Damp Patch On An Internal Wall

What's happening?

There are three different faults that all look like a damp patch, and telling them apart decides who you need. Penetrating damp is water crossing the wall from outside: it tracks the weather, so the patch darkens during driving rain and dries between spells, and it can appear at any height. Condensation forms on cold surfaces from moisture already inside the house, so it shows on the coldest spots, in corners and behind furniture, usually with black spot mould and no clear link to rain. Rising damp comes up from the ground and stays low on the wall with a tide mark. A patch that follows the weather and sits mid-wall on a cavity wall is very often a bridged cavity, which is a construction fault a builder fixes.

What usually causes it?

• Mortar droppings or debris left on a wall tie or in the cavity base, bridging the gap so water crosses to the inner leaf
• Cavity insulation slumped, wet or badly installed, providing a continuous damp path across the cavity
• Missing or badly lapped cavity tray over a window, door, extension abutment or air brick
• Cavity not carried far enough below the damp-proof course: Approved Document C expects at least 225mm below the lowest DPC, or a tray instead
• Failed or bridged damp-proof course, including render or paving carried across it
• A leaking gutter, downpipe, flashing or overflow soaking one area of wall repeatedly
• Condensation from inadequate extract ventilation, where Approved Document F expects 15 l/s from a bathroom and 30 or 60 l/s from a kitchen

Is it dangerous?

Not immediately, but it does real damage if left. Sustained wetting of a wall reaches embedded timber (joist ends, wall plates, lintels and skirtings), and the Property Care Association is clear that timber allowed to stay excessively wet will decay. Damp plaster picks up salts and never dries properly, and mould growth from persistent condensation is a health issue for anyone with asthma or a respiratory condition. There is also a heat cost: a wet wall loses far more heat than a dry one.

Can I fix it myself?

Diagnosis, mostly, and it is the valuable part. Keep a note of when the patch darkens and what the weather was doing: a rain-linked pattern points outside, a cold-weather pattern with mould points at condensation. Go outside during heavy rain and look for the overflowing gutter, the split downpipe, the cracked render, the air brick with no tray above it. Clear blocked gutters and gullies. Do not seal the inside face with waterproof paint or a plastic-backed lining: it moves the damp rather than removing it, and traps water against the wall.

When should you call a builder?

Call a builder where the pattern points at the construction: a bridged cavity, a missing tray over an abutment, an opening formed without a tray, render bridging the DPC. Call a roofer for gutters, flashings and roof leaks, and a damp specialist where rising damp or a failed DPC is the likely cause. If mould keeps coming back on cold corners with no rain link, the answer is ventilation and heat, not a damp course.

What will the builder actually do?

• Establish which of penetrating damp, condensation and rising damp is actually happening, using moisture readings and the weather pattern
• Inspect the cavity with a borescope to find mortar bridging, slumped insulation or a missing tray
• Remove bricks locally to clear cavity debris and clean bridged wall ties
• Install or repair cavity trays and weep vents over openings, abutments and air bricks
• Correct external ground levels, render and paving that bridge the damp-proof course
• Replace decayed embedded timber, then replaster in a system suited to a wall that has been wet

Questions people ask

Sources & References

1. GOV.UK: Approved Document C (Site preparation and resistance to contaminants and moisture)
2. GOV.UK: Approved Document F (Ventilation)
3. Property Care Association: Penetrating damp
4. Property Care Association: Condensation