The deadly smoker above your home
With homeowners seeking to increase habitable floor space whilst intrinsically maximising the value their home the conversion of loft spaces and removal of internal stacks are becoming ever more prevalent. Our role as Party Wall Surveyor in this instance is to determine the likely impact of the proposed notifiable work on neighbouring properties.
TK Surveying Group recognises that chimneys are one of the most dangerous elements of a property and yet one of the most neglected. Whilst modern methods of construction have obsoleted the chimney by design, the resurgence of open fire places and wood-burning stoves together with over a 100 years of Victorian architecture, many City skylines silhouette a wonderful array of differing pots. Prevailing winds, frost and driving rain are thrust upon the chimneys’ external fabric, whilst chemical corrosion and the effects of extreme heating and cooling impact on its internal.
Exposed to the harsh external elements for over a century..... the beady eyed Party Wall Surveyor often has one eye on the chimney.
In view of the extreme conditions it’s no wonder that this element of the building is left wanting some love and attention. The following is a list of typical defects to chimneys with some generalised remedies. To note, moreso with period and conservation work no two cases are ever the same and each remedy must be adjudicated on its own merit.
Thermal expansion of the flue lining can cause vertical cracks to appear in the structure of the chimney, particularly where the flue is hidden within the thickness of a wall and its presence in the first instance introduces a point of weakness. Where this has been determined as the most likely cause of failure the usual remedy is to install a flue liner which will provide some thermal insulation for the wall structure. The external face and crack can then be re-pointed and made good.
Demolished chimney breasts
Our role as Party Wall Surveyors is to ensure that any demolition is done in accordance with best practise and remaining loads are accounted for with structural engineer calculations and design.
Unfortunately we have encountered many houses and flats which have been poorly converted and often discovered that the chimney-breasts have been removed in create more living space, without removing the stack above. This omission is committed either through idleness or for the more laudable rationale of maintaing the original roofscape. In either instance, if the remaining stack has not been properly accounted for it and the walls below it will be unstable.
Chimney breasts and fireplaces in particular form a focus for the architecture of the interior and can be rebuilt to suit any style. If, however, this is not an option, the solution is usually to introduce supporting beams bearing onto suitable walls or brackets which takes the load of the stack and disperses it through and down the adjoining walls.
Defective pots and flaunching
Chimney pots are held in place by a cementitious bedding of mortar known as flaunching, the flaunching also serves to act as weathering. Moreso due to frost action the flaunching is more susceptible to decay than the pots they support. The remediation being to replace although caution must be taken not to damage the pots.
If your chimney pot is damaged the most economical solution is to replace. There are a wide range of designs available on the market, some manufacturers may also be able to replicate bespoke designs. Likewise, Architectural reclamation can be a good source for replacement pots.
The ingress of moisture however it enters causes multifarious problems for building owners and occupants. The two most likely suspects being the ingress of rain and condensation.
Rain can permeate through defective flashing located between the roof and chimney, through defective or inadequate (cover) brickwork/render to stack wall, or simply by entering the flue from the pot.
Flashings are best formed using lead, however many Victorian and properties had their flashing formed from Zinc or cement mortar fillets which are inferior and prone to failure causing dampness to the rooms below.
Weathered brickwork that has suffered frost damaged (spalled) can be cut out and replaced, localised rebuilding is sometimes necessary where bricks have become loose because of soft mortar. Where there is extreme cracking or erosion the stack may need to be demolished and rebuilt using low sulphate content bricks and sulphate resistant mortar.
Condensation inside the stack often results in damp stains on the chimney and walls of fire places and rooms below. Where still in use the burning of fuel produces water vapour which then condenses onto the cold inner surfaces, naturally, very tall, wide or cold facing walls are more susceptible.
Condensation related damp can be mitigated by introducing a flue liner which adds insulation and reduces its surface area. Even if the chimney is sealed and redundant a cross flow of air from the bottom to top is key to prevent the leeching of damp though brickwork and causing damp and sooty stains to internal finishes.
Open chimney pot and flues exposed to the rain can result in similar defects to those in relation to condensation. The rainwater soaks the internal brickwork and mixes with the old soot. There is a number of caps, hoods or cowls that can be used to not only prevent rain penetration but also provide ventilation.
Good Chimney Design
A stack should be built so that it deflects rainfall away and onto the roof away from the main structure as so to prevent damp and damp related issues occurring within the fabric of the building.
A sailing course projection of no less than 30mm at high level – this deflects rainfall away from the structure beneath
Flaunching should be sloping outwards- located above the sailing course.
Integration of damp proof course to prevent moisture permeating through stack into the body of the structure
Suitable flashing dressed between chimney stack and roof abutment; preferably in lead
The pointing to brickwork should preferably be in a weatherstruck finish.
Whilst not strictly party wall, understanding building pathology and causation of defects is paramount whilst under appointment. Assessing notifiable work and its potential impact and then adjudicating on disputes relating to damage are best placed with experienced practitioners.
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