The short answer
Lead wedges are small strips of lead, folded and tapped into the mortar joint beside the flashing's top edge. They grip the sheet so it cannot slip out, then the joint is pointed over them. Flashings fixed this way outlast ones held only by sealant or nails, because the wedges allow the lead to move slightly with the weather without pulling free.
What a lead wedge is and what it does
A flashing needs something to hold its top edge against the wall. The lead is tucked into a channel opened up along a joint, which the trade calls the chase, but lead is soft and heavy, and left alone it will slowly creep down and fall out of the groove. Wedges are the answer. Each is a small strip of lead, folded so that it forms a tight spring, and it is driven into the chase beside the flashing. It jams the sheet against the back of the groove and holds it there.
The wedges are spaced at regular intervals along the chase. Once they are in, the groove is filled with mortar, which covers the wedges and seals the gap against water. From then on, the mortar keeps the rain out while the wedges do the mechanical holding. If either one fails, the other cannot do both jobs.
How the chase is cut and the wedges are driven
Before a wedge goes in, the chase has to be right. The roofer or mason cuts or rakes a clean slot into the joint, deep enough for the lead to sit securely and no deeper than the brick can bear. A groove that is too shallow gives the wedges nothing to bite on, and one that is too deep weakens the course above.
The top edge of the lead is turned into the slot, and the wedges are pushed in between the lead and the masonry and tapped home. Each one is a snug fit, tight enough that you cannot pull the sheet out by hand. They are spaced evenly so that the pull on the sheet is shared, and extra ones go in at corners and steps, where the load is greatest. Only then is the joint pointed over the top. The rest of our lead roofing pages describe the other pieces that depend on this same fixing.
Why nails, screws and sealant are poorer substitutes
Nailing through lead makes a hole, and a hole in a flashing is a place for water to enter. Lead also swells in warm weather and draws back in the cold, so a rigid fixing at several points makes the sheet tear in the gaps between them. Wedges grip without piercing, and they let the lead flex slightly as it warms and cools.
Sealant in the groove is a stopgap. It can hide a gap for a while, but it sticks to the surfaces and gives them nothing to hold, and weather, sun and movement work it loose. Our page on tape and sealant against lead looks at where that kind of fix has a place. For a lasting flashing, the combination is the one roofers have used for generations: a clean chase, lead wedges, and sound pointing over the top.
Signs that wedges have gone
Wedges are out of sight once the pointing is on, so you see the effects instead. The top edge of the lead stands away from the wall, or sags into a curve. A line of mortar has dropped out, leaving a dark slot with the lead hanging in it. Rainwater has run behind the sheet and a damp stain shows inside, level with the roof line.
Old wedges can corrode or lose their spring, and a chase that was cut too shallow never held them well. Hard cement pointing on soft brick often cracks off and takes the wedges with it. A stack or a long wall can lose a few wedges at a time, and the fault creeps along until a stretch drops. A roofer can lift an edge and see how many are still gripping. The survey photographs record what is there.
Which kinds of wall take wedges best
Wedging works in any masonry that has a mortar joint to open, and that covers most brick and block walls and many stone ones. Rendered walls are the awkward case. The render hides the joints, so the lead is usually tucked into a cut made in the render itself, and the finish needs sealing with care. Where a chimney has been rendered over old brick, the roofer works out where the real joints lie before cutting.
Soft old brick needs a gentler touch than hard engineering brick, because a tight wedge can split the face of a crumbling edge. In those cases the joint is raked by hand and the fixing is spread over more wedges. Stone walls, including limestone, are treated the same way, with the cut kept inside the mortar bed and away from the stone itself.
Refixing wedges, and when more is needed
If the lead is sound, the repair is simple. The loose mortar is raked out, the lead is pressed back into the chase, fresh wedges are tapped in at the right spacing and the joint is pointed. That is a modest repair, and it is our preference whenever it solves the fault. Our page on lead flashing cost sets out what drives the price of that and larger jobs.
If the sheet is cracked, worn thin or too short for the run, wedges cannot save it. In that case the flashing is replaced. Lead lining a parapet gutter is judged the same way, and our page on lead parapet gutters covers that setting.
Doing it yourself, and what it costs
Wedging looks like a job a confident householder could manage, but it is work at height beside an edge, usually on a chimney or a high wall, and a loose scaffold board or a wet slate has caused many a fall. Getting the chase to the right depth without damaging the brick is a skill, too. Our page on doing lead flashing yourself covers what can safely be done from the ground.
MyBuilder's chimney repair guide, updated in August 2026, gives a national typical range of £250 to £1,200 for repairing a chimney flashing, and £500 to £2,000 for repointing a chimney. Your own figure depends on height, access and how much needs doing. Once the survey is done, our written quote arrives, and we stand behind the work with a ten-year guarantee, in writing. Ring 01793 626096 or use WhatsApp.