A Surveyor’s View for Kent & East Sussex Homeowners and Purchasers

As RICS Building Surveyors, we regularly encounter a range of issues affecting properties across Kent and East Sussex, particularly those charming older homes that form so much of our region’s character. Among the more insidious and often misunderstood problems is “acid (sulphate) attack” on chimneys. This silent chemical process can slowly but surely compromise the integrity of your chimney stack, leading to significant structural concerns and costly repairs if left unchecked.

So, what exactly is sulphate attack, why is it prevalent in older chimneys, and what should you, as a homeowner or prospective buyer in our historic counties, be looking out for?

The Chemistry of Deterioration: What Causes Sulphate Attack?

Sulphate attack is a chemical reaction that primarily affects the mortar and, less commonly, the bricks within a chimney. It’s a complex process, but here’s the simplified explanation:

1. Combustion By-products: When fuels like coal, wood, or even gas are burned, they produce acidic flue gases, including sulphur dioxide.
2. Moisture & Condensation: These gases, especially in older, unlined chimneys, can condense on cooler internal surfaces of the flue. This moisture combines with the acidic by-products to form sulphuric acid.
3. Hygroscopic Salts: Over time, these acidic compounds react with the calcium hydroxide in the cement mortar (or with sulphates naturally present in some clay bricks) to form gypsum and other expansive sulphate compounds. These new compounds are “hygroscopic,” meaning they attract and absorb moisture from the air or from within the masonry.
4. Crystallisation & Expansion: As the masonry dries out, these salts crystallise and expand within the mortar joints. This expansion exerts immense pressure on the surrounding brickwork, leading to gradual deterioration.

Chimneys are particularly vulnerable because they are exposed to the elements externally and corrosive flue gases internally. This makes them a prime location for the conditions necessary for sulphate attack to flourish.

Recognising the Red Flags: Signs of Sulphate Attack

Identifying sulphate attack early is crucial for mitigating damage. As experienced surveyors, we look for several key indicators. For instance, a typical 1930s semi-detached property we recently surveyed presented clear signs of sulphate attack:

• White Salt Deposits (Efflorescence): While efflorescence can have other causes, persistent white, powdery salt deposits on the external brickwork or internal chimney breasts can be a sign of sulphate migration.
• Staining and Damp Patches: On the 1930s semi-detached property, we observed significant brownish staining and damp patches on the internal chimney breast, particularly within the loft space. This type of severe discolouration and moisture ingress is a strong indicator of sulphate contamination.
• Crumbling or Eroding Mortar: The mortar joints within the affected chimney breast were noticeably soft, powdery, and showed severe erosion. This extensive crumbling, revealing the underlying brickwork, was a direct consequence of the sulphate expansion.
• Cracking and Expansion: The chimney breast in the roof space of this 1930s semi-detached home exhibited pronounced cracking, including diagonal and horizontal cracks along the mortar joints and across the face of the breast. This indicated the powerful expansive forces at play, causing the masonry to ‘jack up’ and displace.
• Damaged Flaunching and Water Ingress: Critically, on the external stack of this property, the flaunching (the concrete weatherproofing at the top of the stack, designed to shed water away from the flue) was severely cracked and breaking apart. This deterioration of the flaunching was allowing a significant amount of rainwater to penetrate directly into the chimney structure, exacerbating damp and sulphate attack within the stack and chimney breast below.
• Bulging or Spalling Brickwork: On the external stack, several bricks were visibly spalling – their faces detaching – and sections of the masonry appeared to be bulging outwards due to the internal pressure. This type of damage, frequently captured in close-up survey images, clearly illustrates the physical breakdown of the brickwork, often intensified by water ingress through defective flaunching.

The Impact on Your Home and Wallet

Beyond the aesthetic damage, sulphate attack can have serious implications:

• Structural Instability: The progressive weakening of mortar joints can compromise the entire chimney structure, potentially leading to partial collapse.
• Mortgageability Issues: Properties with active or severe sulphate attack can be flagged by mortgage lenders as high-risk, potentially making it difficult to secure a loan.
• Gas Leaks: If internal flue integrity is compromised, there is a risk of dangerous flue gases escaping into the property.
• Costly Repairs: Remedial work often involves taking down and rebuilding affected sections of the chimney, installing flue liners, and using specialist sulphate-resisting mortars, all of which can be a significant expense.

Your Essential Protection: An RICS Level 3 Building Survey

For any homeowner or prospective purchaser of an older property in Kent or East Sussex, particularly those considering a period home or a bungalow with its exposed chimney, an RICS Home Survey Level 3 (Building Survey) is paramount.

Our detailed surveys go beyond a superficial inspection. We meticulously examine the chimney stack, both externally and internally where possible, looking for the tell-tale signs of sulphate attack and other common chimney defects. We provide:

• Clear Identification: A precise diagnosis of any issues found.
• Severity Assessment: An understanding of how serious the problem is.
• Remedial Recommendations: Practical advice on necessary repairs and estimated costs.
• Impact on Value/Mortgageability: An assessment of how defects might affect the property’s value or your ability to secure a mortgage.

While local weather patterns in Kent and East Sussex contribute to general wear and tear on chimneys, the underlying issue of sulphate attack is often due to the original construction methods and the lack of modern flue linings.

A crucial preventative measure for maintaining chimney health is to ensure that any disused flues are properly capped and ventilated. This simple step prevents the build-up of cold, damp air within the flue, which can lead to condensation and contribute to both damp issues and the conditions necessary for sulphate attack to develop or worsen.

Conclusion

A chimney is more than just a decorative feature; it’s a critical structural and functional element of your home. Ignoring the subtle signs of acid (sulphate) attack can lead to escalating problems and unforeseen expenses. By investing in a professional RICS Building Survey, you gain invaluable insight into the true condition of your chimney and the wider property, allowing you to make informed decisions and protect your investment.

If you have concerns about your chimney, or are considering purchasing an older property in Kent or East Sussex, don’t hesitate to contact Weald and City Surveyors Limited for free initial advice and guidance regarding specific defect reports or pre-purchase home surveys. sulphate attack signs