What is the tide mark on my damp wall telling me?

People photograph a damp patch and ask what it is. The photograph almost always contains the answer, because the three causes leave physically different marks — one is governed by how far water can climb, one by where the fault in the outside face is, and one by where the wall is coldest. Learning to read the four pieces of evidence is worth more than any meter.

What you'll need

• Torch raked across the wall at a shallow angle
• Pencil
• Camera
• Tape measure

Step by step

1. Measure the height of the top edge.From the outside ground level, not from the floor inside. Ground-water damp stops at around a metre because that is roughly as far as capillary action can lift water against gravity in ordinary masonry. Anything materially higher has another cause.
2. Check whether the top edge is level.Sight along it, or hold a straight edge or a level against it. A horizontal top edge that holds the same height for several metres is the single strongest indicator of ground-water damp. Penetrating damp does not produce level lines because it has no reason to.
3. Look at the edge quality.A ground-water mark has a defined, often slightly darker edge with a paler bloom above it. A penetrating mark has a soft irregular boundary that follows the defect above it. Condensation has no edge at all — it is a growth on the surface.
4. Look for salts.Rake a torch across the wall. A white, fluffy or crystalline fringe at and just above the top edge means water has been evaporating there over a long period, carrying dissolved salts out of the masonry. Condensation does not produce salts, because the water arrives from the air rather than from the wall.
5. Feel the plaster along the line.Press gently. Ground-water damp softens and blows plaster at low level, and the plaster along and below the tide line often sounds hollow and crumbles at a fingernail. Condensation leaves the plaster sound.
6. Note the season and the weather.Worse in winter and in the morning means condensation. Worse after a storm and better a week later means penetrating. The same all year, and sometimes worse in warm humid weather, points to salts and ground water.
7. Check where it is worst horizontally.Worst behind furniture and in corners means condensation. Worst under a window, beside a downpipe or opposite a cracked render panel means penetrating. Uniform along the whole base of an external wall means ground water — or, far more often, a bridge along that whole wall.
8. Write the four findings down before deciding.Height, edge, salts, seasonality. Judging from one of them is how misdiagnosis happens; three or four agreeing is a diagnosis you can act on.

Tips

• Mark the outline of the patch in pencil with the date. Two or three dated outlines across a wet and a dry spell are better evidence than any instrument.
• A tide mark that has been painted over still shows in raking light, because the salt crystals deform the paint film.
• Old tide marks are common on walls that are now dry — a mark left by a problem that was fixed decades ago. If the wall is cold rather than wet and the plaster is sound, you may be looking at history.

Common mistakes

• Diagnosing from the colour of the stain — Brown staining can come from ground water, from a chimney, from a nicotine-contaminated wall, from an old leak or from timber preservative. Colour tells you almost nothing; height and edge tell you a great deal.
• Assuming a level line proves rising damp — A level line proves the water is arriving from below at a consistent level. A bridged damp proof course, a raised path along the whole elevation and a high internal floor all produce the same level line for a fraction of the cost to fix.

Questions people ask