Lead Wedges: How Stepped Flashing Is Held in the Chase and What to Check

A roofer dresses a smart run of stepped lead flashing up the wall where your new extension roof meets the house, smears a bead of sealant along the top edge, and packs up. Then the first proper gale arrives, the suction lifts the unsupported top edge, the sealant peels, and rain tracks down behind the flashing into the cavity. The thing that should have stopped it takes minutes to fit: a handful of lead wedges driven into the joint to hold the lead before it gets pointed. Skipping them is one of the most common shortcuts in domestic leadwork.

What a lead wedge is and what it does

A lead wedge is a small piece of folded or rolled lead, cut from an offcut, that is driven into a slot in the wall to pin the top edge of a flashing in place. It is not a manufactured product you buy by the box in the way you buy screws or wall plugs. It is made on site from the same lead the flashing is cut from, which is why you will rarely see "lead wedges" priced on a shelf. The wedge is a consumable that exists for one job: to hold the turned-in edge of the lead in the wall mechanically, so the flashing cannot pull out before, or after, the joint is sealed.

To understand where the wedge sits, you need to picture the junction it works in. Where a sloping roof meets a vertical wall (the abutment), the weatherproofing is built in layers. Hidden under the tiles sit the soakers, one small piece of lead per tile course. Over the top of those, dressed against the wall and stepped down the slope, sits the visible stepped cover flashing. The top edge of that cover flashing is turned back on itself and tucked into a slot cut into the wall. That slot is the chase. The wedges go into the chase, pinching the turned-in lead against the back of the slot so it stays put.

That distinction is the whole point of this page. Sealant keeps water out. The wedge keeps the lead in. A junction relying on sealant alone, with no wedges behind it, will eventually let go at the most exposed line on the roof.

The chase: what it is and how it's cut

The chase is the slot the flashing turns into. On a brick wall it is cut into a horizontal mortar joint (the bed joint), never into the face of a brick, because cutting the brick weakens it and looks wrong. The roofer rakes out the old mortar from a bed joint that runs roughly parallel to the slope, following the steps of the brickwork up the wall so the chase climbs with the flashing.

Cutting it is the noisy, dusty part of the job. The mortar is raked out with an angle grinder fitted with a diamond blade, or with a dedicated mortar raking tool, to a depth of at least 25mm into the joint. That depth matters: too shallow and there is not enough room to turn the lead in, seat the wedges, and still get pointing on top. A chase cut only 10mm or 15mm deep gives the wedges almost nothing to bite against, and the whole detail relies on the pointing alone, which is exactly the failure this is meant to avoid.

On a rendered wall the rule shifts. The chase is cut about 35mm back from the face of the render so there is room for a 30mm turn on the lead inside the slot, and the render is made good afterwards. The principle is identical: a proper depth so the lead can turn in and the wedges can grip.

How the flashing is held with wedges

With the chase cut, the roofer turns the top edge of the stepped cover flashing back on itself, a small fold, and pushes that folded edge up into the slot. Folded lead is springier and grips better than a single thickness, and it gives the wedge something solid to bear against.

The wedges are then driven in. Each wedge is a strip of lead, rolled up tight into a small cylinder or folded into a tight pad, tapped into the chase with a hammer and a bolster or a wedging tool so it jams the turned-in lead hard against the back and top of the slot. They are spaced along the chase at roughly 450mm centres, with a minimum of one wedge per step of the flashing. On a stepped run climbing a brick wall, that usually means at least one wedge in every visible step, sometimes two on a long step.

The reason for the spacing is wind. The gable and the top edge of an abutment are the most wind-exposed lines on a roof. Wind hitting the wall is forced up and over, and the suction it creates tries to peel the unsupported top edge of the lead away from the wall. Wedges at sensible centres hold the edge down along its whole length so there is no long unsupported span for the wind to get under. Too few wedges, or wedges only at the ends, leaves the lead free to flap, work the sealant loose, and eventually lift.

Wedges are made from offcuts

Because a wedge is just rolled or folded lead, it is made from the offcuts left over after cutting soakers and step pieces. Leadwork generates a lot of awkward scrap, and the wedges use it up. This is one reason you almost never see "lead wedges" sold as a packaged product: the roofer cuts a strip off an offcut, rolls it between gloved fingers or against the wall, and drives it home. The material cost is effectively nil because it would otherwise go in the scrap bin (where, being lead, it still has resale value).

What this means for you as the homeowner is simple. There is no line item to chase on a quote and no product to specify. What you are checking is that the wedges are there at all, that they are lead and not some improvised substitute, and that they are at the right spacing. A roofer who uses timber offcuts, rolled-up bits of plastic, or mortar dabs in place of lead wedges is bodging it. Timber rots and swells, plastic perishes, and a mortar dab is not a mechanical fixing at all.

Pointing over the wedges: mortar vs sealant

Once the wedges hold the lead, the chase is sealed to keep water out of the slot. There are two ways this is done, and they are not equal.

