Identifying damp

Damp or dampness, is unwanted and excessive moisture. There are four distinct forms;

  • Rising damp is below ground water that rises up a wall,
  • Penetrating damp is moisture from defective roofs, gutters, pipes or a “bridge”,
  • Plumbing leaks; from mains or internal pipes, waste, drainage and overflows,
  • Condensation, the most common cause of dampness.

Rising damp

Rising damp is defined as the deleterious vertical flow of water, derived from below the original ground level, through a property’s internal masonry wall, to above the base of the ground floor. Anyone can replicate rising damp by placing the bottom of a clay brick in water, amp will rise by about 200mm. The same effect is more rapid in a clay tile where damp can be seen to rise by as much as 400 mm. Water moves to fill the pores in the most porous substrate first. The force of attraction is equal in all directions, but gravity pushes a majority of the water downwards until all the lower pores are filled.

Profiling rising damp is important. Like a swimming pool filling up, the damp profile is horizontal, not random patterns often associated with condensation. Rising damp is in constant equilibrium with evaporation. The greater the evaporation, the lower the rise of dampness. The greater the relative humidity the greater the potential rise. The effect of evaporation is to increase the rise of dampness into a corner and lower it by a door opening.

Rising damp can be positively identified as it is the only form of damp containing nitrate salts.  The presence of mould quickly eliminates rising damp, as nitrates, found in ground and waste water, inhibit mould growth. Rising damp cannot by itself cause rot.

Rising damp needs a constant source of water, such as a high water within a meter of the brick wall. It is exceptional rare in London as water is pumped out of the ground, and rarely within a meter of a building. Furthermore, London benefits from a by-law introduced in 1877 requiring a damp-proof courses (“DPC”) “beneath the level of the lowest timbers”.

Penetrating damp

Gutters and Drains: Leaking or overflowing gutters and drains are common causes of dampness. They can be difficult to identify in dry weather, so we ask you to look closely at the gutters and drains when it rains.

There are often tell-tale signs, such as a damp stain, greenery or “efflorescent” white streaks. The resolution is often easy, involving a ladder and time to clear the blockage or fix the leak.

External Coverings; Roofs, chimneys, flashings, render

Our survey is not an assessment of the state of roofs, chimneys, flashing, render etc. We look externally for defects and then search internally for signs of dampness. We recommend regular annual integrity checks of external coverings including roofs, chimneys, flashings, render etc.

External Vents: It is important for timbers to be properly ventilated, either in the subfloor void, for floorboards or in the loft for roof timbers. Vents can become blocked over time providing inadequate circulation of air to ensure vapour movement from timbers.


Exterior Ground Levels: The ground immediately surrounding a property is often raised by successive owners to the point where there is very little clearance between the ground and ventilation grills. In the worst cases water flows under the floorboards. Vents should be clear of the ground, ideally higher than a rain drop bounces. Rain water bounces 100-500mm depending on any overhangs, such as window ledges, the type of ground and run off of water. Vents should be observed and recorded during a rain storm – we are happy to analyse results.

If there is sufficient ventilation, a small amount of water entering will evaporate without causing rot. If too much water is entering, then the resolution is a small trench (French drain) about 150mm by 150mm, which can be dug around the vent or perimeter of the exterior wall and filled with shingle or similar material, with drainage. This need not be laborious.

Plumbing leaks

Most plumbing leaks are sudden and obvious. Slow leakage such as from a slightly ruptured pipe is difficult to identify, as are below ground level leaks in the subfloor void, from a mains water pipe, main sewage pipe, rain water pipe or similarly from a neighbour’s pipe.

We do not perform a plumbing survey and may not identify waste water, below ground level leaks or other plumbing leaks. If we suspect that damp is caused by faulty plumbing we will recommend a plumbing survey.


Vapour condensing into water on cold surfaces is the most common form of dampness in the home. It is most prevalent on the lower surfaces of external ground floor walls. Warm moist air from a kitchen, bathroom, washing machine or drying clothes will condense rapidly when meeting a cold external wall, window or pipe. Add to this humid breath from human and pets.

A wall will be relatively cold at the point where both skins of a cavity wall meet.  This is most pronounced at the base of a ground floor wall, which is often more than 5°C cooler than the ambient temperature. The temperature differential can be much greater at night.

The dew point is the temperature that water starts to condense. Humid air from a warm moist kitchen readily condenses on the cooler surfaces of external walls. Typically, a surface only has to be 5°C lower than the ambient temperature for condensation to start to form.

Glass and metal are good conductors of heat and therefore lose thermal energy much more rapidly than timber, brick or plaster. Condensation runs down cold windows and frames onto walls beneath them. Metal objects embedded in walls such as behind an electrical socket, cable or pipe can initiate condensation. Cold metal can cause condensation, even in summer.

Although condensation is inevitable, it can be manged with ventilation out at source, combined with sufficient heat, air circulation and regular wiping of wet surfaces.

Ideally clothes should be dried outside, or with an externally vented clothes drier. Double glazed windows should have trickled vents kept open.

An alternative is to designate wet areas, then manage humidity in those wet areas, by wiping off surface moisture and opening windows often. Victorians used to tile their entrance halls, at the point where cold air meets warm humid air. Bathroom paints and tiles evaporate moisture readily and are easy to wipe down. Top tip; use an electrically heated bathroom mirror.

A common mistake is to increase ventilation into a building. This can be counterproductive as the outside air is likely to be cooler than the warm moist internal air, and will cause, rather than alleviate condensation. Positive flow ventilation systems do not necessarily reduce condensation.

In the worst cases, condensation can form within a wall. This is known as interstitial condensation. We will not be able to identify interstitial condensation unless it visibly affects internal decoration.