Why does my chimney leak?

Step 1: When does the water appear?

Step 2: Inspection from the loft

If you can get into the loft, try to find the spot where water first enters the structure. Water runs along the underlay and rafters, even a metre from the chimney. Check whether the underlay is turned up the chimney and bonded – Všetkoprestrechu.sk describes taping the underlay around the chimney to a height of about 150 mm above the roof plane, with a drainage channel above the chimney. If the underlay ends loose at the chimney, every defect in the metal will show up as a leak.

Step 3: Inspection from outside

• counter-flashing: is it firmly fixed and not standing proud, is the top joint sealed and is the sealant still elastic?
• the back: is the back gutter deep enough, free of leaves, and is water not ponding there?
• front apron: does it lie on the covering and lap over it far enough?
• side flashings: do they have an upstand channel, and does the covering sit tight against it?
• the sheet: is it rusted through, holed by a screw or damaged?

Všetkoprestrechu.sk states that most chimney complaints are connected with a missing or badly fitted movement strip.

Step 4: Repair

If the repair shrinks to “a bit of sealant” but the cause lies elsewhere, the leak will come back. A detailed description of the parts and method: Chimney flashing.

Questions and answers

How can I tell whether it is a leak or condensation in the chimney?

A leak appears in rain or when snow melts and often depends on the wind direction. If the chimney masonry is damp even in dry weather, or brown stains and efflorescence appear on it, it may be condensation of flue gases in the chimney – a matter for the chimney system, not the flashing.