The traditional method is to repoint the chase with a sand-and-cement mortar, pressed in over the wedges and tooled flush with the brickwork. Done well, a mortar-pointed chase looks like part of the wall and lasts for years. The weakness is that mortar is rigid and the lead behind it moves with every change in temperature, so over decades the mortar can crack along the lead and let water seep in behind it. It also needs a skilled hand to point neatly.

The modern method is to seal the chase with a lead sealant, a flexible mastic (typically a low-modulus polyurethane or hybrid sealant) gunned into the top of the slot over the wedges and tooled off. Because it stays flexible, it moves with the lead instead of cracking, and it is faster and more forgiving to apply. Many roofers now prefer it, and a number of products are sold specifically as lead flashing sealants.

Here is the part that gets confused. Sealant pointing is fine, often better than mortar, but only over wedges that are already holding the lead. The sealant is sealing the slot, not fixing the flashing. Where roofers go wrong is treating the sealant as the fixing: no chase cut, no wedges, just a fat bead of mastic smeared along the top edge of the lead against the wall face. That fails quickly. The bead has nothing mechanical behind it, the lead lifts in the wind, and the sealant tears away in a strip. If you see a flashing whose only attachment to the wall is a line of mastic on the surface, with no chase cut into the brickwork, that is the bodge to reject.

Where wedges sit in the whole abutment system

It helps to see the wedge as one component in a layered detail rather than a thing on its own. Working from the roof up the wall, the order is: soakers tucked under each tile course and turned up against the wall; the stepped cover flashing dressed over the soakers and down onto the tiles; the turned-in top edge of that cover flashing folded into the chase; the wedges driven in to grip it; and the pointing, mortar or sealant, sealing the slot. Remove any one layer and the junction weakens. Pull the wedges and the cover flashing has no mechanical hold. Pull the soakers and wind-driven rain gets under the cover flashing at the tile line, which the cover flashing alone cannot stop.

The components, codes, upstands, and laps of the flashing itself are covered in detail on the lead flashing page. The wedge is the small fixing that makes the top edge of that flashing stay where the roofer dressed it.

The table above makes the point that there is no "lead wedge" product to budget for. The wedges come free out of the offcuts. What you are really paying for around the wedges is the lead the flashing is cut from, the sealant or mortar that points the chase, and the patination oil that keeps the finished job from streaking the wall. As part of a full abutment, this is all inside the roofer's price for the roof covering; you would only see it itemised if you were paying for standalone leadwork such as a chimney re-flash.

Who does this and why it isn't a DIY job

Lead wedging is done by the roofer or, on bigger or more exposed jobs, a specialist leadworker. It happens at height, on a roof slope, against a wall, usually off a scaffold or tower, while also dressing lead and cutting a chase with an angle grinder. The skills, raking out a clean chase, folding and rolling lead, driving wedges so they grip without splitting the lead, are trade skills built up over years.

You will not be fitting wedges yourself unless you already work on roofs. The value of understanding the detail is that you can check the work before the access comes down and the evidence is hidden behind sealant and pointing. Once the chase is pointed up, you cannot see whether the wedges are there or how many. So the time to look is while the flashing is dressed and before the final pointing goes on, or to ask the roofer to walk you through what they have done at the abutment.

Common mistakes

No wedges at all, sealant only. The headline failure. A bead of mastic along the top of the lead against the wall face, with no chase cut and nothing holding the lead mechanically. It looks finished, lasts a season or two, then lets go in wind. Insist on a chase with wedges, then sealant or mortar over the top.

Too few wedges. Wedges only at the top and bottom of the run, or one every metre, leave long unsupported spans of lead edge for the wind to lift. The spacing should be around 450mm, at least one per step of the flashing.

Chase too shallow. A slot raked out only 10mm or 15mm deep gives the wedges almost nothing to bite against and forces the junction onto the pointing alone. The chase wants to be at least 25mm into a mortar joint, or 35mm back from the face on render.

Wedges cut into the brick face. Raking the chase straight across the bricks instead of following a mortar joint weakens the wall, looks wrong, and is harder to point. The chase follows the bed joint, stepping up the wall with the brickwork.

Substitute wedges. Timber, plastic, or mortar dabs used instead of rolled lead. Timber swells and rots, plastic perishes, and a mortar dab is not a mechanical fixing. Wedges are lead, cut from the flashing offcuts.

Lead left unpatinated. Not a wedge fault as such, but the same shortcut mindset. New lead left without patination oil throws white carbonate staining down the wall and tiles within weeks. A coat of patination oil on the finished flashing the same day prevents it. If white streaks appear below a new abutment, missing patination oil is the cause.

Where you'll need this

• Roof covering - wedges hold the stepped cover flashing into the chase where the new extension roof abuts the existing house wall, fitted as part of the lead abutment detail
• Roof structure - the abutment detailing where the new roof meets an existing wall, the junction the wedges and flashing weatherproof

Lead wedges turn up wherever stepped flashing meets a wall on any extension, re-roof, or chimney job: side abutments, chimney stacks, dormer cheeks, and parapet junctions. The detail is the same regardless of project type, a chase cut into the mortar joint, the lead turned in, wedges at sensible centres holding it, and pointing or sealant sealing the slot over the top.