Water ingress due to failed flashing around the chimney

Flashing is a kind of lead skirt around the base of the chimney and is there to make a watertight seal between the masonry of the chimney and the roof tiles so that rainwater running down the tiles and the chimney does not leak into the loft and ceiling below. The most common area for water ingress is behind the chimney where water is running down the pitched roof into the chimney. This part of the flashing is called the back gutter. Flashing goes all the way round chimney and is made from overlapping sections of lead.

Most of us don’t spend all our time in the loft looking out for water ingress, so often the first sign will be when we notice damp patches on the ceiling around the chimney breast. If you notice these then that is definitely the time to investigate the loft.

If you have an issue with flashing it will be obvious because water entering the loft by the flue will be seen almost immediately it starts raining heavily and it will stoop soon after it stops raining. Cure: the lead flashing should be replaced.

Water ingress due to brick saturation and failed DPC

A damp proof course (DPC) for the brickwork of the chimney is also traditionally made from lead and called a tray. This should stop any damp that has penetrated the exposed bricks from seeping down through the bricks, or down the inside of the flue, into the chimney breast below.

In the illustration below you can see the tray has a “skirt” (or apron) on the outside which overlaps the back gutter flashing and goes all the way round the chimney. On the inside it is dressed upwards to stop any water on the inside, however this was often not the case on older houses, many didn’t even have a tray at all. However back in the days when everyone had coal fires it wasn’t so much an issue because the chimneys would dry out due to the heat from the coal fire. However since the advent of central heating this is now much rarer and these areas are often a cause of damp.

Cross section of a 1920s chimney
A simplified cross section of flashing and DPC tray. The weep holes drilled through the bricks or mortar just above the DPC tray allow water to exit into the back gutter and then flow round the chimney and off the roof. Older houses may not have the tray dressed upwards internally.

In an ideal world you might think there is no reason for the lead tray to fail, however if an old property needs roofing repairs to battens or tiles, then very often instead of replacing the old lead they may work with it. This would not be an issue except for the fact that bending it up and down to accommodate new battens and tiles etc. may be enough to weaken or cause splits in lead that otherwise would just sit there for centuries with no problems.

This problem is more common with older pre-war houses that have more porous clay bricks and lime mortar, and is of course particularly an issue where there is a high chance of wind blown rain

Unlike issues with the back gutter flashing, rain saturation and damp course problems often show signs of damp some time after it has started to rain, or even after it has stopped, as it can take time for the water to seep down into the house. This can be made worse by poor ventilation which is very common with houses have capped chimneys and sealed up fireplaces.

Cure: the damp course needs repairing or replacing. If bricks are getting saturated due to a lot of wind blown rain a breathable sealant such as Emperor Masonry Creme can eliminate any moisture soaking down through the bricks themselves (see above re: cavities) and is well worth doing before any structural repairs as there is a good chance this is all that is required (along with insuring adequate ventilation inside the flue).

However, if this doesn’t solve the problem, there are two possible solutions:

  • Have a damp course tray installed which usually involves rebuilding the chimney.
  • Demolish the chimney and tile over.

NB: although external inspection should reveal failed flashing, but may not show any obvious issues with the damp course. Many roofers will only guarantee a cure if they replace the chimney rather than trying to repair.

Capped chimneys and sealed fireplaces

The problem of brick saturation can be worse especially if the fireplace itself has also been sealed off and the chimney has been capped completely, as there may be very little airflow to help it dry out naturally. It is always a good idea to have some ventilation within the internal chimney breasts in this situation.

Fireplaces can be sealed in a couple of ways:

  • Within the actual chimney in order to retain the fireplace for cosmetic reasons. In this case you should leave a small air gap in the insulation or install a vent in the face of the chimney breast above where it is sealed.
  • By bricking up or plasterboarding over the  fireplace. You should add an air brick or vent just above the skirting.

See draft excluding chimneys.


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