5 Common Sources of Chimney Leaks

Constantly exposed to the elements, chimney stacks are a common source of leaks and other problems when not properly maintained. Here are five of the most common areas where chimneys fail with an explanation of what each of the components are and the role they perform in preventing chimney leaks.

1. Defective flashings
Flashing is the general term for weathering joints at penetrations and abutments. For domestic chimneys, these are typically formed from sheet lead because of its durability and flexibility. In older properties and where temporary, low-cost repairs have been carried out, a mortar fillet is sometimes applied instead of lead. The effectiveness of mortar fillets for weathering chimneys is often very short-lived because the material is brittle and prone to cracking. Replacement of mortar fillets with lead flashings is highly recommended in almost all cases to ensure that chimneys are watertight.

A range of lead flashings are combined at different areas of a chimney stack to offer protection against leaks.

Chimney Front Apron: In pitched roofs, aprons and installed at the front of a chimney stack, chased into the chimney brickwork, and dressed down over roof tiles in front of the stack to ensure that water does not penetrate beneath.

Chimney Back Gutter: Installed behind a chimney stack, the job of a lead back gutter is to collect rainwater from the roof behind the chimney and divert it around the stack and down the roof.

Chimney Saddle: A lead saddle is used to weather abutments where a chimney stack meets a ridge line.

Chimney Step Flashing: Step flashings are installed at the sides of a chimney stack to weather the joint where the stack meets roofs tiles. Step flashings are chased into brickwork mortar joints, forming a step like appearance as the name suggests.

Installed correctly, lead flashings can be effective against leaks for 100 years and most issues arise from poor and incorrect installation. Flashings should be checked periodically to ensure that they remain securely fixed. Problem often arises when lead flashings are pointed with mortar which is prone to cracking. Consider replacing mortar joints with lead sealant for a more durable long-lasting joint.

2. Damaged Bricks & Missing Mortar Pointing
Constant exposure to wind and rain inevitably takes its toll on the weakest part of the chimney stack structure which is invariably the mortar joints between bricks. Missing chimney pointing allows water into gaps which is absorbed by porous masonry. Fluctuations in temperature cause freezing and thawing of water, causing expansion which leads to cracking of the brick face brick known as spalling which can lead to further water ingress and potential structural issues. Monitor chimney stacks regularly to check the condition of the pointing. Chimney repointing is a cost effective treatment to prolong the life of a chimney stack and avoiding more expensive repairs.

3. Defective Flaunching
Flaunching is the mortar bedding at the top of a chimney stack into which pots are fixed. Made from a strong cement mix, chimney flaunching also serves as a waterproof barrier to quickly expel water from the top of a chimney stack and prevent leaks. Unfortunately, flaunching is difficult to check without easy access to the roof and cracking is not uncommon. With regular inspection, small cracks in chimney flaunching can be repaired cost effectively using epoxy resin liquid fillers before they develop into larger cracks that require complete replacement of flaunching.

4. Uncovered chimney pots
A chimney cowl is a ventilated cap installed at the top of a chimney pot, traditionally made from clay and more recently, metals including aluminium and stainless steel. The main purpose of a chimney cowl is to prevent down draught which can allow smoke from a working chimney to be blown back into a building. Cowls also prevent wildlife from entering via the chimney pot and nesting in the flue. Finally, and importantly in the context of this article, chimney cowls provide protection against rain entering via open chimney pots which is a problem in disused chimneys which lack the heat of fires to dry the chimney out.

5. Condensation
Last on the list but by no means the least common is condensation, which is a particular problem in disused chimneys where there is a lack heat from fires to warm a chimney's inner surfaces. It is critical that unused chimneys are adequately ventilated to allow circulation of air within the void. Avoid capping disused chimneys and use ventilated cowls instead. Combined with internal vents in the centre of chimney breasts internally, this should provide enough airflow to avoid the build up of condensation.

Regular checks and maintenance of chimney stacks is essential for avoiding costly repairs. Left unchecked, minor issues including gaps in mortar pointing and cracks in flaunchingā?Ts can develop into structural issues that require chimney stacks to be rebuilt completely. Removal of chimney stacks is included under permitted development (except for conservation areas where approval must be sought) and is often a good option for disused chimneys to prevent the problems associated with this part of a building for good.

Article kindly provided by retainingwallcentralcoast.com