Chimney, Chimney breast & Flue Survey and The Party Wall Act

The property was built with Chimney breasts which passed through the rooms above into the loft and emerged as chimney stacks above the roof.  The chimneys would be supported off the natural foundations.

The chimneys are a structural part of the house and should not be altered without professional advice, building regulation approval and party wall notices agreements and awards.

The chimneys will be toothed into the party wall and are therefore subject to the party wall act.  The Flues are enclosed within this brickwork and de facto I would not include them within the requirements of the party wall act and if i was the responsible surveyor flue liners could be installed outwith the act.

The flues are brick built and would have lime mortar.  This lime mortar may have decayed and allow combustion gasses to permeate through the flues which is known as sulphate attack and can often be seen visibly as a bowing stack often bowing in the direction of the prevailing wind and rain.

This sulphate attack occurs when soluble sulphates react with calcium compounds in lime mortar. The resulting chemical reaction creates expansive minerals—such as gypsum and ettringite—causing the mortar to swell, crack, and crumble.  This process requires moisture to transport the sulphates. 

The Key Chemical Reactions are:

Gypsum Formation: Sulphate ions react with hydrated lime (calcium hydroxide) to form gypsum. This causes softening and dissolution of the mortar.

Ettringite Formation: Sulphates interact with aluminates in the presence of calcium hydroxide to produce ettringite. This reaction can cause massive internal volume expansion, leading to structural cracking.

Sulphate Issues can be managed with Fly Ash.  While sulphates are highly damaging, Class F fly ash was often used in mortar and concrete mixes to increase resistance to chemical attack.  It binds with the free calcium hydroxide, limiting the calcium available to react with sulphates.  It reduces the porosity and permeability of the mortar, restricting the movement of aggressive groundwater or moisture.

Sources:  The sulphates responsible for this reaction can come from a few distinct areas:T  he Environment: Groundwater, acid rain, and soil and the Masonry Itself: Certain clay bricks contain high levels of water-soluble sulphates that leach out when wet also from contaminated Fly Ash: Fly ashes with high free lime and unburned sulphur content can sometimes lead to internal sulphate expansion.

If the stack bows beyond limits it requires re-building.

When sulphate attack occurs within the property the combustion exhaust gasses can enter the property and cause death.  

Current regulations encourage flue lining and stoves which is the sensible approach.  It is often very difficult and costly to correctly install the 8 inch flue liner and the required support components required for an open fire and the 5 inch flue liner required for stoves can be easily installed.