Preventing or curing dampness is still the biggest, most frequently encountered maintenance problem, and it is essential to find the real trouble before spending money on remedial work.
The four main areas of dampness in buildings are rain penetration, condensation, rising damp and moisture introduction during construction.
Damp can be unsightly and lead to problems later on.
the UK, the ground is almost always damp (to a greater or lesser extent) and
normal building practice includes the provision of some form of damp proof
course to prevent this damp rising to the main structure. It is not uncommon for this damp proof course
to become ‘bridged’ as a result of poor rendering or to become defective with
age. Many old buildings were built
without a damp proof course and if the foundations rest in waterlogged soil,
rising damp can be the result.
Curing the problem involves the installation of a new damp proof course
using either a mechanical barrier (plastic sheet, metal, slate, etc.) or by
impregnating the wall with chemical solution. This will set in the wall and provide a moisture proof zone within the
Rising damp usually carries hygroscopic (moisture
attracting) salts from the ground into the walls and floors. These salts may cause damp patches whenever
the air is moist (through weather conditions, cooking or washing etc). Dampness due to hygroscopic salts is
persistent and will show indefinitely unless the walls are lined or
The slightest defect in the substrate or construction can let water in.
building design and construction is almost primarily aimed at keeping out
rain. To do this effectively needs
considerable attention to detail at all stages, so therefore it is not
surprising that defects in construction often occur. The defect might seem quite trivial, for
example, dropping mortar on a cavity tie – but the result can still be
damp. Slight misfits between window or
door frames and the walls result in moisture penetration.
commonly, rain penetrating into a house, especially where it has been dry for
some years, is the result of something like a slipped tile or flashing
lifting. The cure is usually a repair.
It is also worth noting that in some forms of
construction a damp proof course might be there to prevent moisture penetrating
in a downward direction. In some cases rain might penetrate a wall because the
material used in the construction of the wall has become porous – this is most
common in older buildings with solid wall construction. If so, a cure may be effected by applying a
water-repellent treatment to the outside wall.
Condensation is a common issue.
– where humid air condenses on cold surfaces – often presents the greatest
difficulty in diagnosis. It is often
responsible for surface mould growth on walls, wood and even furniture. When the warm, humid air finds its way into
cold roof spaces even in ordinary domestic dwellings, condensation can raise
the moisture of the timbers to the level where decay can occur.
risk is greatest in unventilated flat roofs, particularly when they are of the
‘cold roof’ type of construction. This
is where any insulating layer is placed immediately above the ceiling and there
is no thermal insulation immediately below the roofing.
The risk of condensation in pitched roofs
is increased by thermal insulation at ceiling level keeping temperatures in the
roof void low; reduction of ventilation within the roof space by blocking eaves
etc with insulation material; water evaporation from uncovered water tanks,
central heating header tanks, etc. In houses the moisture that condenses mainly
originates in either the kitchen or the bathroom, but it can condense anywhere
there is a cold surface – cold bedrooms, on water pipes, cold lintels above
windows etc. The best cure is to provide
ventilation by removing the water vapour at source and so reduce humidity.
Possible causes of dampness
Slightly leaky joints behind bath panels keep floor
Water runs down wall tie that tilts downwards
Cavity bridged by mortar droppings
Earth built up over damp-proof course and airbricks
Damp-proof course bridged by mortar droppings
Damp-proof course punctured by stone in mortar
End of joist touching wall
No damp-proof course under wall plate
Poor construction and lack of or damage to cavity tray
can lead to moisture penetration – water can soak through wall and into the
Poor construction and lack of or damage to cavity tray
can lead to moisture penetration. Moisture can reach inner leaf of wall and
joist above as well as head of window frame
No horizontal damp-proof course in sub-floor plus tile
floor bridges damp-proof course in wall
No through ventilation
Wood on building sites will absorb moisture when exposed to the elements.
Much ‘dampness’ is put into a building
during its construction. Traditional
building methods use considerable quantities of water in concrete, mortar and
plaster. This takes several months to
fully dry out.
Timber skirting or other fittings fastened to wet plaster will absorb
water from the material to which they have been fixed and may then decay. Window frames, roof trusses, pre-fabricated
wall and floor sections, timber for wall and partition studding, floors, etc
must all be stored on site prior to installation. If laid on wet ground or not protected from
the rain they will take in moisture. A
particular hazard of timber frame building is where the timber frame, often
including floor timbers, is exposed to heavy rain before the roof is complete.