External Tanking Systems: Cementitious Slurry, Sheet Membranes, Drainage Board and Drainage Layer Details

Quick Answer: External tanking (BS 8102 Type A) is applied to the outside face of below-ground walls before backfill. Cementitious slurry coatings (e.g. Sika Aqua Tank, Vandex) build 3-5mm thick and bond chemically to the substrate. Self-adhesive bituminous sheet membranes (Sika Bituthene) are pre-formed sheets bonded with primer. HDPE sheet membranes (Newton 909) are mechanically fixed. All systems require a drainage layer (drainage board or geotextile-wrapped gravel) on the outside face to relieve hydrostatic pressure. External tanking is rarely viable for converting existing basements because the structure is already backfilled — Type C cavity drain is the standard alternative.

Summary

External tanking is the traditional and conceptually simplest waterproofing approach: keep the water out by wrapping the structure in a continuous waterproof barrier on the outside. It works well when correctly installed, but the demands on installation are high. A single defect in the membrane or detail can compromise the system. After backfill, the membrane is hidden and inspection is impossible without re-excavation.

For new-build basements, external tanking is the standard primary defence. Combined with internal Type C cavity drain (per BS 8102 best practice), it provides defence in depth — the external tanking does the bulk of the work, and the internal Type C catches any breaches.

For converting existing basements, external tanking is usually not viable because excavating around an existing structure is expensive, disruptive, and may compromise foundation stability. Type C internal cavity drain is the dominant solution. External tanking on existing structures is only feasible where excavation is already happening for other reasons (e.g. landscaping or retaining wall reconstruction).

Key Facts

Quick Reference Table

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System Material Thickness Substrate Tolerance Best For
Cementitious slurry Polymer-modified cement 3–5mm Low — needs sound substrate Concrete, blockwork
Bituminous sheet Bitumen + reinforcement 1.5–2mm Medium Smooth concrete
HDPE sheet High-density polyethylene 0.5–1mm High — corrugated dimples Variable substrates
Bentonite mat Bentonite clay 6–10mm Very high — clay seals defects Imperfect substrates
Liquid-applied Polyurethane / polymer 1–2mm dry Medium Complex shapes

Detailed Guidance

Substrate preparation

A successful tanking installation depends entirely on substrate quality. BS 8102 requires:

Without proper preparation, even the best material will fail. This is the single most common cause of premature tanking failure.

Cementitious slurry coatings

Most common Type A system in the UK. Polymer-modified cement powder mixed with water (sometimes a polymer admixture); applied with brush or trowel.

Application sequence:

  1. Prepare substrate (clean, dampen, repair cracks, fillets)
  2. Apply primer/bonding coat (some systems)
  3. First coat — typically 2-3mm thick, applied with brush
  4. Second coat — applied perpendicular to first; total 3-5mm
  5. Cure for 7+ days (depending on conditions)
  6. Apply protection board
  7. Backfill

Common products:

Cementitious slurry is forgiving of damp substrates (works when other systems fail). Limitation: rigid bond means it tracks substrate cracking; if the substrate cracks, the coating cracks with it. Engineer's design should minimise post-installation cracking.

Self-adhesive bituminous sheet membranes

Pre-formed sheets bonded to substrate with primer. The most common is the Sika Bituthene 8000 series.

Application sequence:

  1. Prepare substrate (clean, dry, smooth)
  2. Apply primer (Sika Bituthene Primer S2 or similar)
  3. Allow primer to dry (typically 1-2 hours)
  4. Position sheet, peel release film, press onto substrate
  5. Roll seams to ensure full adhesion
  6. Lap subsequent sheets minimum 100mm with sealant at lap
  7. Detail penetrations with separate flashing pieces
  8. Apply protection board
  9. Backfill

Sika Bituthene 8000 — 1.5mm bituminous, polymer-reinforced, self-adhesive. The reference UK product for external tanking on residential and commercial.

Bituthene LM/HM — high-performance variants for higher water tables.

Bituminous sheet is faster to install than cementitious but requires a dry substrate and careful detailing at penetrations. Vulnerable to high temperatures during install (membrane sags); not used in high-summer applications without temperature control.

