The short answer
Lead flashing comes away from a wall when what holds its top edge fails. That is usually the pointing in the groove, or the lead wedges that grip the metal, or both. Once the edge sags, rain runs behind the lead and into the roof. Pushing it back or smearing sealant over the gap hides the fault briefly. Re-wedging and re-pointing, or re-chasing, puts it right.
Failed wedges and pointing
The top edge of a wall flashing is let into a slot in the brickwork. Small lead wedges jam the edge tight, and mortar pointing fills the slot and seals it. Those are the two things that keep the lead where it should be.
Both wear out. Mortar is the weaker of the two, since it is exposed to weather, can crack as the wall moves and can shrink away from the lead as it ages. Once the pointing falls out, the wedges are loose in an open groove. Wedges can also fall out on their own, and lead that is no longer gripped begins to sag outwards under its own weight.
From the garden the signs are a dark line at the top of the lead where light or shadow shows the gap, mortar crumbs on the roof below and a flashing that bellies outwards or rattles in a gale. The lead itself may look in good order. Our page on how lead flashing is fixed explains the method that has stopped working.
Often the lower edge has lifted too, as the clips there have worked free, and a flashing held only at one end moves in the wind.
Movement, and why it comes back
If the pointing is repaired but the lead keeps coming away, something is moving. There are a few common reasons.
- The lead was fixed too rigidly along its length, so it tugs at its own fixings as it warms and cools.
- The wall or chimney has shifted a little, opening the joints.
- The slot was cut into a joint that was already crumbling, so the repair never had a firm base.
- Damp or frost has loosened the face of the brickwork behind the groove, leaving nothing solid for the wedges to grip.
In each case, re-pointing alone treats a symptom. The cure for rigid fixing is to refix the lead in shorter, overlapping sections. The cure for a crumbling groove is to cut it back into sound brick.
Gaps appear first at the corners and steps, where the most force falls on the lead, and a run that failed once will usually open at the same points again until the cause is dealt with.
Lead that tears under that movement is a related fault, and it is a different repair.
How water gets in, and what to do first
Water enters a lifted flashing in two ways. Rain that runs down the wall no longer meets the lead, because there is a gap at the top where it can slip behind. Wind-driven rain can also be pushed up behind a loose lower edge. Either way the water reaches the roof timbers, the wall plate or the plaster, and it often travels along a rafter before it shows.
That is why a stain on your ceiling can sit well away from the flashing that caused it. Photographing the damp patch and the flashing together helps trace it, and we do that at the survey.
If the roof is leaking now, a temporary measure is the right first step. A leaking roof is made safe first, and the repair follows once the area is dry enough to work on. A bead of sealant along the top may hold for a short while, but it ages fast, as our page on sealant on lead flashing explains.
Where the flashing is still sound but unfixed, re-seating it is a small job, and delay turns small jobs into plaster, paint and timber work.
Re-chasing and the lasting repair
A lasting repair starts by removing the old pointing and loose wedges and checking the groove. If the brick behind it is sound, new wedges are fitted and the groove is re-pointed in mortar that suits the wall. If the groove is broken or too shallow, it is re-chased: the old cut is opened out or a new one is cut slightly higher, in sound brickwork, and the lead is tucked into it.
The lower edge is checked at the same time, and any missing clips are replaced, because a flashing held at one end only will lift again. If the lead is split or buckled as well, that section is replaced, and the repair goes no further than it has to. Our lead flashing repair page shows where patching stops and renewing starts.
On the survey we photograph the groove, the wedges and the pointing, and you see what is there before anything is decided. The quote is one total in writing.
If the flashing is part of a bigger leadwork job, the lead roofing hub explains the rest of what we do across Farnborough, Camberley and Fleet.
