Leaking chimney flashing: when tape works and when it doesn't

Chimneys cause more roof leaks than almost anything else. They sit right in the path of the weather, they're surrounded by joints, and their flashings are often decades old. When the stain appears on the ceiling beside the chimney breast, it's tempting to reach for a quick fix. Sometimes that's exactly right. Sometimes it isn't.

Where chimney leaks come from

Not every chimney leak is the flashing. Check these before you decide on a repair:

• The flashing itself: lead that's split, lifted or slipped, or that has come out of its mortar joint.
• The mortar joint the flashing is tucked into: if the pointing has gone, water runs behind the lead.
• The back gutter on the uphill side of the stack, which collects water and debris and often fails first.
• The brickwork: porous or cracked bricks and failed pointing let water soak down through the stack.
• The top of the chimney: a cracked flaunching or an uncapped, unused flue lets rain straight in.

If the damp appears after rain blows from one direction, or only after long wet spells, the brickwork or the top is often the cause rather than the flashing.

When tape is the right fix

Self-adhesive flashing tape is a good repair when the problem is local and the rest of the flashing is sound:

• Splits and cracks in lead that's otherwise in good shape.
• A lifted edge where the flashing has pulled away from the tiles or slates.
• Joints between lengths of lead that have opened up.
• An emergency seal to keep the weather out until a roofer can get to it.
• Stolen lead. Lead theft is common, and tape gets the roof watertight the same day, with no scrap value to attract thieves back.

When it needs doing properly

Tape isn't the answer if:

• The lead is badly corroded, missing in places, or has been patched many times already.
• There's no proper back gutter, or the stepped flashing has never been detailed correctly.
• The pointing has failed all round the stack.
• The water is coming through the brickwork or the top of the chimney.

In those cases, the flashing or the stack needs rebuilding by a roofer. Tape can keep the weather out in the meantime, but it won't fix a detail that was never right. For new work, use lead or a certified lead alternative.

How to tape a chimney flashing repair

1. Stay safe. Chimney work is at height on a pitched roof. Use roof ladders or scaffolding, and get a roofer if you're not comfortable.
2. Clean it back. Wire-brush the lead and the area around it to remove moss, dirt and loose material, then let it dry completely.
3. Dress down any lifted lead gently, so it sits flat before you tape over it.
4. Cut the tape to overlap the split or joint by at least 50mm on every side, and round the corners.
5. Work from the bottom up, so each piece laps over the one below and water runs off, not under.
6. Roll it firmly with a seam roller, pressing the tape into the profile of the tiles or slates.
7. Check every edge and re-roll anything that's lifting.

On crumbly old brick or mortar, the tape will only hold as well as the surface. Brush it back to something sound first. Our guide to when flashing tape needs a primer explains more.

Which tape?

Supraseal® Weatherproofing Tape bonds to lead, brick, concrete, clay tiles and slate. It's bitumen free, UV stable, flexible from -30°C to +80°C, paintable, and backed by a 10-year warranty. 200mm suits most chimney flashings, and 300mm suits wider back gutters; see our width guide. On dark slate roofs, Supraseal® Black blends in.

Weighing up tape against lead for a bigger job? Read flashing tape vs lead flashing.