Why lead flashings fail — and why hand-dressed lead matters

Lead has been the standard material for roof flashings on British buildings for centuries. Properly installed, it can last 100+ years. Improperly installed, it can fail in under 10. Here’s what’s going on.

What lead flashings do

Lead is used wherever the roof meets something that isn’t roof — a chimney stack, a wall abutment, a dormer cheek, a valley where two slopes meet. The lead is dressed (shaped by hand) so that it bridges the joint, with the upper edge tucked into the brickwork or chimney and the lower edge lapped onto the tiles.

If the lead is doing its job, water can’t get in. If it’s not, water gets in.

Why lead fails

The main causes:

1. Mortar pointing instead of lead. A common shortcut. The roofer or builder uses mortar to “seal” the joint instead of lead. Mortar is rigid, the building moves slightly with the seasons, the mortar cracks. Water gets in.

2. Acrylic or self-adhesive flashing instead of lead. Sometimes sold as a “modern alternative” to lead. Cheaper, easier to install, dramatically shorter service life. Most acrylic flashings are guaranteed for 10-15 years. Lead is guaranteed for 100+.

3. Lead that’s too thin. Lead comes in thicknesses called “codes”. Code 3 is for soakers (small pieces). Code 4 is for flashings. Code 5 and 6 are for valleys and biggish jobs. If thin lead has been used where thick lead should have been, it will fail.

4. Poorly dressed lead. Machine-rolled lead looks tidy but doesn’t always fit the building well. Hand-dressed lead is shaped to the specific contours of the chimney, abutment, or valley. The difference matters when the building moves or when wind drives rain at the joint.

5. Failed laps. Lead flashings should overlap each other by at least 100mm. If they don’t, water can get between them.

6. Patina damage. Lead develops a protective patina (a dull grey surface layer) over time. If the patina is damaged — by scraping, by aggressive cleaning, by incompatible materials — the lead underneath corrodes faster.

How to tell hand-dressed from machine-rolled

Hand-dressed:

• Each piece shaped to fit the specific chimney/wall
• Slight irregularity in the lines (a sign of being shaped by eye)
• Wedged into the brickwork with lead wedges, not mortar
• Seams overlapped and dressed into the wall

Machine-rolled:

• Comes off a roll, often in long pieces
• Tidy and uniform but generic in shape
• Often bedded in mortar rather than wedged
• Faster to install, easier to mess up

If you’re not sure what you’ve got, we can tell from a photo. We can also tell from the sound — lead that’s been dressed properly has a different feel underfoot than lead that’s just been stuck on.

What the right code of lead looks like

Code 4 lead (1.8mm): the standard for chimney flashings, abutment flashings, step flashings. Most common.

Code 5 lead (2.24mm): for larger flashings, parapet flashings, bigger valleys.

Code 6 lead (2.65mm): for large valleys, flat roof detailing, big commercial flashings.

Code 7+ lead (3.0mm+): for very heavy work — large commercial flat roofs, complex detailing.

The wrong code is usually too thin. Thinner is cheaper, easier to work with, fails faster.

What proper leadwork looks like

1. Lead is the right code for the job.
2. Pieces are shaped by hand to the specific location. Not just rolled out and stuck on.
3. Laps are at least 100mm. Often more on exposed sites.
4. The top edge of the flashing is tucked into a chase in the brickwork and wedged with lead. Not pointed over with mortar.
5. The bottom edge of the flashing is dressed over the tiles or slates. Not behind them.
6. Step flashings are individual pieces per course of brickwork. Not one long strip bent up and down.

A chimney that’s been properly re-leaded with hand-dressed code 4 lead should be watertight for 50+ years. Often longer.

When the existing lead needs to come off

Lead that’s been:

• Mortared in
• Painted over (paint prevents patina formation, accelerates corrosion)
• Damaged by impact or foot traffic
• Replaced with acrylic in a previous “repair”

…usually needs to come off and be redone properly. There’s no good way to dress new lead to match failed leadwork.

What leadwork costs vs. what it saves

Leadwork is more expensive than the alternatives, mostly because the labour is skilled and the lead itself isn’t cheap. But it’s also the only material that genuinely lasts.

A proper chimney re-flash in hand-dressed code 4 lead should be the last re-flash the chimney ever needs. The acrylic alternative is a third of the price and might need doing three times in your lifetime.

If you’ve got a flashing question, get in touch. We can usually tell from a photo whether what you’ve got is sound lead or a shortcut, and what the right answer is.