Plugging Chisels: How a Lead Flashing Anchors Into a Brick Wall
What a plugging chisel is, the chase it cuts that turns into a lead flashing's anchor, and why this detail outlasts silicone or sealant. Buy from £8.
A homeowner inspects an extension roof six years after completion. Most details look fine. But the lead flashing where the new roof meets the existing house wall has lifted at the top edge, water has tracked behind it, and the brickwork below shows a damp stain that wasn't there at handover. Investigating, the homeowner finds the lead was held in place with silicone sealant rather than tucked into a chased mortar joint. The silicone has aged, cracked, and lost adhesion. There's no second line of defence.
A plugging chisel is the tool that creates the second line of defence. By raking out a horizontal slot in the mortar joint of a brick wall, it forms the chase that the lead's upturned edge tucks into. Once the chase is filled with mortar pointing, the lead is mechanically retained behind the brick face, not just stuck to the surface. The detail is unchanged in principle from Victorian leadwork; modern materials haven't found a substitute that lasts as long.
What a plugging chisel is
A plugging chisel (also called a joint raking chisel, mortar raking chisel, or chase chisel) is a narrow, hardened-tip chisel designed to remove mortar from horizontal joints in brick or block walls. The blade is typically 6 to 10mm wide, narrow enough to fit into a 10mm mortar joint, with a fluted or hollowed cutting tip that allows mortar debris to escape as the chisel works.
The chisel is used with a club hammer. You position the cutting tip in the mortar joint, strike it firmly, advance the chisel along the joint, and the mortar is dislodged in chunks. After three or four strikes, you sweep out the loose mortar and continue. A 1.5m run of chase takes 15 to 30 minutes depending on mortar hardness.
The two distinguishing features compared to a cold chisel are the blade width (narrower for plugging chisels, sized to match a typical 10mm mortar joint) and the fluted relief at the cutting tip (which allows debris to escape; a flat-tipped cold chisel jams in the joint).
Why a chase matters
This is the architectural detail that distinguishes a long-lasting lead flashing from a short-lived one.
Lead flashings have an upturned vertical edge that needs to be sealed to a wall surface above the roof line. Three approaches are used:
Silicone or polysulphide sealant (the budget approach). A bead of sealant between the lead's upturned edge and the wall face. The sealant ages, loses adhesion, and fails. Typical lifespan: 5 to 10 years depending on UV exposure and movement. Common on quick budget jobs; not specified by any major warranty body.
Lead flashing tape (the modern approach). A self-adhesive lead-and-aluminium tape that bonds to the wall face. Better than silicone but still surface-applied; eventually loses adhesion as the wall face weathers. Typical lifespan: 15 to 25 years.
Chased mortar joint (the traditional approach, still trade default). The lead's upturned edge is folded and tucked into a chase cut into the mortar joint. The chase is then re-pointed with mortar. The lead is mechanically retained in the joint, not glued to a surface. Typical lifespan: 40 to 80 years (the same as the lead itself).
The plugging chisel is the tool that creates the chase. Without the chase, the lead has no mechanical anchor. With it, the lead is held by the mortar pointing that fills the chase, and the only failure mode is the mortar pointing itself eroding (which is a separate maintenance task, replaceable without disturbing the lead).
Tip
The visible test for a properly chased flashing on an existing wall is whether the lead's top edge appears to be cut flush with the brickwork (chase tucked) or applied across the brickwork surface (silicone or tape applied). Look closely at the junction; the chased detail shows mortar pointing along the lead's top edge.
Cutting a chase: the technique
The chase is a horizontal slot in the mortar joint, 25 to 30mm deep, the height of the mortar joint (typically 10mm). The slot must be clean, square, and continuous along the run.
Mark the chase line on the wall
With the lead flashing held in position against the wall, mark the top edge of the lead on the brickwork. The chase will be cut along the nearest mortar joint above this line. Use a pencil or chalk; the mark gets removed during cutting.
Position the plugging chisel in the mortar joint
Hold the chisel at a slight angle (around 30 degrees from horizontal) with the cutting tip in the mortar joint. The angle helps the chisel bite into the joint rather than skating across it.
Strike with a club hammer at controlled force
A 1kg club hammer is the right weight. One firm strike per position should remove a chunk of mortar around 20 to 30mm deep. Heavier hammers risk cracking the surrounding brickwork.
Advance the chisel along the joint
After each strike, slide the chisel 20 to 30mm along the joint and repeat. Work systematically along the entire chase length.
Clear debris from the chase
After the first pass, sweep loose mortar out of the chase with a stiff brush. The chase needs to be clean for the lead to seat properly.
Square the chase walls with a second pass
Run the chisel along the chase a second time, removing any uneven mortar from the chase walls. The chase should be parallel-sided and square, not angled.
Inspect with a torch
Shine a torch into the chase to confirm depth and squareness along the full length. The chase should be 25 to 30mm deep throughout, with no narrow points where the lead won't tuck fully.
The chase is now ready to receive the lead. The leadworker folds the lead's top edge to a 90 degree return, pushes the return into the chase, and points the chase with mortar to lock the lead in place. Once the mortar cures, the lead is mechanically retained.
