What Causes Damp?

mushroom fungus
Mushrooms found during survey damp cause by condensation from cold water pipe

Damp Cause – excess humidity

Whether damp in humid air, soil or brickwork, by definition damp is caused by excess humidity.  Most things contain water – damp. Something is considered damp when humidity is sufficient to damage, spoil or disrupt. Bricks, render, mortar and plaster can hold damp before they spoil. Typically this is about 5{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} by weight, but it varies.

Effects of damp cause in properties

Damp cause the following;

  1. Decorative spoiling – annoying and a reasons to have a damp survey, could be hiding a bigger issue, but damp in plaster is not inherently dangerous.
  2. Mould, which can be very disturbing, anxiety inducing, but has not been conclusively demonstrated to cause ill health in the quantities found on walls. Mould grows when humid air excess 85{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} see MouldPoint.co.uk.
  3. Damp in wood or timber. Plant cells split open, allowing rot to convert cellulose back into energy, water and carbon dioxide – respiration, or reverse photosynthesis. The water generated accelerates the process. Rot will stop as soon as the source of damp is stoped and the area ventilated.
Dry rot
Cause of Dry rot (Serpula lacrymans) yellow damp tear drops. We found rot treated by contractor, grew back as he didn’t stop damp.

Identifying cause of damp is part science, part imagination and part persistence until investigation complete.

Evaporation – damp cause?

  • Damp evaporates – obviously.
  • It rains – clearly.
  • Bricks get wet – naturally.
  • If it wasn’t for constant evaporation, rain water would flood our houses.
  • In winter the rate of evaporation reduces, therefore in winter the effects of damp are greater.
  • Water evaporates well below 0˚C, its call sublimation.
  • The key to speeding up evaporation is the flow of dry air (ideally warm).
  • Rising damp is limited by evaporation, as well a gravity and porous material at lower levels.
Condensation London
Damp cause – condensation – London


The opposite of evaporation is condensation, the two are in constant equilibrium. Condensation starts when evaporation stops and vice versa – like a car acceleration or breaking. Condensation starts when the surface humidity of reaches 100{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} – see our free Mould Point Calculator; MouldPoint.co.uk to calculate humidity. In winter we publish regular condensation alerts on twitter.com/DampSurveys – follow us and request a condensation risk kit. Be aware;

  • Condensation need to be managed. The greatest risk is at night, even in summer, around cold spots like metal wiring, and shaded walls from October to April.
  • 80{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} of dampness in houses is caused by condensation.
  • Most condensation results from humidity not being extracted at source.
  • Rising damp normally disappears when humidity is reduced.
  • Mould only results from excess humidity (85{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}).
  • Nitrates found in rising damp inhibit mould – if you have mould you don’t have rising damp.
damp cause penetrating damp
Damp cause – penetrating damp

Penetrating damp

  • Damp from roofs, chimneys, leaking windows, sills and bricks, travel downward.
  • Look for signs of damp on the outside, dark (damp) patches of brickwork, green algae, grey streaks or white efflorescent streaks (salts).
  • Observe the gutters and rainwater pipes during a rainstorm.
  • The key risk with penetrating damp, is that damp is causing timber to rot, the key risk is structural timber.

Bridging the damp proof course (DPC)

  • Damp bridging the  DPC is commonly mistaken for rising damp.
  • Something as simple as old tiles or wooden boards resting on the outside wall, can cause damp to bridge, or for rain to bounce up the wall.
  • Most properties in London have settled into soft clay soil. Over time, the ground has been built up. This can cause water to bounce onto the wall above the damp proof course.
  • The solution is simple, and inexpensive, build a French drain around the property.
Rising damp
This was misdiagnosed as rising damp. The stain isn’t horizontal. A damp streak was on the outside, a sign of penetrating damp.

 Rising damp

  • Moisture is is drawn in every direct by the relative attraction of water to silicone in mortar, plaster and to a lesser extent, brickwork. Gravity forces water down to the point of saturation. The effect is that damp will rise with a very defined horizontal pattern.
  • When the ground immediately below a house is saturated with water, small amounts of moisture can start to rise up the wall.
  • The effect is unhelpfully known by some as “capillarity” or capillary action. Unhelpful, because an impression is given that water rises first, before moving downwards, or sideways. Whereas, the reverse is true.
  • There needs to be a constant source of water for damp to rise up a wall, the only source of water that can’t be separately stop (penetrating damp or a leak), is a high water table.
  • Rising damp is therefore exceptionally rare, especially in London where flooding is unsual and water is constantly pumped out of the water table.
  • Moisture from rising damp is insufficient to cause rot. However, penetrating damp or condensation can cause rot.
  • Therefore the greatest risk is a leak misdiagnosed as rising damp, which is then hidden behind impermeable plaster, resulting in spalling to brickwork, rot to structural timber and or damage to a neighbours property.
Damp Survey Cost
Dry rot, the real cost of not having an independent damp survey.

Risk of rot

  • The greatest risk of damp to a property is rot, especially dry rot of structural timber.
  • Misdiagnosing damp can result in rot going untreated.

Top 5 tips

  • Measure humidity over the course of a day or week. Look for signs of condensation such as mould or isolated damp areas by cold spots.
  • If the wall looks damp inside, look on the outside or neighbour’s side for signs of damp coming from above, or below. Look for clues, such as discolouration, growth, damaged gutters.
  • Misdiagnosing damp can result in rot going untreated. If you are unsure call in an expert.
  • Don’t ask a contractor to “survey” the property, as this will be treated as an opportunity to quote, not to diagnose the root cause of dampness.
  • It’s not in a contractor’s interests to find the real reason for damp, as it rarely costs much to fix.