Chimney Damp Gravimetrics

Rising damp on a chimney breast!

Chimney breast damp is very common, particularly in homes built before the midpoint of the 20th century. Often, dampness evident on a chimney breast surface may lead a building owner or surveyor to believe weather tightness at roof level must be an issue. This may lead to roofing repairs such as tile replacements, lead work and chimney re-pointing. Chimney damp may often be resolved by identifying such penetrating damp issues however in some cases, wall surface moisture may be as a result of a far more complex issue.

The long term burning of fossil fuels such as coal, can lead to the migration of salts throughout chimney breast masonry. Such salts may include chlorides, nitrates and ammonium-sulphate. These salts are ‘hygroscopic’ which means they can effectively withdraw moisture from the air. The effect of this may lead to surfaces appearing damp. It is quite common for chimney breasts to have isolated areas of wall surfaces which only appear to be ‘damp’ & not show any signs of staining or paint spoiling. This is because moisture is withdrawing from the atmosphere into the breast rather than being pushed out.

Experienced surveyors may understand these principles however due to the possibilities of alternative damp sources, there must be an intrusive aspect of inspection to conclude with an accurate diagnosis. Conclusive evidence of a diagnosis is an important aspect of our investigations to enable the correct recommendation for remedial action. We often take mortar or brick samples to establish how much of a total moisture content is hygroscopic moisture (salt) and how much is free moisture (actual water). The only way to practically obtain this information is via gravimetric sampling, in a scientific laboratory environment.

The pie chart below is an example of how much hygroscopic moisture can be found within a mortar sample in respect to actual free moisture. This is normally what we would expect to see when investigating chimney damp. If the free moisture (blue) was higher than the hygroscopic content (red) then this would indicate an alternate source of moisture such as penetrating or rising damp. If the hygroscopic moisture (red) was higher than the free moisture (blue), we could conclude the main source of dampness is salt contamination. We understand how to interpret results, which can lead to a very accurate diagnosis of moisture source.

Although the most common sources of damp on chimney breasts are in fact salt or rainwater penetration related, we do come across cases of rising damp. The photographs below relate to an investigation of evident dampness on a chimney breast and its reveals. The wall surface at low level indicated what looked to be a case of moisture ingress from below. This can often be determined by the pattern or form in which decorative surfaces are spoilt. Paint bubbling can often suggest trapped moisture within a wall which subsequently breaks the surface via evaporation. 

We believe a holistic approach should be adopted towards all damp diagnosis inspections therefore an educated guess of the source of moisture, in this instance would not suffice. The adjacent exterior wall to the chimney breast sparked cause for concern in respect to rising damp. In short, the wall had been rendered to the floor, subsequently bridging the damp proof course. As well as this, a drain stood in close proximity to the affected area. The drain was tested with a hosepipe & the gully simply filled to the top and saturated the soil surrounding. A combination of classic rising damp related defects and deficiencies. 

So, was the chimney breast damp a result of penetrating moisture, rising damp or salt? The only way to distinguish between free moisture and hygroscopic moisture is via a method we use called gravimetric analysis. This can sometimes be referred to as the ‘oven dry’ method which involves taking mortar/wall samples in increments, to build up a profile of moisture. The graph below includes the results from this analysis which proves evidence of great free moisture content at the base of the wall & a minimal hygroscopic content. The predominance of moisture at the base of the wall and the lack of hygroscopic content suggests that the source of damp is working in a mechanism known as rising damp. In this instance, the chimney breast  included a low hygroscopic salt contnent. Displaying this evidence alongside the typical rising damp defects and deficiency’s identified, allowed me to provide accurate remedial advice.

This example is testament to the fact that all cases of damp should be treated individually & the only way to obtain conclusive evidence of a moisture source is to engage in intrusive methodology. O.C