Penetrating damp is water arriving from outside through the fabric of the building. It is the easiest of the damps to fix and the easiest to misdiagnose, because by the time it shows on a wall inside, it has usually travelled some distance from wherever it got in. Finding the entry point is the work; the repair is often small.
What is water ingress? What the term means in a survey
In a survey, water ingress simply means water entering the building where it should not, as distinct from moisture generated inside it or drawn up from the ground. The usual routes are a failed or overflowing gutter soaking a wall, a defective flashing at a roof abutment or chimney, cracked render or pointing, a bridged cavity, a blocked weep hole, a failed window seal, or ground levels raised above the damp proof course outside. Approved Document C is the requirement the building was meant to meet.
Telling it apart from the other causes of damp
Penetrating damp responds to weather: it darkens during and after driving rain and dries between. It is often on one elevation, the one facing the prevailing wet wind, and it can appear at any height including upstairs, which rising damp cannot. Condensation is seasonal and surface based. Rising damp is limited to the bottom of a ground floor wall. Watching a patch across a wet week tells you more than most meters.
Efflorescence on walls, salts and the staining that persists
Water moving through masonry carries salts with it and leaves them behind as it evaporates. Efflorescence is the white fluffy deposit that results and it is a sign that water has been moving, not a problem in itself; it brushes off and returns until the water stops. Hygroscopic salts are different and matter more: they absorb moisture from the air, so a chimney breast contaminated with salts from years of flue condensation stays damp even after the flue is dealt with, and the plaster usually has to come off and be replaced with a salt resistant system.
The repairs in order, ending with a cavity drainage system
Start with rainwater goods, because a leaking gutter or downpipe is the single commonest cause and the cheapest to fix. Then flashings and the chimney. Then pointing, render and any cracks. Then ground levels and whether a path or a raised bed has bridged the damp proof course. Only after all of that does an internal solution make sense, and where a solid or below ground wall genuinely cannot be made watertight from outside, a cavity drain membrane manages the water rather than resisting it, draining it to a sump instead of pretending it is not there.
Questions people ask about water ingress through walls
How do I know if damp is penetrating rather than rising?
Penetrating damp changes with the weather and can appear at any height. Rising damp is confined to the bottom of ground floor walls and does not fluctuate with a single shower. Height and timing are the two tells.
Is efflorescence a problem?
It is evidence that water has been moving through the masonry rather than a defect on its own. Brush it off, fix the water, and it stops returning.
What is a cavity drain membrane?
A studded plastic sheet fixed to the inside face of a wall that lets water run down behind it to a drainage channel and sump, instead of trying to hold it back. It is the standard approach below ground and on walls that cannot be sealed externally.
Why is my chimney breast still damp after the flue was capped?
Hygroscopic salts left in the plaster absorb moisture from the air indefinitely. The contaminated plaster normally has to be removed and replaced with a salt resistant render and plaster system.