How High Should Waterproofing Go on Internal Basement Walls?

Direct answer

Where a below-ground structure has waterproofing at foundation level — whether a tanked or coated system (Type A) or a cavity drain system (Type C) — the internal walls sitting on or within that structure may need waterproofing treatment applied to their lower faces. As a working starting point: 300-500mm above finished floor level for reinforced concrete internal walls, and up to the full height of the below-grade portion for internal masonry walls, particularly in existing buildings. The appropriate height depends on the wall material, the water pressure conditions, and whether the waterproofing system accepts that water will be present at foundation level — as a cavity drain system does.

Full explanation

Why internal walls need consideration

A below-ground structure waterproofed with a Type A (tanked or coated) system is designed to resist water at the external face and at the floor slab. In principle, the waterproofing shell encloses the structure, and internal walls within that shell are assumed to be dry.

This assumption holds when:

• The waterproofing is intact and performing as designed.
• No water is present at foundation level within the protected envelope.

It breaks down when:

• The waterproofing is of uncertain integrity — as is typically the case in a refurbishment or an existing building.
• A cavity drain system (Type C) has been adopted as the waterproofing strategy, because this system explicitly accepts that water will pass through the external structure. Water in the drainage cavity at foundation level can migrate into any internal wall that sits on or adjacent to the floor at the drainage level.
• Water has been present at foundation level for an extended period, saturating the lower portion of internal wall construction.

How water rises in walls

Water below ground does not need a defect in the waterproofing to reach internal wall surfaces. In masonry walls, moisture travels through the interconnected pore structure of brick, block, and mortar by capillary action — a physical process driven by surface tension in the pore network. In unreinforced or low-strength masonry, this process can carry moisture well above the waterline given sufficient time and a continuous pore network.

In reinforced concrete, the same process occurs more slowly. Normal-strength concrete has a pore structure that permits slow moisture migration, particularly at construction joints and at the interface between the slab and any wall construction above it. Waterproof concrete with a water-to-cement ratio below 0.45 and adequate cover resists this significantly better — but internal walls in basement structures are not always specified to waterproof concrete standard.

In Victorian or older masonry basements, internal walls may have been in contact with ground moisture for decades. The pore structure of long-wetted masonry has a different moisture distribution from new construction, and the effective height to which moisture has migrated may be considerably higher than might be assumed from the water table level alone.

Practical height guidance

Reinforced concrete internal walls (new build): 300-500mm above the finished floor level, applied as a cementitious waterproof render or slurry to the internal face at the base of the wall. This is sufficient to prevent capillary rise in concrete of reasonable quality under typical conditions. Where water ingress is known or suspected at the base of the wall, the height should be increased.

Concrete or blockwork internal walls (refurbishment or existing building): Apply the treatment to 500mm minimum. Where moisture evidence is visible above this level — tideline, carbonation, spalling plaster — the treatment should extend to the highest observed moisture evidence, plus a margin of at least 150mm.

Masonry internal walls, below-grade portion, new build: 300mm is the minimum; 500mm is more conservative and appropriate where the water pressure regime is more severe. If the wall sits on a mass concrete or strip foundation that extends below the main waterproofing plane, a longer treatment height should be considered.

Masonry internal walls, below-grade portion, existing Victorian or pre-war buildings: Full height of the below-grade portion of the wall is the more defensible specification. Internal walls in Victorian basements typically sit on brick or stone footings without a damp-proof course, and moisture has often been active in the wall for the building’s lifetime. Taking treatment to full below-grade height and overlapping with any above-grade rising damp treatment removes ambiguity about the transition zone.

The cavity drain interface: a specific detail requirement

This issue arises most acutely where a cavity drain system has been adopted as the primary waterproofing strategy — because the system explicitly accepts that water will be present at drainage channel level, against the back of the membrane.

If the drainage channel at the base of the wall sits at finished floor level, moisture can migrate from the wet cavity through the floor slab construction and into the base of any internal wall that is not protected. The result can be damp appearing on the internal face of that wall, above the cavity drain line, with no obvious connection to the external waterproofing — which the occupant or surveyor may misattribute to condensation, services leaks, or a defect in the membrane.

The correct detail is a cementitious waterproof render or screed applied to the base of internal walls within the drainage zone, dressed over the drainage channel upstand and onto the floor slab to provide continuity. This is a simple, low-cost detail that eliminates a common moisture pathway. It should be specified explicitly — it is not inherent in the membrane installation and is often absent from standard cavity drain specifications.

Why this is commonly missed

Waterproofing design drawings typically focus on the external envelope, the drainage route, and the pump installation. Internal wall treatment at the base is often left to the contractor’s judgment or omitted entirely from the specification.

In refurbishment projects, the boundary between the waterproofing specialist’s scope and the builder’s or finisher’s scope frequently falls exactly at this junction — the membrane terminates at the drainage channel level, and the builder is assumed to apply whatever internal finishing is appropriate. If neither the specification nor the waterproofing design drawings address the treatment of the internal wall base, it is omitted.

An independent waterproofing designer co-ordinating across scopes will identify this junction and specify the treatment. Where there is no independent designer, the gap is likely to become a defect.