Masonry walls built from brick, blockwork or porous stone which stand in water or saturated soil and which have been built without a physical barrier to the upward movement of moisture, can have rising damp to a height in excess of 1 m.
The height depends on several factors including:
rate of evaporation from the wall,
pore sizes of the masonry,
salts content both of the materials and from
groundwater level and degree of saturation,
use of heating within the property.
3. How high will the moisture rise?
Bricks, stone and mortars have a complex pore structure in which the diameter of the pores varies widely.
This has an important effect on the height to which the damp will extend because, the smaller the pores the greater the vertical rise of moisture.
Old walls may contain pores as
small as 0.001 m diameter, which
can theoretically support
a column of water far higher
than 1 m
The Public Health Act of 1875 introduced the requirement for a dpc in walls to prevent rising damp. The adoption of this Act into local byelaws took some time, If the building has a dpc, it is unlikely that it has failed as most dpc materials have a very long life.
Possibility that the dpc has been bridged should be checked. This can include rendering over the dpc externally or bridging by plaster internally. Raised soil levels or paths, or rubbish or fuel piled against the wall, all provide possible routes for moisture.
In cavity walls, mortar droppings within the base of the cavity will allow moisture to bridge the dpc .
Physical dpcs can fail occasionally, particularly those formed from engineering bricks or overlapping slates. Bitumen felts can become brittle with age.
Diagnose the source of dampness. Where the dampness has existed for some time, there are likely to be visible signs such as damp patches on walls, peeling and blistering wall decorations, areas of efflorescence and possible rotting or splitting of woodwork as a result of wet or dry rot.
There is a need to distinguish between rising
damp, rain penetration, condensation and
Diagnosis 6. Causes of dampness in walls
Rain penetration through the walls, defective window sills, cracked render and defective pointing, result in damp patches particularly where cavity insulation is present.
Defective rainwater goods: cast iron pipes cracked adjacent to the wall. Internal dampness will match the external position of the rainwater pipes.
Condensation: often showing as mould growth just above the skirting, crescent pattern in a corner will confirm the condensation diagnosis.
Leaking central heating or plumbing pipes.
Construction water: the plastering of the wall down to the slab surface and subsequent screed lying against the plaster will allow moisture to rise up the plaster, resulting in disruption to the finishes just above skirting level.
A perfectly satisfactory dpc can be made ineffective by bridging by a variety of methods illustrated on the next slides.
7. Examples of Bridged DPCs 8. Examples of Bridged DPCs 9. Examples of Bridged DPCs 10. Examples of Bridged DPCs 11.
Bricks- have an impervious characteristic, referred to as engineering bricks. Laid in three courses and should go through each wall that is supported on the damp ground. Lower three rows of brickwork are usually blue in colour.
Slate- inserted both horizontally and vertically within the wall. Easily breached if minor wall movement fractures the slate or if the slate has been bedded in too thick a mortar.
Lead- still is used, particularly where a tray is needed around a chimney. Where it has been used as a damp course it can be punctured by the pressure of the wall weight on an uneven bed coat.
Bitumen - impregnated materials - used from the 1950s onward. They are vulnerable to being punctured, and some loss of bitumen does occur close to the wall face when the wall became very warm.
Types of DPC 12.
Trading Standards North Yorkshire undertook a survey of damp proof course material in the 1980s.
Found it was often defective before it was installed through poor quality manufacturing control and even poorer site handling and control.
Once rising damp has occurred chlorides and nitrates and in some areas sulphates are drawn up in solution from the earth below the building and thus enter the porous structure of the masonry
Some may argue rising damp is rare. In theory this may be right, but in practice for reasons noted above it is not especially rare and the effect of contaminants such as salt and sulphate is often of catastrophic risk to the decorations and the structure.
The use of siphon tubes, air bricks, electrical polarity reversal to prevent capillary action work to some degree but the presence of salts will defeat their purpose. These devices depend on porous material themselves and can be attacked by salts therefore the salt content of the masonry is still an issue.
Earth piled against external walls removed and external render and internal plaster cut back to just above the dpc line.
Improve land drainage at the base of the wall may also help to cure the problem.
Air Bricks should be cleared of any obstruction as subfloor ventilation is important.
Original dpc may be effective; a fact often overlooked when treatment is being considered.
New dpc: traditional (physical damp-proof course)
15. Non-traditional Treatment Methods.
The only method which BRE considers suitable is chemical injection.
Chemical injection systems can be used in most types of structure, although flint walls and rubble-infilled walls can be difficult to treat successfully whereas Physical dpcs can only be placed in brickwork or coursed stonework.
Once an effective damp proof course has been created by retrospective injection the presence of internal salt remains and for this reason the plaster is removed and replaced with a salt resistant sand and cement.
Should have as high a vapour permeability as possible to help the evaporation of residual moisture and should be weaker than the background to which it is applied.
Rich cement-based undercoats are effective moisture barriers in their own right. However, their vapour permeability is low and their strength is very high.However their high strength may preclude their use direct on weak backgrounds where lathing is needed to provide a base for the plaster.
Renovating plasters are available but should be used only if the cause of the dampness has been removed. Gypsum-based undercoats are not effective moisture barriers and must not be used. While the wall should be allowed to dry for as long as possible, replastering can follow, providing porous decorations are selected.
Application of gloss and vinyl paints or wallpapers should be delayed for at least one year.
The ISSE is evaluating and commissioning research via nanotechnology for a solution to this problem by the creation of a nano masonry preservative.
As the information and concept is commercially sensitive no information can be provided here.
New reclaimed material to displace sand and cement and mitigate against salt damage are also being investigated and developed.
All information contained in this presentation has been taken from:
BS 6576:2005 - Code of practice for diagnosis of rising damp in walls of buildings and installation of chemical damp-proof courses
BRE Digest 245 (2007) : Rising damp in walls. Diagnosis and treatment