Plugging chisel versus cold chisel: do not confuse them
The two tools look similar; the plugging chisel is usually narrower and has the fluted relief at the cutting tip.
| Feature | Plugging chisel | Cold chisel |
|---|---|---|
| Blade width | 6-10mm narrow | 6-25mm; usually wider than plugging chisel |
| Tip relief | Fluted or hollowed for debris escape | Flat solid tip |
| Hardness | Hardened cutting tip, softer body | Fully hardened blade |
| Use | Raking mortar joints | Cutting mild steel, cutting masonry, splitting bricks |
| Will it cut steel? | No (the fluted tip is too narrow) | Yes |
| Will it work in a tight mortar joint? | Yes (the fluted relief allows debris escape) | Marginal (the flat tip jams) |
For chase work specifically, the plugging chisel is the right tool. A cold chisel can be used in a pinch but jams in the joint as debris accumulates, slowing the work and producing rougher chase walls.
For homeowners who own a cold chisel and don't want to buy a separate plugging chisel for a single job, an angle grinder with a 4mm thin diamond blade cuts mortar joints faster than any chisel. The trade-off is that grinder work generates respirable silica dust (a documented respiratory hazard) and requires PPE including FFP3 mask, eye protection, and ear defenders. For a single short run of chase, a plugging chisel and 30 minutes of work avoids the dust hazard entirely.
Warning
Cutting mortar joints with an angle grinder generates respirable crystalline silica. Sustained exposure causes silicosis, a recognised occupational lung disease. If you must use an angle grinder for chase work, wear an FFP3 dust mask, ensure good ventilation, and consider a water-fed grinder that suppresses dust. Hand chiselling generates much less airborne dust and is the safer option for occasional homeowner use.
Tools that compete: the angle grinder option
For trade roofers running multiple chases per day, an angle grinder fitted with a thin diamond blade is the production tool. The grinder cuts a chase in 60 seconds where a chisel takes 30 minutes. The trade-off is the dust and noise; the productivity is the win.
For homeowner use, the calculation is different. One chase per build, low time pressure, dust hazard avoidance: the chisel is the right choice. The plugging chisel and a club hammer cost 15 pounds combined and produce clean chase work without dust generation.
If multiple chases are needed (say, a long valley plus chimney junctions plus parapet capping), a hire grinder for the day with a diamond blade handles the work in an hour. Hire shops typically charge 20 to 30 pounds for a day's grinder hire plus 10 pounds for a diamond blade. The dust PPE adds 20 pounds for an FFP3 disposable mask and goggles. Around 60 pounds for the day, against 15 pounds for the chisel; the time saving justifies the difference for trade volume but rarely for homeowner volume.
What to buy
For a homeowner doing one extension with a few short chases, a plugging chisel and club hammer are the right purchase.
| Item | Approx price | Models | Use |
|---|---|---|---|
| Plugging chisel single | £8-15 | Faithfull FAIPLG, Roughneck 31-450, Wickes own-brand plugging chisel | Standard purchase for one or two chases |
| Chisel and bolster set with plugging chisel | £20-30 | Wickes 4-piece chisel set, Roughneck 4-piece set | Better value if you also need a cold chisel and bolster |
| Club hammer (1kg / 2.5lb) | £8-15 | Estwing, Stanley, Fiskars club hammer | The striking partner for any chisel work |
| Plugging chisel + club hammer kit | £15-25 | Buy as separate items from any merchant | The minimum kit for chase cutting |
| Angle grinder hire + diamond blade | £25-40 day rate + £10-15 blade | Brandon Hire, HSS | Trade speed for multiple chases; requires dust PPE |
For a recommendation: the Roughneck 31-450 Plugging Chisel at around 10 pounds from Screwfix or Toolstation is the trade-default homeowner purchase. Hardened tip, fluted cutting profile, comfortable hex shaft. Pair with a 1kg club hammer (Estwing or Stanley around 15 pounds) and you have the complete kit for 25 pounds.
If you already own a cold chisel from a 4-piece chisel set, a single plugging chisel adds the right tool for chase work specifically. Don't try to substitute the cold chisel; it will jam and slow the work.
Where you'll see this work
Plugging chisels appear at lead detailing stages of a build:
- Roof covering for chasing mortar joints to receive lead step flashings, valley flashings, and parapet capping
- Walls and blockwork where existing walls are being chased to receive new flashings on tie-in details
- Snagging checklist for inspecting the chase pointing and lead retention at completion
The chisel is a roofer's tool primarily; homeowners may use one if they're cutting their own chases for a small detail. Most homeowners watch the work rather than do it.
Common mistakes
Confusing plugging chisel with cold chisel. Different tools, different jobs. Use the right one for chase work.
Cutting the chase too shallow. A chase less than 25mm deep doesn't retain the lead properly; the upturned edge can pull out under wind load. Aim for 25 to 30mm depth throughout.
Cutting the chase too wide or too narrow. The chase should match the lead thickness plus a small mortar bed. For Code 4 lead (1.8mm thick), a chase 8 to 10mm wide is correct. Wider chases require more mortar pointing; narrower chases don't accommodate the lead easily.
Cracking surrounding brickwork by striking too hard. A 1kg club hammer with controlled strikes is enough. A 2kg lump hammer hit hard can crack surrounding bricks at the chase line. Match hammer weight to chisel size.
Skipping the chase and using sealant. Already covered. Silicone or sealant on a lead flashing is a 5 to 10 year detail; chased lead is a 40 to 80 year detail. The cost difference is hours of work, not pounds of materials.
Not cleaning debris from the chase before fitting the lead. Loose mortar in the chase prevents the lead from seating fully. Sweep the chase with a brush and check with a torch before the lead is fitted.
Pointing the chase with the wrong mortar mix. Use a lime-rich mortar (1:2:9 cement:lime:sand or similar) for chase pointing; pure cement mortar shrinks and cracks, allowing water ingress. The roofer or bricklayer pointing the chase should specify the mix.