What is external waterproofing?

External waterproofing involves excavating around the outside of below-ground walls and applying a waterproof membrane or coating to the external face. This is the most effective method (Type A protection to BS 8102) because it keeps water away from the structure entirely. It is more disruptive and costly than internal methods.

What membrane is used externally?

Self-adhesive bituminous membrane, torch-on modified bitumen, or liquid-applied rubber coatings are commonly used. The membrane must be continuous, lapped at joints, and sealed at all penetrations. A protection board (such as Platon or polystyrene) is placed over the membrane to protect it from damage during backfilling.

Do I need a land drain at the base?

Yes. A perforated land drain pipe should be laid at the base of the excavation, bedded in gravel, to collect any water that reaches the base of the wall and direct it to a sump, soakaway or surface water drain. This prevents hydrostatic pressure building up against the membrane.

How wide should the excavation be?

The excavation needs to be wide enough for safe working — minimum 600mm clear width from the wall face. For deeper excavations (over 1.2m), the trench must be battered or supported to prevent collapse. Health and safety requirements for trench work must be followed.

What about the backfill material?

The lower 300mm should be gravel or clean stone to assist drainage to the land drain. Above this, the original excavated material can be used if it is free-draining. Clay soils should not be placed directly against the protection board — use imported granular fill instead.

TradeCalculator.co.uk

External Waterproofing Calculator — Excavation, Membrane & Drainage

Calculate external basement waterproofing requirements: excavation volume, membrane area, protection board, land drain pipe and backfill quantities.

Wall Length

Total length of walls to waterproof

Excavation Depth

Depth to base of wall

Price Per m² (optional)

All-in cost per m² of wall treated

How We Calculate This

This calculator estimates quantities for external waterproofing based on wall length and excavation depth.

The formula

Excavation = Length × Width × Depth

Membrane = Wall Area × (1 + Overlap)

Gravel = Length × Width × 0.3m

Components

Membrane: Bituminous or liquid-applied
Protection board: Over membrane
Land drain: Perforated pipe in gravel bed
Backfill: Gravel base + general fill

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.