What is rising dampness ?
Most building materials which are used for walls are porous,
that is, they will absorb water if they are in contact with it. This is because
the materials are not solid, but contain many very small holes called pores
and capillaries
into which water can pass. As water
spreads through porous materials it draws more water along behind it - even
against the force of gravity - in much the same way that a kitchen towel will
absorb a small spill of water if just one edge of it is dipped into the water.
This mechanism means that a wall in contact with wet ground
will absorb water and the water will also pass up the walls - this is rising
damp. Rising dampness is the
result of capillarity, this being the
process in which water rises up the very fine tubes formed by the pores.
As the water passes up the wall, it evaporates away from
the surface at a rate mainly depending on the temperature and type of wall
covering. Eventually the amount of
water passing into the wall is balanced by the amount which can evaporate and
the water does not reach any higher up the wall. This may result in a tide
mark being seen across the wall - below it the wall is constantly damp, but
above it is relatively dry. The height of the tide mark depends on the dampness
of the ground and how quickly water can evaporate from the wall. If the wall is coated with a water resistant covering, such as tiles,
gloss paint or vinyl paper, the damp may reach much higher before it can
evaporate. Rising damp rarely rises above a height of 1 metre above the external
ground level and / or the internal solid floor level.
Often the rising water will carry salts into the
wall from the ground. These can
react with plaster or brickwork and a deposit of crumbly white crystals may be
seen on the surface. They can be
brushed off, and may build up again, and affected plaster will eventually perish, becoming soft and
falling off.
Rising damp can be most costly when timber floors are
affected. In older houses floor
joists are often seated directly into walls, with little or no protection from
the dampness. Joists are usually supported directly on to the physical damp
proof course in more modern building constructions. Where rising dampness
exists, masonry, on which the timber joists are supported, becomes wet. Eventually damp wood will become
infected with wood decaying fungi such as wet rot or dry rot, and may also
become attacked by wood boring beetles. These
cause complete breakdown of the structure of the wood and the floor may
eventually collapse. This can occur over a short period, or take many years,
depending on the degree and speed of development of the dampness and the
resultant fungal or insect attack.
Joists can be protected by chemical treatments and wrapping
the ends in a damp-proof membrane before inserting them into a damp wall. (Even a wall which has been damp-proofed will still contain some residual
moisture and it is important that new replacement timbers are also treated in
this way.) Better than chemical treatments, is to ensure joist ends remain dry -
but this is not always possible.
Treating Rising Damp - Damp-Proof Courses
Old buildings were constructed with little or no protection
from rising damp, but in more recent times rising damp has been prevented by
inserting a layer of water-proof material into the wall as it was built. This was often slate, poured bitumen or bituminous felt, but nowadays is
most likely to be a layer of PVC. Although
some of these have a long life, it is possible for these substances to perish and allow water
through. This is not the case in modern PVC damp-proof courses.
Luckily, it is possible to damp-proof a building without
dismantling it. The main methods used today are chemical injection, mortar
injection, electro-osmosis and sometimes the insertion of physical damp-proof courses or other
types such as evaporative clay pots set in to walls. (Insertion of physical
damp-proof courses and the clay pot method are not discussed further here). Chemical injection is the most suitable and cost effective method for
solid brick and cavity brick walls. The other methods are necessary for thicker stone
and rubble-filled walls. They may also be suitable for breeze block walls, which
are difficult to treat because of their open structure and brittle nature when
drilled.
Chemical Injection
This method involves drilling 10 mm holes into the wall at
approximately 100 mm intervals and injecting a solvent or water-based chemical into the
wall under pressure, until the wall material is soaked with the chemical. The chemical layer then controls water rising past it. There is also a
newer system involving the injection of a damp-proofing 'cream' into 12 mm
holes. Here the cream in injected into the wall from a caulking gun via a long
nozzle. Solvent based materials have largely been replaced by water-based
fluids (primarily on safety grounds - solvent damp proofing chemicals are
flammable).
With this type of water-based or cream chemical the injection is
carried out into the mortar layer between the bricks (solvent-based chemicals
may be injected into the bricks themselves). This is usually at a perpendicular
joint to give approximately 2 injection holes per standard brick width. This is a spacing
of about 110 mm. The holes on the outside of the property are filled, but those in
internal walls are usually left open.
If possible external walls are injected from both sides to
ensure complete penetration of the chemical, but where access makes this
impossible the wall is double drilled from one side, with each hole being
drilled and injected, and then drilled deeper and re-injected to ensure the
chemical reaches all parts of the wall. This is sometimes called double drilling
/ double injecting the wall. An advantage of the cream system is that it is
designed to treat thicker brick walls and can be inserted from one side only via deep drilled holes.