HDPE sheet membranes

Heavy-duty embossed plastic sheets, mechanically fixed to substrate. Often used as the membrane component of a combined drainage-tanking system.

Newton 909 — 0.5mm HDPE with raised dimples; fixed mechanically with plugs and washers; laps sealed with butyl tape.

Delta MS500 — 0.6mm HDPE; similar fixing detail.

HDPE membranes are very forgiving — they accept rough substrates, work in cold conditions, are not affected by minor punctures (the dimples create air gaps that limit damage propagation). Often combined with drainage boards for combined Type A + drainage layer.

Bentonite mats

Bentonite is a swelling clay; when in contact with water, it expands by 15× its dry volume to form a self-sealing barrier. Bentonite mats wrap the bentonite in geotextiles for handling.

Volclay Voltex — 6-7mm bentonite layer between geotextiles Cetco Volclay — similar product

Bentonite is highly forgiving of substrate imperfections — small cracks in concrete are sealed by clay swelling. Used on heritage and difficult-to-prepare substrates. Limitation: requires sufficient confinement (backfill weight) to develop full sealing pressure.

Drainage layer

A drainage layer on the outside of the tanking is essential to relieve hydrostatic pressure. Without it, water table presses against the membrane and seeks any defect. With drainage, water is intercepted and channelled to a perimeter drain.

Drainage board options:

Geotextile-wrapped gravel:

Perimeter drain:

Without the drainage layer, the tanking faces full hydrostatic pressure. With drainage, the membrane only needs to handle water that has bypassed the drainage system.

Penetration detailing

Penetrations are the weakest point of any tanking system. Common penetrations:

Approved details:

Always specify penetration detail in the specification, and inspect on site before backfill. Photograph each penetration before backfill — this is your only evidence if a leak develops later.

Backfill protection

After tanking is cured/set, install protection board to prevent damage during backfill:

Backfill in 300mm lifts, compacted with light pneumatic plate compactor. Do not use heavy compaction equipment within 600mm of the wall. Use clean granular fill (e.g. Type 1 or 6F2) — no sharp stones that could puncture protection.

Combining with internal Type C

Modern best practice for Grade 3+ habitable basements:

Belt-and-braces approach. Costs are additive (no saving from combining), but failure modes are uncorrelated — if Type A fails, Type C catches; if Type C pump fails, Type A still defends. IBG providers prefer combined systems for habitable applications.

Common failure modes

  1. Inadequate substrate preparation — cementitious cracks tracked, bituminous failed adhesion
  2. Penetration leaks — service pipes, drainage pipes, fixings through membrane
  3. Backfill damage — protection board missing or punctured
  4. Lap failures — sheet membranes with inadequate laps or no sealant
  5. No drainage layer — full hydrostatic pressure overwhelms even good membrane
  6. Settlement cracking — substrate moves after install, breaks membrane bond
  7. Premature backfill — cementitious not fully cured

Frequently Asked Questions

Can I tank an existing basement externally?

Technically yes, but rarely cost-effective. Excavation is expensive (typically £150-£300/m³), disruptive (lose use of garden, scaffolding, plant access), and may require temporary shoring. For most existing basements, internal Type C is the practical solution — see cavity drain membrane systems.

Which is better, cementitious or bituminous?

Each has uses. Cementitious is forgiving of damp substrates and works on rough surfaces. Bituminous is faster to install and gives a continuous membrane. For new-build, both work well; choose based on programme and contractor preference. For damp/heritage substrates, cementitious is usually better.

How long does an external tanking system last?

Properly installed cementitious or bituminous systems should last 50+ years. Failure typically occurs at penetrations or details rather than within the bulk of the membrane. With internal Type C as backup, the consequence of any failure is significantly reduced.

What's the typical cost premium for external tanking on new-build?

For a new basement, external tanking + drainage layer typically adds £40-£100/m² to the cost (membrane + labour). On a 50m² wall area, that's £2,000-£5,000 — modest premium for the additional defence.

Can the drainage discharge to a soakaway?

Yes, subject to ground conditions. Approved Document H requires soakaway design per BRE 365. Test the infiltration rate; if soil is clay-heavy with low infiltration, soakaway may not be viable and discharge to surface water sewer is required.

Regulations & Standards