How a Bitumen dampcourse can fail
Bitumen Dampcourse Failure
Here’s how a bitumen dampcourse fails.
How Asbestos-Bitumen Dampcourses Fail.
Many older homes used bitumen or asbestos-bitumen dampcourses. The bitumen offers a waterproof membrane, while asbestos fibres were added to strengthen the sheet. When intact, these perform reasonably well.
Damage, abrasion, or cracks in the bitumen exposes the substrate to moisture and dissolved salts. The failure mode is mostly mechanical, not chemical—though salts (especially chlorides) accelerate deterioration of adjacent materials and promote pathways for moisture.
Steps of failure:
Installation damage or aging — bitumen may crack, tear, or delaminate.
Moisture ingress — rising damp water penetrates weak points.
Salt transport — dissolved Na⁺/Cl⁻ reach the exposed zone.
Crystallisation — as moisture evaporates, salts crystallise, widening microcracks.
Progression — the barrier gets increasingly breached.
Over many years, the bitumen may become brittle, and asbestos fibres may break down or pose hazards if disturbed.
A typical asbestos-bitumen DPC cross-section might show: intact black sheet, a scratch or tear exposing masonry behind it, moisture moving upward through cracks, salt crystals forming at those cracks.
Aluminium Dampcourse Corrosion Process
How Salt Travels Upwards Through A Wall.
How Salt Travels Upwards Through A Wall.
When rising damp occurs, moisture from the ground moves upward through the tiny pores in bricks and mortar — a process known as capillary action. Each pore behaves like a narrow tube, drawing water higher as surface tension and adhesion between water molecules and mineral surfaces pull it upward.
This rising moisture often carries dissolved salts such as sodium chloride (NaCl). Inside the wall, the salt separates into sodium (Na⁺) and chloride (Cl⁻) ions, each surrounded by a hydration shell — a small cluster of water molecules that allows them to stay suspended and mobile. These hydrated ions help moisture migrate deeper and higher through the masonry.
A bitumen dampcourse is intended to block this upward movement. However, with age, heat, and building movement, the bitumen layer can become brittle, develop micro-cracks, or lose adhesion to surrounding mortar. As this happens, water and hydrated ions can find pathways through the weakened areas, continuing the rise of moisture.
When the damp evaporates above the damaged section, salt crystallisation occurs. These crystals expand as they form, widening cracks and causing further breakdown of the bitumen layer — accelerating failure over time.
The diagram below illustrates how sodium and chloride ions travel within their hydration shells, allowing moisture to pass through and around compromised sections of a bitumen dampcourse.
Our rising damp injection products help stop this process by repelling water and salts from your masonry, restoring the wall’s ability to stay dry.
Hydration Shell Illustration