Mon - Sat : 8:30am - 5:30pm


The latest blog from Prinsegate surveyors
Kingston Building survey

Penetrating damp: causes of moisture in buildings

Penetrating damp is one of the most common causes of moisture in buildings. It includes any damp which has penetrated into the building however also includes plumbing leaks. On our building surveys in Kingston, we see the causes of penetrating damp on nearly every property we inspect. Cavity walls (generally post 1920s)

Penetrating damp from outside sources namely rainwater has a very different effect on solid walled buildings than it does cavity walls. Cavity walls were designed specifically to prevent penetrating damp in exposed areas.

They allow damp to penetrate into the first layer of brickwork however this cannot get to the inner leaf. This means that the inner leaf remains dry as well as any decorative finishes inside the property.

This means that many defects which would cause penetrating damp in solid walls do not in cavity walled construction. This leaves the main issues with cavity walls such as wall tie corrosion and bridging as the few consequences of penetrating damp to cavity walls.

Related: How to negotiate a house price down after a survey

Solid walls (generally pre-1940s)

Solid walls act very differently. Solid walls rely on the thickness and breathability of the walls to remain dry. When driving rain hits a solid wall the outer skin of brickwork will become saturated. When it becomes saturated additionally rain will not penetrate into the wall and will simply run off. When the driving rain stops the wall must breath to allow any moisture within to evaporate out.

This means that solid walls are a lot more susceptible to penetrating damp as if there are any defects to the wall the damp may be able to penetrate right through the wall causing damp internally within the property. these defects can be anything such as receded mortar or spalling brickwork. Additionally, a permeable mortar should be used in between the brickwork to allow for the wall to dry and moisture evaporate. At Prinsegate Chartered Surveyors we tend to see a lot more damp in solid-walled buildings because of this.

Kingston Building survey showing damp penetration in solid wall

Photograph taken during a building survey in Kingston

Above photograph: damp penetration into a solid wall due to defective guttering. The wall is clearly saturated externally however no damp was found to the property internally. At present, the solid wall is staying watertight due to the process described above however over successive wet periods this may not remain the case.


Defects to the roof structure can be a significant cause of penetrating damp. Missing tiles or dropped slates can allow penetrating damp in the loft structure and then into the internal areas of the house. The roof structure is always a susceptible area for wood rots, due to an exposed location and exposed wood. This means it is very important to keep all tiles or slates in good condition. As soon as it is noticed that one has slipped or eroded away it should be replaced.

Related:  What is a homebuyer’s report?

Having said this, there is some protection provided by the overlap of tiles/slates. Wherever there is one tile/slate there is normally another (or part of another) underneath preventing damp penetration into the loft space. Additionally, most properties nowadays have a sarking felt. These were originally designed to keep the majority of the internal roof space dry whilst a roofer was fitting tiles. Now they are used as a second line of defence to prevent penetrating damp when the roof covering has failed.

Damp in roof

Photograph taken by our Kingston building surveyors

Above photograph: damp penetration due to a roofing defect in a valley gutter. This has caused wood rot (wet rot). Timbers are showing over 30% wood moisture content on the moisture meter.

Rainwater goods

Defects to the rainwater goods are another common cause of penetrating damp. Cavity walls are less susceptible to this and are only normally affected when there is bridging of the cavity. Solid walled buildings are more susceptible. A major defect to the guttering of a property will allow rainwater to flow onto the wall. A solid walled building will not be able to cope with this whereas a cavity may prevent this severe damp penetration.

Undulating rainwater goods

Above: Prinsegate photo showing undulating rainwater goods. This is very likely to lead to damp penetration into the solid walls below.

Building surveys

There are many more sources of damp penetrating not mentioned above however the above are the most common that we see daily. In our RICS full building surveys (level 3) we are always on the lookout for any defects that could cause damp penetration into the building and recommend the remediation necessary. Sometimes these are minor defects which could lead to more major damp penetration in the future.

If you feel you need a penetrating damp survey or have any questions, contact us today.