How to diagnose damp problems

Testing dampness in a wall

Damp damage to house walls is frequently encountered by HouseCheck inspectors.  HouseCheck inspectors are equipped with sensitive moisture meters which enable them to determine whether the visible damp damage is old, unrepaired, damp damage or whether the surface is still wet.  Wet surfaces indicate that the problem is on-going.  HouseCheck inspectors also use their experience and powers of observation to trace the source of the damp.

There are three main types of damp:

Penetrating damp:  This is when water enters the structure in one of the following ways:

  • Via roof leaks.   These can be tricky to trace because the water may run down roof rafters and the damp showing on the wall or ceiling can be a long way from the source of the actual leak.
  • Through masonry cracks – these are often cracks on the tops of parapet walls or other horizontal surfaces.
  • As a result of leaking plumbing pipes – often inside walls.
  • Defective waterproofing behind showers, baths or sinks.
  • Defective seals around windows and doors.

Rising damp:  This is when ground water seeps up into the wall.   There are usually two possible causes for rising damp:

  • Failed or absent damp proof course (DPC).  A DPC is the layer of plastic, malthoid or slate between the foundation and the bottom of the wall.  Just like water can “wick” up a piece of string, so also ground water can rise up within a masonry wall – usually to a maximum of 1.2 meters, before gravity halts its upward progress.
  • Incorrect outside ground levels.  If the level of the outside ground is higher than the damp proof course, then water accumulating against the walls in flower beds, lawns or incorrectly sloped paving, can seep into the walls and then wick upwards.

Condensation:  Moist air – mostly in warm, badly ventilated bathrooms or bedrooms – will condense on walls and other cooler surfaces resulting in mould.  Mould is a fungus growth and its spores are dangerous to humans.   The solution is to clean off existing mould with a weak bleach solution and improve ventilation –by opening windows or installing a fan.

With any damp damage, the following course of action is recommended:

  •  First determine the cause and source of the damp and solve the problem at source. This may involve repairing a roof leak or plumbing problem, sorting out waterproofing, combating rising damp through chemical damp-proofing or altering outside ground levels; or simply improving ventilation (in the case of mould).
  • Second, do not repair and redecorate the damp damaged area until the problem has been fixed and the damp area is thoroughly dry.

John Graham is CEO of HouseCheck ( Readers are invited to contact John at  Tel. 083 3109 766 or email  with any questions on houses.

Scroll to Top
Scroll to Top
× Say, Hello! on WhatsApp