The short answer
Lead flashing is held by a cut called a chase, made into a mortar joint or a groove in the wall. The top edge of the lead is turned into that groove, jammed tight with lead wedges and sealed over afterwards. The lower edge is held down by lead or copper clips. Nails through the face are avoided, as they leak.
The chase: where the top edge goes
At the point where a slope abuts a wall or stack, the upper edge of the flashing needs an anchor in the masonry. The roofer cuts a groove, called a chase or a raglet, into a mortar joint or the brick or stone itself, using a cutter on a grinder or a hand tool in softer work. The lead is turned in at right angles into that groove, forming a small return called a tuck or a turn-in.
Chasing into a mortar joint rather than the stone is the usual rule, because a joint is easier to repair than a cut face, and it keeps the damage out of the brick or stone. On old buildings with soft lime mortar, the cutting has to be gentle. A rough cut can crack the masonry, and in a listed building you may need to speak to the conservation officer before any cutting begins.
Depth is a judgement. It must be enough to lock the lead and little enough to leave the wall sound, and the roofer makes that call by eye, joint by joint, because no two walls cut alike. Hard modern bricks, soft old stone and crumbly lime mortar each behave differently under the cutter.
Lead wedges: the part you will never see
Once the edge of the lead is in the chase, it must be held tight. The traditional method is a small wedge made of rolled lead, driven into the groove beside the turned-in edge so that the metal is jammed against the masonry. The wedges are spaced along the run and are what takes the strain, not the mortar.
Lead wedges are chosen over timber or plastic because they are made of the same material as the flashing. They do not rot, shrink or swell, and they do not react with the lead. They also stay put under the movement of heating and cooling.
Wedging is usually the step that tells a careful job from a rushed one. Too few wedges leave slack, and the lead can lift and flap in wind, which in time works the mortar loose. Wedges forced in without care can split the flashing. Once wedged, the groove is filled, a step dealt with in our answer on pointing lead flashing with mortar or sealant. That step seals the chase, but the wedges are what hold the lead.
Clips on the lower edge
Wedges and clips work as a pair. The wedges resist the pull of the lead away from the wall, and the clips resist the wind trying to curl the other edge upward.
The bottom edge of a flashing or soaker rests on the roof covering below, and wind can get beneath it. To prevent lifting, strips of lead or copper called clips are fixed to the roof structure and folded over the lead's lower edge, holding it down while still allowing it to slide as the temperature changes.
Because the clips are hidden beneath the overlapping lead or tile, they are rarely noticed. They are fixed with nails into timber, never through the exposed sheet. Where a flashing has no clips, you may see the edge curled up and pale underneath, or a flapping sound in a gale. Replacement flashings should have clips as a matter of course, and when we refit an old one we look for any that have rusted through or pulled out of the timber behind.
Lead lengths are kept short for the same reason. MyJobQuote's guide refers to flashing fitted in 1.5 metre lengths, and shorter pieces cope better with movement. Each length laps the next, so a lost clip affects less metal.
Why not just use sealant or nails
People sometimes ask why a gun of silicone or a few clout nails will not do. There are three answers.
- A nail through the face of the lead makes a hole, and the lead moves around it until the hole becomes a crack.
- Sealant relies on adhesion. It hardens, shrinks away and splits as the lead expands and contracts, and it can trap water behind a skin that looks sound.
- Neither lets the lead slide, and slide it must. A rigidly held sheet in sun and frost will split.
Sealant has a limited, honest role as a supplement in an awkward detail, but it does not replace the chase and wedges. Our answer on replacing lead flashing yourself takes this further for owners who are tempted to patch with a tube.
What we check when a flashing has come loose
On site we tap the mortar, test whether the lead has any slack in the chase, and look for missing wedges, rusted clips and open joints. We also look for the reason it came loose. Chimney movement, brittle mortar or a gutter overflow can undo a good fixing. Our answer on lead parapet and box gutters covers a flat gutter where the same wall chase detail appears.
If a flashing can be re-wedged and repointed, the written quote says so simply. If the lead has to be renewed, the photographs show why. Refixed work carries our 10-year workmanship guarantee in writing. To book a look, ring 01453 933613, or send a photograph through WhatsApp or the form. The full range of leadwork is on the lead roofing page.