Waterproofing and Tanking

BS 8102 Types A, B and C, roof membranes and wet-area tanking — keeping water out, or accepting it and managing it, without kidding yourself about which one you are doing.

Waterproofing and Tanking — construction process cover

Last updated 2026-07-28 by the BuildPedia Editorial Team.

What is Waterproofing and Tanking?

Below-ground waterproofing is the one package where a defect cannot be patched from outside afterwards — the earth goes back, the landscaping goes on, and the next time anyone sees the membrane is when water is coming through the wall inside. BS 8102 sets out the three recognised approaches: Type A, barrier protection — a membrane or coating applied to the structure that the water never gets past; Type B, structurally integral protection — the concrete box itself is the waterproofing, with waterbars and crack control doing the work; and Type C, drained cavity protection — the water is allowed through the structure, collected by a cavity drain membrane and pumped or drained away. Most serious basements combine at least two, because every system has a failure mode and belts and braces are cheap compared with digging up a finished garden.

The commercial reality on site is that Type A wins tenders and Type C wins arguments. A bonded sheet or liquid membrane is cheap to price and quick to apply, but it depends absolutely on substrate preparation, laps, and workmanship that is buried and uninspectable within weeks — and when it leaks, there is no maintenance route that does not involve excavation. A Type C cavity drain system accepts minor ingress and manages it behind a maintainable, accessible layer; it costs more up front but it is the only approach with a genuine whole-life answer. That is why warranty providers and basement specialists increasingly push drained systems, and why the specification argument at pre-construction is worth having properly.

Above ground the same logic applies in miniature: flat roof membranes live or die on falls, laps and upstands, and wet-area tanking in bathrooms and showers fails at corners, penetrations and the tray junction — almost never in the middle of the wall. In the UAE, below-ground work faces high saline water tables on coastal plots, and roof waterproofing takes a beating from UV and thermal cycling; liquid-applied and torch-applied membranes dominate, with DCD and municipality approval of systems and applicators.

When and why is Waterproofing and Tanking used?

Waterproofing follows the excavation and structure: below-ground systems go on as the basement box completes and before backfill, roof membranes follow the deck and insulation build-up, and wet-area tanking happens just before tiling at second fix. The choice between Type A, B and C is driven by ground conditions, water table, the use of the space (a plant room tolerates risk a bedroom does not), the warranty requirement, and honest assessment of who is maintaining it in year ten.

Types of Waterproofing and Tanking

Type A — barrier protection

Bonded sheet membranes, liquid-applied coatings or bentonite applied to the outside (or inside) of the structure to stop water at the face. Performance depends entirely on substrate preparation, laps and protection before backfill.

Type B — structurally integral

The concrete box itself is designed as the barrier: waterproof concrete with waterbars at joints, crack-width control, and sealed service entries. Common on UK basements and standard practice for UAE basement boxes.

Type C — drained cavity

Studded cavity drain membranes on walls and floors collect any ingress and channel it to a sump and pump or a gravity drain. Maintainable and accessible — the interior stays dry even if the structure leaks a little.

Roof waterproofing systems

Reinforced bitumen membranes (torch-applied), single-ply sheets (PVC, TPO, EPDM) and liquid-applied systems over the deck and insulation build-up. Falls, outlets, upstands and penetrations are the battleground.

Wet-area tanking

Cementitious or liquid-applied tanking under tiles in showers, bathrooms and plant rooms — full-height in shower zones, with preformed corner tapes, pipe collars and sealed tray junctions.

Waterproofing and Tanking: step by step

Step 1: Set the strategy and the grade

Set the strategy and the grade — Waterproofing and Tanking, step 1

BS 8102 grades the internal environment — Grade 1 tolerates some seepage (car parks, plant rooms), Grade 2 wants no visible water but tolerates vapour, Grade 3 (habitable rooms) demands a dry space. The grade, the ground conditions and the water table drive whether you need one form of protection or two combined. Write the strategy down before the box is designed, because Type B has to be in the concrete design, not bolted on after.

