Penetrating Damp

Penetrating Damp

What is Penetrating Damp?

Penetrating damp (or lateral damp) is the horizontal ingress of moisture through the building walls. Penetrating damp normally presents on external walls and forms as isolated patches of dampness which increase in size after periods of heavy rain. This is because when rainwater comes into regular contact with the wall saturation occurs. Consequently, penetrating damp predominantly happens in areas exposed to severe weather conditions, such as properties near the sea or in areas open to the elements.

Penetrating damp can affect roofs, ceilings and walls and it can, unlike rising damp, happen at any level. Penetrating damp is far more common in older properties, since they are more likely to have solid walls. A new build property with cavity walls offers much more protection and, as such, is unlikely to suffer from this type of defect.

Why Does Penetrating Damp Occur?

Penetrating damp is usually due to one or more building defects, for instance a leaking rainwater pipe or faulty joints between windows and walls. However, the most likely cause is defective brickwork, such as bricks which have become porous with age, frost damaged bricks, deterioration of mortar joints and/or cracked renders and pebble dashing.

These faults allow moisture to soak completely through the wall. The failure or indeed absence of a waterproofing system will also aggravate the situation.

In addition, penetrating damp normally occurs on external walls where the external ground levels are higher than the internal floors (bridging). As a result dampness within the soil bears against these walls and penetrates through them. Consequently, this type of damp is particularly prevalent in basements and cellars. If you do have damp issues in your basement please refer to our guide.

Penetrating damp can easily be recognised as a watermark generally appears which is separate from windows, the floor and roof and this area will provide high readings on a damp detector). These damp patches will also grow if the water continues to enter and will be more prevalent in periods of heavy rain. There may be mould growth with this form of damp, however, this will depend upon conditions. There may also be water droplets or free flowing water on the surface depending on the severity of the damp. No salts will be present with this type of damp.

Problems caused by Penetrating Damp

Penetrating Damp can lead to further problems, such as:

  • Wet rot
  • Mould
  • Damp plasterwork
  • Damp smells
  • Frost damage to masonry
  • Visible water damage
  • Rooms feeling cold and uninhabitable

Quick Overview of Penetrating Damp Symptoms

  • Ingress of water
  • Damp staining
  • Salt crystallisation
  • Plaster breakdown
  • Timber decay


The first step to treating penetrating damp is identifying the source and fixing it. Since penetrating damp forms when water gets in from the outside, the first necessary step is to check everything in and around the property. Look at gutters, downpipes, flashing, rendering and window frames in detail. Always make sure that downpipes are unobstructed and if required think about replacing the guttering. A badly fitting roofing felt on a flat roof will also cause damp, as will a cracked wall, so these would need to be fixed. The rendering may have cracked, letting in water, or it might be necessary to re-seal any gaps around window frames. Also, check under the window sills to see if the drop groove is blocked with moss or dirt.

Contact the association and our members can then weatherproof using specialist coatings and solutions to stop any further degradation to your walls.