Diagnosing rising damp

Misdiagnosed rising damp is both expensive and damanging to a property. Follow our tips…

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1 Tips for diagnosing rising damp

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

1. 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.
2. 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.
3. 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.
4. Rising damp is exceptionally rare. A property has to be in constant contact with groundwater. Groundwater is the water contained below the water-table.
5. 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.
6. The profile of rising damp is horizontal with dips in the corners and around opening.
7. 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.
8. 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.
9. 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.
10. Test for a main-water leak.
11. Search for damaged rain-water goods, gutter, hoppers, downpipes, gullies, drains.
12. 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).