The point where a chimney or flue passes through the roof concentrates two problems at once: it is a discontinuity in the waterproofing, and it is subject to minimum-height regulations that aren't always respected in older installations.
Minimum height under the regulations
The regulations require the flue to project a minimum height above the roof and above any nearby obstacle (such as the ridge itself if it is close), calculated according to the distance to taller elements, to ensure proper draught and prevent smoke returning indoors or to neighbouring homes.
Flashing and sealing: the critical waterproofing point
Around the chimney a flashing is installed (a metal or flexible lead piece) that wraps the flue and overlaps the surrounding tile or waterproof membrane, ensuring rainwater doesn't get in at that joint. A badly fitted flashing, or one degraded by the years, is one of the most frequent causes of localised leaks on roofs with a chimney.
When to check the flashing
Faced with any damp stain near a chimney, or before re-laying an old roof, it is advisable to check the state of the flashing and the height of the flue, since both are points that deteriorate or become out of date over the years.
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