What Happens When Chimney Flashing Fails?

Where the Water Goes First

A failed flashing lets water into the roof at the point where the stack passes through it. What happens next depends on which of the four pieces of lead has gone.

If the back gutter above the stack has failed or silted, water goes into the roof on the uphill side and runs down the rafters. That is the worst of the four, because the back gutter is catching everything coming down the slope above the chimney.

If a side flashing has pulled out of its chase, water runs down the face of the stack and gets in wherever the lead stops.

If the front apron below the stack has split, water gets under the covering below it and runs down the slope inside.

And if the failure is not the lead at all but the masonry - eroded pointing or spalled brick on the weather face - the water is inside the stack itself and comes down the flue or through the breast.

Stage One: Weeks to a Season

Damp timber around the stack. On a Neston or Parkgate property with an older roof, and particularly on the unlined slate roofs common here, there is no underlay to catch it, so it goes straight onto rafters and battens.

Nothing shows inside yet. That is the point of this page: the expensive part of a chimney leak is almost entirely invisible for the first few months.

Wet insulation in the roof space, if there is any near the stack. Mineral wool that has been soaked compresses and does not recover, so it stops insulating even after it feels dry.

Stage Two: One to Three Winters

The battens around the stack soften. That matters more than it sounds, because the slates or tiles in that area are nailed into them, so the covering around the chimney starts to slip - and now you have a second, apparently unrelated problem.

Staining appears on the ceiling near the breast, or on the breast itself at the top floor.

Rafter tops begin to decay if the roof space is not ventilating well. Timber needs to sit above roughly twenty per cent moisture content for decay fungi to establish, and a persistent chimney leak in a poorly ventilated loft achieves that comfortably.

Stage Three: The Chimney Breast

This is the stage people actually notice, and it is the one that costs the most to put right relative to the roofing.

A chimney breast that has been wet repeatedly develops a damp patch that comes and goes with the weather. Then it starts to blow the plaster - the surface bubbles, the paint lifts, and a brown or yellowish stain comes through however many coats go over it.

On an old flue there is a specific reason for that. Soot deposits contain hygroscopic salts, which draw moisture out of the air. Once water has carried those salts into the plaster, the plaster itself absorbs damp from the room whenever the humidity rises - so the patch keeps reappearing long after the leak has been fixed.

That is why fixing the roof does not make the stain go away. The affected plaster usually has to come off and be replaced with a salt-resistant render or a membrane, and that is a plastering job, not a roofing one.

The Arithmetic

Renewing a chimney flashing is a defined job with a defined cost, and the biggest single element of it is usually getting safe access rather than the lead.

Renewing a chimney flashing plus a strip of battens plus insulation plus a ceiling plus hacking off and replastering a chimney breast is several trades and several times the number.

None of the extra work is roofing. That is the whole case for dealing with a chimney leak in the season you notice it rather than the season after.