Damp proofing in construction is a type of moisture control applied to building
walls and floors to prevent moisture from passing into the interior spaces.
Damp problems are one of the most frequent problems encountered in homes.
Overview Damp proofing is defined by the American
Society for Testing and Materials as a material that resists the passage of
water with no hydro-static pressure and waterproof as a treatment that resists
the passage of water under pressure. Generally damp proofing keeps moisture
out of a building where vapor barriers keep interior moisture from getting into
walls. Moisture resistance is not necessarily absolute: it is usually
defined by a specific test method, limits, and engineering tolerances.
Damp proofing is accomplished several ways including:
A damp-proof course is a barrier through the structure by capillary action such
as through a phenomenon known as rising damp. Rising damp is the effect of water
rising from the ground into your property. The damp proof course may be
horizontal or vertical. A DPC layer is usually laid below all masonry walls,
regardless if the wall is a load bearing wall or a partition wall.
A damp-proof membrane is a membrane material applied to prevent moisture
transmission. A common example is polyethylene sheeting laid under a
concrete slab to prevent the concrete from gaining moisture through capillary
action. A DPM may be used for the DPC. Integral damp proofing in concrete
involves adding materials to the concrete mix to make the concrete itself
impermeable. Surface coating with thin water proof
materials for resistance to non-pressurized moisture such as rain
water or a coating of cement sprayed on such as shotcrete which can resist water
under pressure. Cavity wall construction, such as
rainscreen construction, is where the interior walls are separated from the
exterior walls by a cavity. Pressure grouting cracks and joints in
masonry materials. Materials
Materials widely used for damp proofing include:
Flexible materials like butyl rubber, hot bitumen, plastic sheets, bituminous
felts, sheets of lead, copper, etc. Semi-rigid materials like mastic asphalt
Rigid materials like impervious bricks, stones, slates, cement mortar or cement
concrete painted with bitumen, etc. Stones
Mortar with waterproofing compounds Coarse sand layers under floors
Continuous plastic sheets under floors Masonry construction
A DPC is a durable, impermeable material such as slate, felt paper, metal,
plastic or special engineered bricks bedded into the mortar between two
courses of bricks or blocks. It can often be seen as a thin line in the
mortar near ground level. To create a continuous barrier, pieces of DPC or DPM
may be sealed together. In addition, the DPC may be sealed to the DPM around the
outside edges of the ground floor, completely sealing the inside of the
building from the damp ground around it. In a masonry cavity wall, there is
usually a DPC in both the outer and inner wall. In the outer wall it is
normally 150 millimetres to 200 millimetres above ground level. This
allows rain to form puddles and splash up off the ground, without saturating
the wall above DPC level. The wall below the DPC may become saturated in rainy
weather. The DPC in the inner wall is usually below floor level,, or, with a
solid concrete floor, it is usually found immediately above the floor slab
so that it can be linked to the DPM under the floor slab. This enables
installation of skirting boards above floor level without fear of puncturing
it. Alternatively, instead of fitting separate inner and outer DPCs, it is
common in commercial housebuilding to use a one-piece length of rigid
plastic,, which fits neatly across the cavity and slots into both walls. This
method requires the need for weep vents to enable rainwater ingress to drain
from the cavities otherwise rising dampness could occur from above the DPC.
Concrete walls and floors Concrete normally allows moisture to
pass through so a vertical damp proof barrier. Barriers may be a coating or
membrane applied to the exterior of the concrete. The coating may be asphalt,
asphalt emulsion, a thinned asphalt called cutback asphalt, or a rubber
polymer. Membranes are rubberized asphalt or epdm rubber. Rubberized
products perform better because concrete sometimes develops cracks and the
barrier does not crack with the concrete.
Remedial damp proofing Until the 20th century masonry buildings
in Europe and North America were generally constructed from highly
permeable materials such as stone and lime-based mortars and renders covered
with soft water-based paints which all allowed any damp to diffuse into the air
without damage. The later application of impermeable materials which prevent the
natural dispersion of damp, such as tiles, linoleum, cement and gypsum-based
materials and synthetic paints is thought by some to be the most
significant cause of damp problems in older buildings.
There are many solutions for dealing with dampness in existing buildings, the
choice of which will largely be determined by the types of dampness that
are affecting the building. These are described in more detail under Damp.
Health and safety Some DPC materials may contain asbestos
fibres. This was more commonly found in the older, grey sealants as well as
flexible tar boards. Other possibly hazardous materials
include the use of lead sheets as a DPC material.
References