Dampness in building elements

There are more questions asked by surveyors on the subject of damp coursing existing walls than any other area of surveying, probably because there is a dearth of published advice, apart from information supplied by those marketing patent damp-proofing systems.

Is a new damp-proof course needed?

Over-emphasis on this question diverts attention from several others that really should precede it, such as:

• Is the dampness present 'significant'?
• Where exactly is the wall base significantly damp?
• Where is the dampness likely to be coming from?
• What are the available remedy options to control, stop or mask the dampness?
• How should the preferred remedy option be detailed and installed for best effect?

It is not difficult to measure how much moisture there may be in a wall at low level. But confirming the source can be difficult.

Even if you confirm that 'ground salts' are present in low-level wall plasters, this does not mean dampness is coming from down in the ground. Nitrate or chloride salts may be present from a historic damp problem. Nitrates are also found in fertilisers spread over the top surface of flower bedsand from tap water as well as a number of other elements that come into contact with building materials and elements. See Salts Tests and comparison table for more information.

Many wall base damp problems are due to low-level damp penetration, when the wall has been unable to cope satisfactorily with the moisture load from leaky drain gullies, pooling yard water or splashing rainwater. And there may be additional moisture from below ground.

You can sometimes remedy wall base damp by improving perimeter surface water drainage, or the condition and detailing at the wall base to prevent damp-bridging or the wall soaking in too much moisture. The insertion of a damp-proof course may sometimes be a measure to consider. A horizontal dpc will prevent or control dampness that is soaking into the wall base and travelling upwards, but will not on its own prevent lateral movement of dampness through a wall.

In traditional buildings where there is good height between the wet wall base zone and vulnerable decorations, finishes, timberwork, and so on, wetness may often be controlled by natural evaporation from the masonry. However, in buildings where this alone cannot deal with the low-level dampness load, perhaps because wall finishes are too impervious or there is insufficient evaporating wall surface, then a damp-proof course may be needed to help control or stop damp.

Remedies can be found in the Surveyor's checklist - remedies for wall base damp (below). Consider remedies in the light of 2 key requirements:

1. Speed of building response - some remedies act to gradually help dry a wall base, e.g. improving perimeter surface water drainage, but it could take months and years for this to noticeably reduce dampness internally. See Monitoring moisture condition.
2. Ease of remedy application - some remedies applied to internal faces of walls or floors necessitate a considerable initial strip-out of internal fittings, obstructions or wall finishes. Others involve external work, creating far less internal disruption. External remediation is more likely to be feasible to detached property, as semi-detached or terraced property usually offers no access to the neighbour's side of party walls.
   Damp-proofing that is introduced into the thickness of a building element (such as chemical injection damp-proofing) suffers from a number of difficulties. You cannot easily know whether the chemicals have fully impregnated the wall or bed joint thickness. Also, it will be difficult to know whether the wall thickness above the treatment is becoming suitably drier from the damp coursing, and especially when an internal wall face is replastered.

Alternative remedy approaches for wall base damp

Four key means to stop or control dampness are as follows:

• barriers;
• evaporation opportunities;
• drainage opportunities; and
• sheltering.

These mechanisms can act as stand-alone dampness strategies, or can be used working together.

Many cases of damp penetration, particularly to door openings, could have the penetration remedied merely by shielding the door opening by a well-designed canopy or porch.

As a general rule, you will achieve more success in stopping or controlling dampness if more than 1 of the above 4 strategies is used. See case studies for examples of remedies using combinations of strategy.