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Property Defects – Chimneys and Flues

Friday, 21st January 2011 | by: Peter Barry

When carrying out a building survey we usually start by looking at the exterior of the building and work my way down from the top to the bottom of the structure. One of the first things that we usually see are the chimney stacks.

Chimney stacks have been a common feature of the London roofscape since the mid 17th Century and by the 19th Century most London terraces have exhibited a virtual forest of chimney pots as almost every room in a Victorian house had its own fireplace requiring a flue and chimney.

Multiple chimney stacksBy the 1960’s and 1970’s open fireplaces had largely fallen out of use and many fireplaces were removed or bricked up.  As the chimneys were no longer in use they were often neglected and received little or no maintenance.  In more recent years open fires have come back into fashion and now we find that many potential purchasers want to know if they can reinstate the fireplaces to a period property. Consequently we pay particular attention to the condition of chimneys and flues when we carry out a survey.

The vast majority of chimneys and flues are constructed in brickwork and finished with a terracotta chimney pot. The chimneys and pots can be very plain and functional or, in some cases, extremely ornate.  The more ornate flues were generally confined to ‘up market’ properties and were built using purpose made bricks. It goes without saying that these ornate chimney stacks will now be very expensive to repair and maintain. The photographs below show chimneys at both ends of the range.

The left-hand photograph below is a typical domestic Victorian chimney, complete with modern TV aerials, whilst the right-hand photograph is an extreme example of a highly ornate stack built with purpose made clay bricks. Fortunately for most house holders the left-hand stack is very common and the right-hand stack is relatively rare.

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We would say at this stage all chimney repairs tend to be relatively expensive as, due to Health and Safety legislation it will almost always be necessary to erect scaffolding to carry out chimney repairs. In an ideal world chimney repairs would be carried out in conjunction with other works that would require scaffolding, such as roof repairs or external decorations. This is a good reason to consider having either a Building Survey or a Homebuyers Report carried out when you are buying a property, as the Surveyor can often identify repairs that will be required in the future.

Chimneys are by their nature exposed to the elements and receive the full force of any adverse weather such as rain, snow and frost. They are also exposed to intense heating and cooling (if the flues are in use) condensation and chemical reactions from hot flue gases. Typically older chimneys were built using relatively soft, porous clay bricks and lime mortar.  The brickwork and mortar is particularly susceptible to frost damage which often results in erosion and ‘spalling’ of the brickwork. Porous brickwork absorbs moisture which freezes and expands in cold weather and forces off the exterior face of the brick (known as spalling). This is often repaired on a temporary basis using modern cement mortar which often makes the problem worse. The only proper solution is to cut out and replace any badly damaged bricks.

Weathred Victorian Chimney StackThe photograph on the left shows a typical weathered Victorian chimney stack with signs of spalling adjacent to the television aerials. Some patch repair has been carried out to the mortar pointing in the past, but, in the longer term, more extensive repairs will have to be carried out including cutting out and replacing the badly ‘spalled’ bricks.

This type of work should only be carried out by an experienced bricklayer as it is important that the right materials and techniques are used.

Generally with older properties the brickwork is laid in lime mortar which, as the name suggests, consists of a mixture of lime, building sand and water. Modern brickwork is built using cement mortar which consists of a mixture of Portland cement, building sand and water. Cement mortar is much harder and much less porous than lime mortar. Many inexperienced bricklayers will carry out repairs to old buildings using modern cement mortar on the basis that ‘harder’ mortar must be better than ‘soft’ lime mortar and will last longer. This, unfortunately, is not the case. Cement mortar in most cases is not compatible with relatively soft, porous Victorian brickwork and will often cause more damage to the brickwork. If you look closely at the photograph above you will see that worst damage to the brickwork is actually in an area that has been re-pointed using cement mortar.

The next area that we would like to look at are the chimney pots themselves and how these are attached to the stack.  Basically, chimney pots are most commonly terracotta (very similar to garden flower pots) and are relatively weatherproof.  They can be quite plain, as in the photo above, or very ornate. Chimney pots are not really ‘attached’ to the stack in the strict sense of the word. They literally sit on top of the brickwork and are kept in place by a bedding of mortar known as ‘flaunching’. In older properties the flaunching is usually lime mortar and will have deteriorated over the years. The pots themselves may still be sound but they are often not secure as the flaunching may have cracked and fallen away. Unfortunately, the condition of the flaunching usually can’t been seen from ground level and Surveyors will often recommend that it is checked when repairs are being carried out to roofs and gutters.

In extreme cases insecure chimney pots could topple in high winds resulting in damage to property or injuries to passers by. It is, therefore, extremely important that the condition of chimney pots and flaunching is checked on a regular basis. Replacement terracotta chimney pots are now quite readily available from most reclamation yards.

Another common defect found in chimney stacks is that they are leaning (i.e. not vertical). This is a relatively common defect particularly if the stacks are relatively tall and slender.  The technical reason usually given for stacks ‘leaning’ is that one side of the stack is more exposed to the prevailing wind than the other side. The theory is that the side of the stack exposed to the prevailing wind will dry out more quickly than the other side.  The mortar joints on the ‘dry’ side will tend to shrink whilst the joints on the ‘wetter’ side will tend to expand.  As we say, that’s the theory; in practice chimney stacks almost always tend to lean in towards the centre of the building.

Leaning chimney stackThe photograph to the left shows a typical end-terrace Victorian house with the chimney stack on the gable wall ‘leaning’ into the building. In this particular case the inward lean was not considered severe enough to warrant any major works, but the owner was advised to continue to monitor the situation.

In an extreme case the chimney stack would have to be taken down and rebuilt. As before, this type of work should only be done by an experienced bricklayer, using traditional materials and techniques.
From looking at the photos of the chimney stacks you may have noted that a common feature is that almost every chimney stack seems to have a television aerial attached to it. Usually the chimney stack will be the highest point of a building and the best reception will be obtained by attaching the aerial to the stack.

The aerial will normally be attached with metal brackets or straps and the fixings should be checked periodically to make sure that the aerial is secure and that the metal fixings have not corroded or come adrift.
We mentioned earlier that chimneys are subject to chemical reactions from the flue gases. The flue gases contain sulphates which, when combined with moisture in the flue, due to either moisture penetration or condensation, produce a weak sulphuric acid which can weaken the brickwork and mortar.

We think that you can see from this blog that something as seemingly simple as a chimney stack raises a huge number of potential issues. In the next blog we will deal with some of the other issues relating to chimneys and flues, such as the removal of chimney breasts and the practicalities of using the existing flues.