How to Install Lead Flashing: Step-by-Step (Chimney, Wall & Pitched Roof)

What you’ll need

Lead flashing is one of the few roofing jobs where the material has barely changed in centuries, and the reason is simple: properly installed lead outlasts almost everything around it. The technique matters more than the tools, but you need the right tools to execute it.

Materials:

• Code 4 lead flashing roll — the standard weight for most domestic flashings
• Lead clips or lead wedges for fixing into the mortar chase
• Lead sealant / mastic for pointing the chase
• Patination oil to prevent white carbonate staining
• Sand and cement, or a pointing mortar, if repointing the chase

Tools:

• Angle grinder with a diamond blade for chasing the mortar joint (or a plugging chisel and lump hammer)
• Lead dresser and a bossing stick or bossing mallet for working the lead to shape
• Sharp knife or lead shears for cutting
• Chalk line, tape and a straight edge
• Bolster chisel, brush, and dust extraction or water suppression when grinding

What is Code 4 lead?

Lead sheet is graded by code number, which relates to its thickness and weight. The higher the code, the thicker and heavier the lead.

Code 4 is the workhorse. It is thick enough to be durable and hold its dressed shape, thin enough to work comfortably by hand, and it is what the great majority of chimney flashings, abutment flashings and soakers on UK houses are made from. Our Code 4 lead flashing rolls come in 150mm (6 inch) × 3m and × 6m, and 300mm (12 inch) × 3m — the widths that cover most standard flashing details.

How to size and cut lead flashing

The most important sizing rule in lead work has nothing to do with width: never use a single piece longer than 1.5m.

Lead expands and contracts significantly with temperature. A long, continuous piece is restrained at both ends, so the movement has nowhere to go and the lead fatigues. It works itself back and forth over thousands of cycles until it cracks along a line — usually just where it turns out of the chase, and always at the worst possible place.

So the rule is: maximum 1.5m per piece, with each piece overlapping the next by at least 100mm. Cut your lengths before you start, allowing for the overlaps.

For width, the flashing needs to cover from at least 25mm inside the mortar chase, down the wall face, and at least 150mm over the roof surface. A 150mm roll suits most step flashings and cover flashings; 300mm is used where you need more cover, such as an apron flashing at the front of a chimney or a wider abutment.

Cut lead with a sharp knife against a straight edge, scoring and folding rather than sawing, or with lead shears for a cleaner edge on thicker sections.

Step-by-step: wall and apron flashing

• Set out the line. Chalk a line on the wall parallel to the roof slope, 150mm above the roof surface. This is the bottom of your chase.
• Cut the chase. Grind a slot 25mm deep into the horizontal mortar joint nearest to your chalk line. Cut into the mortar, not the brick. Rake out the debris and brush the chase clean.
• Cut the lead to length. Maximum 1.5m per piece, allowing 100mm overlap onto the next.
• Fold the upstand. Turn a 25mm lip along the top edge of the lead — this is what tucks into the chase. Fold it at a slight angle so it wants to spring into the slot rather than fall out.
• Offer up and dress. Push the folded lip into the chase and dress the lead down the wall and out over the roof surface with the dresser, working from the middle outwards so the lead lies flat without stretching or creasing.
• Fix with lead clips. Drive lead wedges or clips into the chase at roughly 450mm centres to hold the upstand securely. Every piece needs at least two.
• Point and seal the chase. Fill the slot above the lead with pointing mortar or lead sealant, tooled to shed water outwards. This is the joint that keeps water out of the top of the flashing — take your time with it.

How to lead-flash a chimney

A chimney is the most demanding flashing job on a normal house, because water attacks it from four directions and each face needs a different detail. The work is done in a specific order, from the bottom up, so each piece sheds water onto the one below.

1. Front apron

The apron covers the downslope face. Cut a piece wide enough to run at least 150mm up the chimney face and 150mm down onto the tiles, with 50mm returns around each corner. Dress it over the tile profile using a bossing stick, working the lead into the shape rather than stretching it thin. Tuck the top edge into a chase in the chimney face and wedge it.

2. Step flashing to the sides

The two side faces are flashed with stepped pieces that follow the mortar courses up the slope. Each step tucks 25mm into the mortar joint, overlaps the piece below it by at least 100mm, and covers the soakers beneath. Cut the steps to match the brick courses and the roof pitch — mark them out on the lead before cutting, as getting the step spacing wrong is difficult to correct later.

3. Soakers under the tiles

Soakers are individual pieces of lead that sit under each tile or slate along the chimney sides, turning up against the chimney face beneath the step flashing. They are the primary waterproofing at the junction; the step flashing over them is the cover. Both are needed.

