The Dangers of Sulphate Attack To Chimneys

In older properties, sulphate attack is one of the most common problems affecting chimney stacks. The chimney stacks of most houses are exposed to the worst of the weather, by their very nature, and have been known to take the brunt of most storms and periods of rainfall.  This can lead to damage to the brickwork, the pointing, the flashings, the flaunchings and the chimney pots. There is also a problem that can develop within the chimney flue itself that can lead to further damage.

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What causes sulphate damage?

Typically, when most fuels are burned in a chimney without a liner, hygroscopic salts are created which collect in the mortar between the bricks and which is responsible for the formation of hygroscopic salts. The salts, as their name implies, are capable of attracting moisture, which may be as a result of condensation inside a chimney stack, or moisture that may be absorbed from outside the stack.  There is a time, when the salts will dry out, at which point they will crystallise within the mortar and expand, resulting in also an expansion in the width of the joint that has dried.

While the weather in this country is notoriously unpredictable, each location has its own prevailing wind.  As a result, each side of a chimney can be in very different conditions and as a result, one side of a chimney will dry out faster than the others, resulting in more crystals forming on that side. Consequently, the stack will start to lean, almost as if blown by the wind, as a result of a greater degree of expansion to the one side. 

However, the good news is that it’s not always the case that a leaning stack means it’s unsafe to use.  There are no set rules when it comes to how much lean can be present in a stack, but generally, a lean of less than 1mm per 100mm of stack height is considered to be safe. One stack could sit at a greater degree of lean for many years while another stack could collapse right away regardless of how long it has been leaning. 

The good news

In order to detect and understand the possible effects of sulphate attack at your new home, it is important to have a good Building Survey performed by an experienced and qualified property surveyor.  The first thing that a surveyor should be able to do is to recognize the problem, to indicate the severity of the problem, and to suggest possible solutions.  In the case of a less involved report, such as the RICS Homebuyers Report (HBR), you may not get much more information than the suggestion that the stack may be unstable. Within a Building Survey, further advice would be offered and provided, it is also important to be able to know how to resolve it as soon as possible.