Damp in chimneys

Chartered Surveyor James Bush takes a look at damp in chimneys and what to look for.


A high percentage of building defects in this country result either directly or indirectly through dampness, particularly with regard to damage to plaster, decorations and wet or dry rot developing in adjoining timbers.

In this regard chimney stacks are usually the most exposed elements of a residential building and so are subject to driving rain exacerbated by high winds. This can, over time, lead to the deterioration of the chimney pointings (the mortar that holds the bricks together ) with a subsequent likelihood of penetrating damp occurring within the flue(s).  These photographs are from a recent survey in London Borough of Ealing, North West London.

When fireplaces were in use daily either within the reception rooms and/or bedrooms, the heat generated within the flues during the combustion process would normally dry out the flues thus lessening the incidence of dampness occurring within the flues and affecting the chimney breasts. Nowadays, most chimneys are rarely in use, never mind daily, and so the likelihood of dampness developing within the flues is heightened. Due to their positioning, most householders rarely or regularly inspect their chimney stacks whilst some faces of the stacks may be impossible to see from ground level. These two factors often combine to cause dampness internally.

Additionally, with chimney pots remaining open and the flues being rarely dried, the propensity for downward damp penetration through falling rain is also commonly encountered. Chimney pots should therefore be capped and vented if no longer in use, preferably using a ‘pepperpot’ type of pot that prevents downward rain penetration but allows for ventilation of the flue via holes around its circumference.

The flaunchings (the mortar by which the pots are secured at the top of a stack) also need to be kept in good condition as they are another place where damp penetration can occur. Once light cracks develop, frost can cause them to widen, seeds can become bedded which, when the plant begins to grow, can also widen the cracks and in extreme cases their deterioration can lead to the bedded pots becoming unstable.

Often, the first instances of internal damp are not seen as they develop within loft spaces that are usually rarely visited spaces. The first a homeowner may know of such an issue is when damp stains begin occurring around the bedroom chimney breasts or on the bedroom ceilings where the chimney breasts have been removed below loft level. (See photos).

Repairs can often be costly due to accessibility issues and commonly the expensive use of scaffolding to gain access to repair. Therefore, when buying a house it is important to commission a survey from a reputable About RICS Chartered Surveyors in order to identify such issues before they become future expensive repairs.

James Bush  BSc (Hons) MRICS is a Director of Woodward Chartered Surveyors

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