Misdiagnosed rising damp is both expensive and damanging to a property. Follow our tips…
RICS surveyors sometimes write in Building Surveys;
- “A reading above 20% WME (wood moisture equivalent) is a cause for concern and is caused by rising damp.”
- “This will require a chemical damp proof course injection.”
- “Note: there may be further defects including wet rot under the floor as a result of the damp” and
- “further investigation is recommended.”
- Some surveyors even refer a named PCA contractor to quote for rising damp treatment.
This is complete baloney and says something about the training of some RICS surveyors.
Tips for diagnosing rising damp
- Damp meter should never be used to determine the source of water. For instance a high damp meter reading can’t be used to diagnose rising damp.
- Water comes from four mains sources;
- groundwater (the root cause of rising damp),
- a main or wastewater leak,
- penetrating damp from rain and
- vapour forming condensation which represents about 85% of damp issues.
- Damp meters are useful for detecting possible damp.
- They were designed for timber.
- Pure water does not register a high dap meter reading.
- Damp maters measure conductance, typically from salts in water, a useful proxy for dampness.
- They are qualitative not quantitative.
- An amount over 20WME is indicative of damp.
- Rising damp is exceptionally rare. A property has to be in constant contact with groundwater. Groundwater is the water contained below the water-table.
- To determine if there is a a risk of rising damp look at local data for the height of the water table.
A property needs all three of the following to be at risk of rising damp.
- low lying land (relative to local water,
- in a flood plain,
- built on ground with aquifers.
- The profile of rising damp is horizontal with dips in the corners and around opening.
- A low meter reading at the base of a wall is as important as a high damp meter reading as none of the wall would be dry if there was rising damp.
- Test for nitrates. Salts concentrated from drying groundwater or mains-water is high in nitrates. Low concentrations is indicative of condensation or direct rain-water (that has not percolated through nitrate rich soil). Low nitrate concentrations excludes long-term contact with groundwater or mains-water.
- Test the sub-floor relative humidity. Groundwater and any other form of water entering the sub-floor void causes high sub-floor relative humidity, unless there is adequate sub-floor ventilation. High sub-floor humidity causes water to condense above the damp proof course.
- Test for a main-water leak.
- Search for damaged rain-water goods, gutter, hoppers, downpipes, gullies, drains.
- If all the above indicates rising damp, then the final test to determine if there is rising damp is by drilling down by a metre below the ground floor, near to the house (back of the house, or into the sub-floor void is best – check for drains, mains-water, gas, cable etc. first) and test for free water in its liquid form (not simply rain-water absorbed into soil).