A chimney lead gutter is a small lead channel fitted against the inside of the part of a masonry flue that stands above the roof, with an outlet discharging to the outside. In Dutch building practice this detail is called a kropgoot and the outlet a kropgat. Usually two or four of these gutters are fitted, facing different directions.
Why it is there
The part of a chimney stack above the roof is fully exposed. Two sources of moisture meet there:
- Driven rain — rain hitting the masonry and working its way in through the joints.
- Condensation — water vapour from the flue gases condensing against the cold inner face of the flue.
Without a collection point, that moisture travels down inside the flue to below roof level. The results are damp patches on the chimney breast in the loft or living space, efflorescence (white salt staining) and, in time, damage to bricks and mortar. The lead gutter catches the water at the transition from above-roof to below-roof and leads it out, above the tiles.
Practical points
You mainly find these gutters in historic, traditionally built masonry stacks. Points to watch during restoration or chimney repairs:
- Let the outlet project slightly. An outlet flush with the brickwork lets water run back down the face; a few millimetres of projection makes it drip clear.
- Discharge above the roof covering. The outlet must clear the tiles — not into the abutment or behind the lead flashings.
- Check the simple things first. Blocked or filled outlets, for instance sealed over during repointing, are a common cause of damp around a chimney.
- Use sheet lead. Lead is malleable, holds its shape and lasts for decades when the detail is properly formed.
If an old stack is fitted with a modern flue liner, the moisture balance changes: condensate is then carried away by the liner. The existing gutters still need to work, though, because moisture continues to penetrate through the masonry itself.
Related terms
- Lead
- Sheet lead
- Lead soaker
- Roof penetration
- Gutter
- Rainwater drainage
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