Internal drainage & moisture control

Solutions & Applications · Internal Drainage & Moisture Management

Blistering render, salt deposits and the smell of mould that returns after every coat of paint mean one thing: the problem is structural and water is being trapped rather than managed. International practice for existing buildings provides a clear answer — we do not attempt to stop the water by applying a coat of paint, but instead allow it to pass in a controlled manner behind a membrane and direct it to a drainage point.

Step Zero: Diagnosis — Three Phenomena That Are Often Confused

Before selecting any material, the cause must be identified. Three different phenomena produce a similar appearance on the wall and require different treatments:

Phenomenon	Field Notes	Correct Answer
Rising damp (capillary)	Horizontal ‘levelling line’ at a height of approximately 0.5–1.5 m; crystallised salts; low-level render deterioration	Prevention of capillary rise + salt isolation
Lateral penetration	Stains at any height, more pronounced after rain or on the side of an embankment	Water management & drainage (Type C)
Summary	Mould in cold corners and thermal bridges; worsens in winter; no salts	Ventilation, heating, elimination of thermal bridges

The International Framework: BS 8102 — Three Types of Protection

The British code of practice BS 8102:2022 (Protection of below-ground structures against water ingress) is the international benchmark for underground structures and defines three types of protection:

Type	Principle of Operation	Suitability for an Existing Building
Type A — Barrier	Waterproofing membrane (coating, membrane, tanking)	Externally, it requires excavation; internally, it withstands full hydrostatic pressure
Type B — Structurally Integral	The concrete itself is designed to be watertight	Not applicable retrospectively
Type C — Drained Protection	Water is allowed to enter a controlled gap and is channelled to a drain	The most practical solution for existing buildings — minimal substrate preparation

The internal drainage method described here is a Type C system. Its advantage is its durability: it continues to function even if the masonry cracks or the water table rises — because it relies not on the watertightness of the structure, but on flow management.

The System’s Layering

From the existing damp wall into the interior space, the system consists of seven layers that function as a whole. Each one has a single role:

Laying — from the wall towards the room	Role	Material
1 · Existing damp wall	It remains damp. It is not plastered over and does not come into contact with the new finish	—
2 · Dimpled drainage membrane — geotextile against the wall	Creates a flow and ventilation gap; water drains in a controlled manner	DANODREN H15 PLUS
3 · Channel with gravel at the base	It receives the flow and filters out sludge before it reaches the pipe	River gravel (garbili)
4 · Perforated pipe, fully encased	Drainage outlet to a drain, manhole or pump	GEODRAIN PLUS
5 · Independent steel frame	It supports the cladding without bridging the damp wall	Knauf CW stud
6 · Moisture-resistant plasterboard	Final surface, ready for skimming and painting	KNAUF Type H2 12.5 mm
7 · Skirting boards	Covers the channel whilst keeping it accessible for maintenance	—

Step-by-step guide

1. Removal of salt-contaminated render: the affected render must be removed to a depth of at least 300 mm above the highest visible sign of moisture or salts. The old render acts as a carrier for hygroscopic salts — if left in place, it continues to draw moisture from the air.
2. Excavation of a channel at the base: a trench is dug along the wall, ensuring a gradient towards the drainage point.
3. Gravel sub-base: a 5–10 cm layer of river gravel (garbili) at the bottom of the channel.
4. Installation of a perforated pipe: the pipe is set on the sub-base and completely encased in gravel up to floor level. The gravel acts as a filter: it allows rapid flow towards the pipe and prevents blockage by silt.
5. Membrane installation: with the geotextile facing the damp wall and the cones forming the gap. The membrane starts at the base, within the channel, so that the flow ends up in the gravel — and rises at least 300 mm above the highest moisture level.
6. Independent framework: a metal dry-construction framework is erected without piercing the membrane in the water flow zone and without bridging the damp wall to the finished surface.
7. Cladding: Type H2 moisture-resistant plasterboard in accordance with EN 520, filled and painted. The skirting board is fitted whilst leaving the access channel accessible.

The Four Key Factors for Success

Point	Why It Is Crucial
Where does the water end up?	If there is no natural gradient towards a drain or an absorption well, a collection well with a pump is required. A system without a drainage outlet simply shifts the problem elsewhere.
Maintainability	BS 8102:2022 explicitly requires that all components be accessible for inspection and maintenance. The duct must remain accessible for cleaning.
Salt isolation	The old wall remains damp — this is acceptable. It is not acceptable to plaster or clad it where it is in contact with the damp, as the salts will migrate into the new finish.
No bridging	Every point where the damp wall comes into contact with the new structure is a moisture pathway — the framework, the skirting, the floor.

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