Sulphate attack one of the more common problems that can affect chimney stacks, especially in older properties.  By their very nature, chimney stacks are often the most exposed part of any house and receive the worst of any weather.  This can cause damage to the pointing, brickwork, flashings and pots but there is also a problem that can start from within the chimney flue itself.

The burning of most fuels result in hydroscopic salts being created and in an unlined chimney these salts collect in the mortar between the bricks.  As the name implies, these salts attract moisture, such as builds up inside a stack because of condensation or that soaks through from the outside.  When the salts eventually dry out they crystallise within the mortar and expand, expanding the width of the joint with it.

Although the weather in this country is famously changeable, in general each location has its prevailing winds.  This means each side of a chimney can be in very different condition and a result one side will dry out slower allowing more crystals to form.  This in turn results in a greater degree of expansion and the stack will start to lean, almost like it is being blown by the wind.

The good news is that a leaning stack does not always mean it is unsafe.  The general rule is that a lean of less than 1mm in 100mm of stack height is considered safe although there are no hard and fast rules.  It is perfectly possible for one stack to sit at a greater degree of lean for many years whilst another could collapse straight away.

This is why a good Building Survey carried out by an experienced and qualified surveyor is important when it comes to spotting and understanding the possible affects of sulphate attack at your new home.  Your surveyor should firstly be able to recognise the problem, indicate how serious it is and suggest possible remedies.  With a less involved report, such as an RICS Homebuyers Report, you may not get much more detail than a suggestion that the stack may be unstable.  Although it is good to know if there is a problem it is also important to know how to get it sorted.

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