4. Back gutter

The upslope face is the hardest detail and the most common failure point. Water runs down the roof and collects against the back of the chimney, so the back gutter has to carry it around to both sides. It needs a sole, an upstand against the chimney, a turn-up under the tiles above, and welted or bossed corners at each end. This is the piece to think carefully about before cutting — and the piece where a professional roofer earns their money.

How to fit lead soakers

Soakers are simple to make and easy to get wrong.

Each soaker is a rectangle of Code 3 or Code 4 lead, folded at right angles into an L. The horizontal leg lies on top of the tile or slate below, the vertical leg turns up against the wall or chimney face. The length of each soaker is the gauge of the tile or slate plus the lap, so that each soaker overlaps the next going up the slope.

Fit one soaker per course, working up the slope as the tiling proceeds — they cannot easily be retrofitted once the tiles are on. The upstand is then covered by the step flashing, which is what you actually see when the job is finished.

Sealing, pointing and patination oil

Three finishing details make the difference between a flashing that lasts forty years and one that leaks in five.

Pointing the chase: the lead’s turned lip goes into the chase, the wedges hold it, and the remaining slot is filled. Use a pointing mortar or a proper lead sealant, and tool the finish so it sheds water away from the joint rather than holding it. Never rely on sealant alone with no mechanical fixing — the wedges do the holding, the sealant does the sealing.

Lead sealant: use a sealant designed for lead. General-purpose silicones do not bond reliably to lead and fail at the joint within a few years.

Patination oil: apply it to all new lead as soon as the work is complete, ideally the same day. Fresh lead reacts with rainwater to form lead carbonate, a white powder that washes down and stains brickwork, tiles and render below. Patination oil prevents that reaction. It takes minutes to apply and saves a job that looks like a mistake for years.

Common mistakes

• Pieces longer than 1.5m — the single most common cause of cracked flashing, from thermal fatigue
• Overlaps under 100mm, which let wind-driven rain track back up the joint
• Chasing into the brick instead of the mortar joint, weakening the masonry
• A chase less than 25mm deep, so the lead works loose over time
• Relying on sealant instead of lead wedges to hold the upstand
• Stretching the lead instead of bossing it — stretched lead is thin lead, and it fails where it’s thinnest
• Skipping soakers and relying on step flashing alone
• Forgetting patination oil, leaving white staining down the brickwork
• Fixing lead over a badly maintained or missing back gutter, hiding the actual leak

Lead vs lead-free alternatives

Lead-free flashing is worth considering, particularly where theft is a risk or where the job needs to be done quickly without specialist skills.

Lead-free flashing is a malleable, permanently waterproof material that installs with a knife or scissors and needs no specialist tools or bossing skill. It has no scrap value, which removes the theft risk entirely on exposed or vulnerable properties. It is lighter to carry and quicker to fit, and it is suitable for flat, pitched and commercial roofs. Our lead-free flashing comes in 300mm × 6m in grey.

Traditional lead still wins on longevity, on conservation and listed work where it may be required, and on complex bossed details where its working properties are unmatched. For most straightforward abutments and simple chimney work, lead-free is a genuine alternative — for a bossed back gutter on a Victorian stack, lead remains the material of choice.

Frequently asked questions

What size lead for flashing — Code 3 or Code 4?

Code 4 is the standard for most domestic flashings, aprons and step flashings. Code 3 is lighter and generally used for soakers and small repairs. Code 5 and above are for valleys, gutters and heavier exposed work.

How long can a piece of lead flashing be?

No more than 1.5m. Longer pieces cannot accommodate thermal movement and eventually crack from fatigue. Use 1.5m maximum lengths with at least 100mm overlaps between pieces.

How do you seal lead flashing?

Mechanically first, chemically second. Turn a 25mm lip into a chase cut in the mortar joint, secure it with lead wedges or clips at around 450mm centres, then point or seal the chase above with mortar or a purpose-made lead sealant.

Do you need patination oil?

Yes, on all new lead. Without it, fresh lead reacts with rainwater to form a white carbonate that washes down and stains brickwork and tiles below. Apply it as soon as the work is complete.

How do you flash a chimney?

In four parts, working from the bottom up: front apron on the downslope face, soakers under the tiles on each side, step flashing over the soakers following the mortar courses, and a back gutter on the upslope face. Each element sheds water onto the one below.

Products mentioned in this guide

• Lead Code 4 Flashing Roll 150mm (6 inch) × 3m
• Lead Code 4 Flashing Roll 150mm (6 inch) × 6m
• Lead Code 4 Flashing Roll 300mm (12 inch) × 3m
• Lead Free Flashing 300mm × 6m Grey
• Roofing Supplies category

Related guides

• How Much Does Lead Flashing Cost in the UK?