight against the wall. We check depth at survey on any existing flashing, because a shallow chase is among the most common faults we find.
Once the lead is in the slot, it is wedged. Wedges are small folded pieces of lead, driven in at intervals along the upstand, which hold the metal against the back of the chase. Wedges are lead because other metals can react with it and corrode.
Our answer on fitting lead flashing yourself explains why cutting and wedging at height is hard to do well without practice, and why a wavy chase shows up as a leak.
Pointing the joint
Pointing is the filling of the slot after the lead is wedged in. The roofer packs the gap with mortar, or with a sealant chosen for the job, and finishes it flush with the wall. It needs to be tight and slightly recessed from rain, not smeared over the surface.
Is mortar or sealant better?
It depends on the wall and the building. On a traditional house, mortar matching the wall suits the look and the breathability of the masonry. A flexible sealant allows for movement in some situations. Sealant is not a substitute for a proper chase, and a tube of it smeared over a flashing with no chase is a patch, not a fix. Our answer on sealant on lead flashing covers the difference.
Mortar needs to cure, so pointing is a dry-weather job, as our answer on fitting leadwork in the rain describes. On a listed home, the mortar mix is part of keeping the building true to its age, and Somerset Council's planning team can say whether consent applies.
Pointing that has cracked or fallen out is the first thing to check on an older flashing.
Signs a chase has failed
You can often spot trouble from the ground. Look for pointing that has dropped out in a line along the wall, lead lifting clear of the brick, or a dark stain running down the masonry below the flashing. Inside, a damp patch near the top of a wall beside a roof can follow the same path.
Can the chase be re-pointed on its own?
If the lead is sound and the chase is deep enough, yes. The old pointing is raked out, the wedges are checked, and the joint is refilled. If the chase is too shallow, the lead has crept out or the flashing is thin, re-pointing only delays the repair. Our guide to the signs of failed flashing sets out what to look for.
The survey looks at the wall and the lead together, with pictures of the real flashing for you to see. One written price follows. Lead can last for over 80 years when it is chased in properly, so a sound chase is worth doing right.
Our lead work page covers the other flashings we handle. A photo of the wall and the lead is the best place to begin.
More on fixing lead to walls
How deep should the chase be?
Deep enough to hold the lead firmly, with room for wedges and pointing. The exact depth depends on the wall and the lead. We check it at survey and do not rely on a single figure, because masonry and mortar joints vary widely from house to house.
Can flashing be fixed to a rendered wall?
Yes, but the render complicates it. The chase may need to be cut through the render into the joint behind, and the render made good around the lead. We look at the wall first and tell you what the repair will involve before pricing.
The full picture: Lead work in Taunton ยท More short answers