Step 2: Prepare the substrate — properly

Prepare the substrate — properly — Waterproofing and Tanking, step 2

Every Type A failure story starts here. The surface must be clean, sound, smooth and dry to the membrane manufacturer's limits: fill tie holes and blowholes, grind off fins, round or fillet internal corners, chase out and repair honeycombing. Sheet membranes need priming; liquid systems need the moisture content checked with a meter, not a guess. If the substrate is wrong, stop — the membrane will faithfully copy every defect into a leak path.

Step 3: Install Type B joints and waterbars

Install Type B joints and waterbars — Waterproofing and Tanking, step 3

For a structurally integral box, the waterproofing is in the pour sequence: waterbars centred and fixed at every kicker, day joint and movement joint, kicker height to spec (typically 150 mm minimum), kickers cast with the slab or properly roughened and cleaned. Service penetrations get puddle flanges or hydrophilic seals set before the pour. Crack control comes from the reinforcement and the pour plan — there is no remedial easy fix for a leaking cold joint.

Step 4: Apply the Type A membrane

Apply the Type A membrane — Waterproofing and Tanking, step 4

Sheets are laid to the lap schedule — typically 100 mm side laps, 150 mm end laps — rolled firm, with seams tested as you go; torch-applied membranes want an even bleed of bitumen at the lap, not a charred edge. Liquid systems go on in the specified coats to the specified wet-film thickness, checked with a gauge. Around penetrations, corners and terminations use the preformed details and tapes. Terminate at the DPC line or above the water table as the drawings show, never short.

Step 5: Protect it before backfill

Protect it before backfill — Waterproofing and Tanking, step 5

A finished membrane on a basement wall has hours of vulnerability: protection boards or a drainage/protection layer go on before a single bucket of backfill. Backfill in layers with selected material — no rubble, no frozen lumps, no compaction plant tight against the wall. Most "mystery" basement leaks are damage done in this step by a machine in a hurry.

Step 6: Install the Type C drained system

Install the Type C drained system — Waterproofing and Tanking, step 6

Wall membrane is fixed with sealed plugs at the specified centres, floor membrane laid with overlaps taped, and the perimeter drainage channel bedded at the wall-floor junction with rodding eyes. The sump chamber, pump, battery backup and high-level alarm go in and are commissioned before the floor finish. Document the system layout — the whole point of Type C is that someone can maintain it, and they can only maintain what they can find.

Step 7: Lay the roof waterproofing

Lay the roof waterproofing — Waterproofing and Tanking, step 7

Check falls to outlets with a level and a hose before any membrane — ponding on a flat roof shortens the life of any system. Torch-on membranes go down in the specified layers with staggered, tested laps; single-ply is loose-laid or bonded with mechanically fixed seams welded and probe-tested. Upstands rise at least 150 mm above finished roof level, into chases or under counter-flashings, and every penetration gets a preformed boot or a proper pitch pocket.

Step 8: Tank the wet areas

Tank the wet areas — Waterproofing and Tanking, step 8

In showers and wet rooms, tank the floor and full height in the spray zone, carry the floor membrane up the wall 150 mm and the wall membrane over the bath rim and tray upstands. Preformed internal and external corner tapes, pipe collars and drain connectors are not optional extras — they are where these systems leak when the corners are brushed over. Flood-test the tray and floor for 24 hours before tiling, with the outlet plugged.

Step 9: Test, commission and record

Test, commission and record — Waterproofing and Tanking, step 9

Roof: flood-test or electronically leak-test the membrane before coverings or finishes. Basement: commission the Type C pump and alarm, record discharge tests, and hand over the maintenance manual with the system layout. Every buried or covered layer gets photographed in progress with laps visible. Warranty providers (NHBC in the UK) and DCD/municipality inspectors in the UAE will want the records — assemble them as you go, not at handover.

Plant and equipment

Quality control checks

Safety considerations

Common defects

Best suited for

How long does Waterproofing and Tanking take?

Typical duration: Membrane on a basement wall elevation: 2–5 days including preparation and protection. Type C fit-out in a house basement: 1–2 weeks with drainage and commissioning. A house flat roof: 2–4 days. Wet-area tanking per room: 1 day plus cure and flood test..

Related processes