Investigating Damp Chimney Breasts

Thursday, 29th February 2024

There are four main causes for this unwanted problem:

  1. Eroded pointing
  2. Exterior water leaks at roof flashings
  3. Rainwater coming down inside the chimney pots
  4. Internal Condensation

Gaps may have formed where mortar joints have been eroded by acidic chemicals from combustion gases. Leaks can sometimes go undetected, particularly within lofts. Wherever possible, suspect mortar joints should be made good prior to lining the flue.

The airtightness of a flue can be tested using smoke pellets which are available from DIY stores.  But even where a flue is already lined, old steel liners eventually suffer from corrosion.  This allows damp to enter which then penetrates down through the porous masonry.

Chimney breasts in older properties pose an even greater problem.  The walls of the stack will most likely not have a damp-proof course.  Older chimneys were often fitted with lead trays, designed to prevent water penetration through the stack.  However, over time these trays can corrode.

Modern chimney stacks do have a damp-proof course, usually about 150m above the roof and another near to the head.  Any condensation within the stack can be controlled by installing a suitable flue liner.  However, this can also bring problems, since any exposed pots and flues can allow rainwater to find its way straight down the chute and puddles will form in the fireplace.

The answer is capping.  There are a wide range of caps, cowls, and hoods on the market which protect pots and flues.  Even if your fireplace has been sealed up and is no longer in use, it is advisable to have a flow of air to prevent condensation. Air bricks inserted in the stack wall will also help.

Fuel Types:

Whatever fuel you choose to burn, it will produce water vapour.  This turns to moisture when it hits cold surfaces.  The amount of moisture is influenced by different factors, such as the position of the stack or its size.  Problems can be greater if the stack is located on a cold outer wall, or if it is particularly tall.

The amount of moisture produced is also influenced by the type of fuel you use.  Freshly cut wood can be particularly wet and will give off an excess of water vapour.  Once this vapour mixes with the soot inside the stack, moisture can seep through your plasterwork, thus leaving the tell-tale staining.

The use of such fuels can also cause the rapid build-up of tar and other resinous deposits on the inside the flue. Flexible flue liners can be difficult to keep clean, so try burning smokeless fuel in a very hot fire.  This may help to burn such deposits away.

Flues need to be swept annually to remove any soot deposits and possible blockages.  You can do this yourself, with the necessary rods, but if you choose a sweeper to do the job for you, make sure they are a member of NACS (National Association of Chimney Sweeps), HETAS approved, or a member of the Guild of Master Sweeps.