How do I check the lead flashing round a chimney?

A chimney is a masonry column standing through a roof, which means it has four separate weathering details around it and any one of them can leak. It is also the wettest, windiest, most exposed part of the building, which is why chimneys cause a disproportionate share of roof leaks.

Safety

Step by step

1. Understand what should be there.A front apron flashing across the downhill face, stepped flashings dressed into the mortar joints up both sides with soakers under the tiles, and a back gutter behind the stack taking water round it. Missing any one of them is a leak.
2. Look for cement instead of lead.A mortar fillet round the base of a chimney is a cheap substitute for lead and it always cracks, because the stack and the roof move independently. A cracked fillet is a confirmed leak path.
3. Check the chase pointing.Step flashings are tucked into raked-out mortar joints, wedged with lead, and pointed. Look for flashings hanging loose, or dark open lines where the pointing has gone.
4. Look for splits and fatigue in the lead.Lead expands and contracts, and a piece that is too long without a joint splits at the fold. Splits look like thin dark lines along a crease.
5. Check the flaunching on top.The mortar fillet that holds the pots. Cracked or missing flaunching lets water straight into the top of the stack, and loose pots are a falling hazard.
6. Check the stack brickwork and pointing.Spalled bricks, open joints and a leaning stack all let water in or signal a more serious problem. A stack that has clearly moved needs urgent professional attention.
7. Look inside for the evidence.In the loft, damp staining on the chimney breast, wet timbers where the stack passes through, and salts on the brickwork. In the rooms below, brown staining or damp patches on a chimney breast, often with a tide line.
8. Check any capped or unused flue.A disused chimney should be capped with a ventilated cowl, not sealed solid. A sealed flue traps moisture and causes damp on the breast inside, which is often mistaken for flashing failure.
9. Get the whole stack looked at together.If access is being set up for flashing work, have the flaunching, pots, pointing and any cowl checked at the same time. The access is most of the cost.

Tips

• A camera on an extending pole gives a genuinely good view of a chimney base from ground level.
• Chimney damp often appears in the room below in a hard, wind-driven rain rather than in steady rain. Noting the conditions helps identify the entry point.
• Salt staining on an internal chimney breast is usually old soot salts drawn out by damp, not fresh water. Both indicate the stack is wet.

Common mistakes

• Sealing a disused chimney completely — The stack still gets wet from rain and the flue has no way to dry. Moisture then shows on the breast inside. A capping cowl that keeps rain out but allows ventilation is the correct detail.
• Repointing or fillet-repairing a stack from a ladder — Chimney work is at the highest point of the roof with nothing to stop a fall and requires a scaffold or a proper roof access system. This is the single most dangerous DIY roof job.

If it doesn't work

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