How a slate dampcourse can fail
Slate Dampcourse Failure
How Slate Dampcourses Fail.
Slate Dampcourse Failure
Slate was once a common dampcourse material in older masonry buildings. It’s a dense, natural stone that doesn’t corrode like metals and doesn’t absorb much water. When intact, it’s an excellent barrier to rising damp.
But over time, slate can crack, delaminate (split into thin layers), or shift due to building movement. These small gaps create capillary paths for moisture to rise. The failure isn’t chemical — it’s mechanical and structural.
Here’s what typically happens:
Movement or Pressure: As the building settles, mortar joints and wall loads can stress the slate.
Micro-cracks and Layer Separation: Slate splits along its natural planes, creating hairline gaps.
Salt Crystallisation: Rising damp water carries dissolved salts that crystallise in these cracks, widening them further over time.
Moisture Pathways Form: Water begins bypassing the slate, wicking up through gaps or damaged mortar.
Progressive Breakdown: The slate itself remains chemically stable, but the surrounding mortar and brickwork deteriorate, making the barrier ineffective.
In short, slate doesn’t rot or corrode — it simply gets breached. You’ll often see this in heritage buildings where walls have fine white salt deposits or damp marks directly above the slate layer.
How Salt Travels Upwards Through A Wall.
Hydration Shell Illustration
When rising damp occurs, moisture from the ground travels upward through the microscopic pores of brick and mortar — a process called capillary action. Each pore acts like a narrow tube, drawing water upward as the surface tension of water and its attraction to mineral surfaces pull it higher into the wall.
But this water isn’t pure — it carries dissolved salts such as sodium chloride (NaCl). Once inside the wall, the salt separates into sodium (Na⁺) and chloride (Cl⁻) ions, each surrounded by a hydration shell — a cluster of water molecules that keeps the ions mobile and suspended. These hydrated ions act like microscopic messengers, carrying moisture deeper and higher through the masonry.
A slate dampcourse is designed to stop this upward flow, but over time small cracks, movement, or salt crystallisation can create gaps. The hydrated ions and their surrounding water molecules can pass through these weak spots, continuing the capillary rise. As the water evaporates above the slate, it leaves behind salt crystals that expand and widen the cracks even further — accelerating failure.
The diagram below shows how sodium and chloride ions are carried within their hydration shells, allowing water to move through and around the slate barrier over time.
Our rising damp injection treatment products can repel water from your building’s masonry