The following points apply to all older chimneys.
Older chimneys were constructed without horizontal damp proof membranes at or just above the roof line or below the chimney base. Any damp that enters into the chimney stack from those areas that are proud of the property (eg. through the sides, flaunching or pot) will tend to soak down into a property. These chimneys can also suffer from rising damp and attract condensating damp as a cold bridge. This can cause problems with walls, roof timbers and the internal environment.
Chimney stacks built of brick, block or stone and their pointing and rendering can all become porous over time. Many chimney stacks, however, limit the amount of damp entry into the property by maximising evaporation therefore removal of the damp and / or reducing the level of damp entry.
The level of damp entry from old chimneys can often be reduced but can rarely be completely prevented. It is very important therefore to make sure that this damp can escape rather than becoming a source of problems internally.
Many chimney stacks contain flues leading from a number of fireplaces. We recommend that you assess which fireplaces and which flues will be used at the subject property.
All unused flues should be covered and vented externally and vented from within the property. These can sometimes be vented into the roof space if the roof space is sufficiently well vented. It is important to allow ventilation at chimney pot level as well as at room level and to prevent direct rainwater entry or birds nesting.
This would prevent or at least limit any direct rainwater ingress whilst allowing any moisture that has soaked through into the chimney stack to evaporate through a continuous slow flow of warm air through the stack itself.
When the brickwork, blockwork or pointing becomes porous, the damp needs to be able to escape rather than descend through gravitational force to adversely affect the adjoining roof timbers, internal environment or other aspects of the property.
Whilst impermeable materials are acceptable to prevent penetration on the top flaunching, the sides should be allowed to breathe and should not be covered with any coating that prevents evaporation of moisture that may have already entered into the chimney stack.
For some years builders have rendered chimney stacks in hard Portland cement based renders to prevent moisture entering into the stack especially as a repair if the bricks / stack have already become porous. This render may become porous with time and lack of decoration thereby allowing penetration of moisture but preventing easy evaporation.
It should also be borne in mind that if a chimney is not to be used, it may be appropriate to take a chimney stack down below the roof surface and thereby remove a weak point in the structure. Legal permission to do this may be required, (e.g. listed building consent etc.).
There are certain aspects of minor repairs that are likely to be required to the flues and stacks of the majority of such properties. The precise extent will depend upon how the occupier(s) wish to live at the property and how the occupier(s) intend to use the flues.
All flues should be swept and checked in the Autumn before use.
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