When soil presses against a wall that is cracked or made from porous materials, groundwater can seep through and saturate the wall, causing lateral damp.
You’ve probably seen lateral damp on a small scale when a flower bed is attached to an external wall. This wall will usually have a wet patch surrounding the flower bed caused by moisture being absorbed from the soil, which is exactly what happens when soil beneath the ground rests against the wall of a building.
Soil becomes wetter the lower you dig down, so the risk of lateral damp is increased in deep cellars and basements. Some areas have a higher water level in the soil than others, such as Fulham, Putney and Richmond, due to their proximity to the thames.
Buildings on sloped ground are also more vulnerable, as the slow flow of groundwater downhill puts additional pressure on any walls in the way.
But it’s not only underground floors that are affected. Soil rises over time, exposing groundwater to walls not originally built to withstand lateral damp.
In such cases, in addition to treating the damp, it’s worth excavating excess soil build up to stop the problem reoccurring.
Tanks for nothing
With enough time, water will always manage to push its way through bricks or cement. This is why building regulations require that all underground developments are equipped with a tanking system.
Tanking is a waterproof seal – either a slurry or a membrane – that allows water to pass into the exterior wall but stops it reaching the interior. This water then drains out naturally, evaporates or is collected and pumped away.
Unfortunately, tanking systems are often poorly installed or, in older properties, missing entirely.
Beware of hygroscopic salts
Lateral damp can become even more serious if hygroscopic salts from the surrounding soil contaminate the walls. Hygroscopic salts appear as a white, chalky residue on contaminated walls.
These salts are constantly absorbing moisture, even from the air, which can rapidly accelerate a damp problem as the wall ends up being attacked from two fronts, rapidly accelerating the spread of damp.