The Anatomy of a Chimney Stack

When assessing chimney repairs and selecting a contractor to carry out work, it is helpful to know what the different parts of a chimney stack are and the role that they perform. A basic understanding will help you to make informed decisions and aid comprehension of the repairs being suggested by your contractor.

The term chimney stack is generally used to describe the external structure of a chimney above the roof line. We’ll start from the top of the chimney stack and work down with a brief explanation of what each part of the chimney is called and what it does.

  1. A chimney cowl is a ventilated cover placed on top of a chimney pot. In working chimneys, cowls prevent smoke from the flue blowing back into the building. Where chimneys are no longer in use, flues should be installed to prevent rainwater entering the structure. They are also a great tool to stop birds and other wildlife from making home in the chimney.
  2. Chimney pots provide ventilation for flues and create additional draft which helps fuel fires with oxygen. In the UK, chimney pots are generally made from clay and as long as they are securely fixed will last many, many years.
  3. Flaunching is the term for the mortar bedding above the top course of brickwork. It is made from a strong mortar mix and has two functions. Firstly, it ensures that rainwater is diverted away from the stack as quickly as possible before it can soak into the brickwork below. Second, flaunching holds chimney pots in place securely. Chimney flaunching is prone to cracking and should be repaired as soon as signs of damage are evident.
  4. The main part of the chimney stack structure is made up of bricks and mortar. Mortar especially is of comparatively low strength and often cracks. When cracks in mortar appear, chimney repointing should be carried out immediately to prevent serios water ingress and damage to bricks that could lead to structural issues, leaving a chimney rebuild as the only option.
  5. Flashings is a collective term for the weathering of chimney stacks where an abutment occurs, most commonly with a roof. Preferably formed from sheet lead but sometimes from tin and even mortar only, they are hugely important in the prevention of chimney leaks. Flashing must be installed to all sides of a chimney stack to avoid water ingress and should be checked regularly for defects.

When considering chimney repairs it’s helpful to be aware of what the components of a chimney stack are and what they do. We hope that this brief introduction will encourage you to look into the subject more and make informed decisions when it comes to chimney repair.

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