Penetrating Damp On An Internal Wall

What's happening?

Penetrating damp is rain finding a path through the outside of the building and appearing inside. It behaves in a way that gives it away: it is tied to the weather, worse after driving rain from a particular direction, and it dries back and returns rather than sitting at a constant level. It can appear at any height, which immediately separates it from rising damp, and it is usually a defined patch rather than a band. Diagnosing it is really a matter of finding the entry point, which is rarely directly behind the patch: water travels along mortar beds, cavity ties, lintels and cavity trays before it reaches a face where it can show. Solid walls are more vulnerable than cavity walls, and exposed gables and chimney stacks worst of all.

What usually causes it?

• Cracked, hollow or missing pointing on an exposed elevation, giving rain a direct route into the wall
• Defective or absent cavity tray over an opening, an abutment or a lean-to roof
• Cavity bridged by mortar snots on wall ties, carrying water across to the inner leaf
• Cracked or blown cement render trapping water behind it, so the wall cannot dry
• Failed sealant or an unsealed cavity closer at a window or door frame
• Overflowing gutter, leaking joint or blocked downpipe soaking one part of the wall repeatedly
• Failed flashing, a leaking chimney stack or defective verge mortar delivering water into the wall head

Is it dangerous?

The damp itself is not, but what it feeds is. A wall that is wet after every storm supports mould growth on the internal finishes and keeps embedded timbers (joist ends, lintel bearings, floorboards against the wall) wet enough to decay. Where the path is a chimney stack or a wall head, the water is also working on the masonry that delivered it, and that becomes a structural question rather than a decorative one.

Can I fix it myself?

The detective work, which is the part contractors most often skip. Note which wind direction brings it on, how quickly it appears and how long it takes to fade, and look at the outside of that wall during and after rain: overflowing gutter joints, water tracking down a wall face, cracked render or a wet patch that spreads from a specific point. Clearing gutters and downpipes is worth doing and fixes a fair share of these. Do not seal the inside of the wall with a waterproof coating or tanking to hide the patch: that leaves the wall wet and pushes the damp somewhere else.

When should you call a damp proofer?

Call a damp proofer when the patch keeps returning after the obvious rainwater goods have been cleared, when it is high on the wall or beside a chimney breast, or when mould has established. If the entry point turns out to be tiles, flashing, a valley or verge mortar, that is roofing work rather than damp proofing: a good damp survey says which, and it is the survey you are paying for.

What will the damp proofer actually do?

• Map the patch internally and correlate it with weather and wind direction
• Inspect the corresponding external elevation for pointing, render, flashings, cavity trays and rainwater goods
• Open up where necessary to find the actual path, including checking for bridged wall ties and blocked or missing trays
• Repair the entry point: repointing, renewing a cavity tray, clearing the cavity, resealing frame perimeters
• Allow the wall to dry and monitor it through a wet spell before any replastering
• Refer roof-origin water to a roofer rather than treating the internal symptom

Who fixes this

• Penetrating damp repair

Questions people ask

Sources & References

1. Property Care Association: Penetrating damp
2. GOV.UK, Site preparation and resistance to contaminants and moisture: Approved Document C
3. Property Care Association: Damp proofing problems