Where fluids are used, it is important that the drilled surfaces of the wall are
visible when injecting - so any plaster or render must first be removed,
otherwise the chemical may be injected down between the wall and its coating
instead of into the wall itself. Seeing the injection point also helps to
determine optimal saturation of the wall area by the injected chemical. Plasters
and renders should be permanently removed in the area of the damp proof course
so that it is not bridged.
Cream-type damp proof course injection has become the standard for the
industry, because of the reliability, ease, and quickness of installation. Now
we use mostly these.
The height of the new damp-proof course in relation to
external ground levels and any floor timbers is also very important to ensure
that floors are protected from dampness - see the section Bridging Of Damp-Proof Courses below. Further information about this
can be found from British Standard 6576:1985 (not reproduced in these web
pages).
Mortar Injection
This is a similar technique to cream chemical injection,
suitable for treating thick stone walls where rising damp is largely carried by the
mortar beds and rubble filling. Larger 20 mm holes are drilled at about 100 mm
intervals and the damp-proofing agent is a pore-blocking chemical contained in a
fine sand / cement mortar. The damp-proofing agent is carried
into the porous mortar beds and rubble by the rising damp, where it crystallises
to form an impermeable barrier to control subsequent water rise. The drilling is
more time-consuming and the materials more expensive than the liquid or cream chemical injection, but the results are much more satisfactory for these types
of stone walls. Mortar injection can also be used in conventional solid 225 mm or cavity 275 mm
brick constructed walls.
Electro-Osmosis
This is a very different technique from the previous two,
and works by the effect a small electric current has on salt-contaminated water.
A channel is cut horizontally along the wall, with deeper holes drilled
intermittently along it. A metal
(titanium) wire with special terminals (platinum coated anodes, positive
connection) is set into the
channel and mortared over, and this is connected to a ground positioned earth
rod (the cathode, negative connection) and a small electric transformer to form a circuit. The circuit is powered by a small current (about the size needed to power
a door bell). The electric current
in the wire and terminals forms a field which repels rising water enough to
prevent it moving up past the level of the wire. This forms a damp-proof course
while the current remains connected. This can be a successful technique in
uneven thick walls which are situated in very wet conditions.
Plastering
Even when a new damp-proof course has been installed and
the rising dampness controlled, the wall will still contain water from the
previous dampness, together with salts carried up from the ground. These will continue to attack plaster and decorations.
Gypsum based plasters will be changed by the action of
the salts so that they actually attract water from the air, and may never dry
out. It is therefore important that the old perished plaster is removed and
replaced with a special plaster which resists attack by the salts. This is sometimes referred to as renovating plaster
or chemical rendering and can be purchased
as a special plaster system or mixed according to a particular specification.
See Replastering after Damp-Proofing.
The replastering therefore forms an integral part of the
damp-proofing work and must be carried out properly if the new damp-proofing
system
is to function effectively.
Bridging Of Damp-Proof Courses
The damp-proof course installed in a wall controls water
from moving up the wall, but this is obviously useless if water can by-pass around
it. When this happens this is referred to as bridging. The most common form of bridging is when the ground level
outside a solid wall is higher than the installed damp-proof course. Other forms
of bridging include internal plastering and external wall renders extending down
over the damp-proof course line.
If soil or paths are allowed to touch the wall above the
level of the damp-proof course, ground water will be in contact with the wall
and rising damp can occur. Exposed timbers bearing onto the wall may also be at
risk from dampness and therefore may rot. Even if the ground level is below the
damp-proof course bridging can occur in a solid wall when rainwater hits the
ground and splashes on to the wall above the damp-proof course.
For this reason the ground level should be at least 150 mm
below the damp-proof course (which should be below the level of any floor
timbers, if at all possible). If a path or driveway is too high, the situation
can be improved by digging out a channel along the wall (about 300 mm wide) and
lining it with gravel, to act as a soak-away.
Tanking and Structural Waterproofing
Sometimes it is impossible to prevent walls being in
contact with high ground levels; an underground cellar or a house built into the
side of a hill, for example. Most common is the positioning of local authority
pavements in relation to internal floors. These are ground levels that can not
be reduced and so some tanking or structural waterproofing must be used.
In these cases the only way to completely damp-proof the
walls is to install a damp-proof course above ground level, and to apply tanking
or structural waterproofing to
the inside of the walls below ground level. Sometimes this water-proofing layer
can be applied to the outside of the structure, as part of construction, or by
digging out ground levels, installing the tanking / structural waterproofing layer, then replacing the
ground levels. Tanking is used as a generic name for structural water-proofing
and takes various forms. These treatments effectively create a barrier to water, much
like the inside of a swimming pool. For more information about this, see Basement
Tanking.
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