Why does waterproofing fail at the details?

Every sheet system is a collection of pieces held together by something else. A plastic DPM under a slab is a series of overlaps, taped and dressed up the upstands, with the tape doing the waterproofing at every joint. A tanking tray relies on a bonded seam at the wall-to-floor junction. A single-ply roof relies on welded laps and a collar around each outlet. The material between the joints is rarely what leaks. The joints are.

The pattern repeats because sheets are flat and buildings are not. A kicker joint, a pile cap, a ground beam, a service penetration, an internal corner in a wet room, a balcony threshold, a rainwater outlet — these are the shapes a flat sheet has to be cut, folded, lapped or collared to fit. Every one is a manual detail whose success depends on a specific person on a specific day, and almost none is inspectable once the screed, the tiles or the paving goes on.

A liquid-applied membrane removes the joints rather than trying to make them perfect. Applied wet — by brush, roller, spray or pour — it cures into a single monolithic layer bonded directly to the substrate, moulding itself around penetrations and awkward junctions with no seams to fail, and it cannot lift, tear or be punctured by following trades. Genuinely high-risk details are reinforced with tape, mesh or sealant and then coated, so the reinforcement sits inside the membrane rather than being the membrane.

Damp proofing or waterproofing — which does this detail need?

The two terms are used interchangeably on site and they should not be. The difference is what kind of water each is designed to stop.

Damp proofing resists moisture moving through the structure from the ground — capillary rise, ground moisture, vapour. It is the right answer where the internal floor level is above external ground level and there is no below-ground habitable space, so no significant water pressure acts on the construction. Approved Document C requires floors and walls next to the ground to resist moisture passing to the inside; a horizontal damp-proof course should sit at least 150 mm above external ground level, and ground floors need a damp-proof membrane.

Waterproofing resists water under pressure. Below ground, groundwater exerts constant hydrostatic pressure, and a system that merely resists capillary moisture will not hold it back. Where a below-ground room is habitable, the design follows BS 8102:2022, the code of practice for protection of below-ground structures against water from the ground — typically Grade 3, a dry environment.

Getting the diagnosis right matters as much as getting the product right. Rising damp shows as a tide mark up to around a metre above floor level, with salts on the plaster and crumbling skirtings. Penetrating damp shows as patches anywhere on the wall after wind-driven rain. Condensation is surface moisture and has a different cure entirely. Our guides work through all three: Damp Proofing vs Waterproofing and How to Stop Rising Damp with a Liquid DPM.

What goes under a floor and against a foundation?

If you have worked on a block-and-beam floor or a kicker block detail, you know the usual approach: loose-laid plastic DPM sheet, taped and dressed up the upstands, hoping every lap holds through the follow-on trades. Waterproof Blue does the job differently — a polymer-modified, water-based, VOC-free liquid membrane applied by roller or brush in two crosswise coats, curing into one seamless, jointless barrier.

It goes anywhere a plastic DPM or DPC would traditionally be specified: block-and-beam floors before screed, kicker block upstands, foundation walls, basement floors and slabs, and behind coping stones. Use the 5 kg brush grade to seal reveals, upstands and junctions accurately. Because it forms a continuous unbroken membrane, it also carries radon barrier certification, which matters where the ground floor build-up has to double as a damp proof membrane and a gas barrier — see our underground gases page.

Where the structure is genuinely below ground and under pressure, the specification steps up to The GOAT, a solvent-free reactive polymer engineered for those conditions. It applies to damp surfaces at 0 °C, where standard coatings need a dry substrate and 5 °C or above — a real programme advantage in UK groundworks. It handles positive and negative water pressure, and is fully bonded and flexible, bridging shrinkage cracks and moving with the structure rather than debonding.

Typical applications are the external faces of foundation walls before backfilling, kicker joints, basement walls and roofs, pile caps and ground beams, and service penetrations. The 18 kg roller grade covers large flat and gently sloping areas quickly; the 7.5 kg thicker grade is mixed to a higher viscosity so it grips vertical surfaces without slumping, and can also be poured. Reinforcement is embedded at junctions and crack-risk areas before the second coat. Full method in How to Waterproof Foundations with The GOAT and Waterproofing Basements and Foundations.

What has to go behind the tiles in a wet room?

Under the NHBC Standards 2023 updates introduced in July 2023, all bath and shower enclosures, wet rooms and bathroom pods must incorporate an impermeable liquid-applied waterproofing membrane behind tiled finishes. The revision to Clause 9.2.5, "Ceramic Wall Tiling", followed a rise in insurance claims relating to failures around baths and shower enclosures. It makes clear that gypsum plasters must not be used in areas subject to repeated or persistent wetting, and that wet areas require backing surfaces protected with a suitable tanking system. The requirements are mandatory for every NHBC-registered home where foundations were begun on or after 1 January 2024, and at 9.2/06, Table 2, liquid-applied systems are identified as a compliant solution.

Poseidon is the product for that job: a polymer-based liquid-applied waterproof membrane curing to a flexible, durable, impervious layer prior to tiling, for concrete and cement-based surfaces, shower enclosures, bathrooms and wet rooms, steam areas and bathroom pods. It is applied by spray, roller or brush as two cross-applied coats to a dry film thickness of roughly 0.4–1.0 mm, with reinforcement at internal corners, wall-to-floor junctions and around pipe and drain penetrations, before tiling with a suitable C2 cement-based adhesive. The step-by-step sequence is in How to Waterproof a Wetroom or Bathroom Before Tiling, and the compliance background in NHBC Standards 2023: Waterproof Membranes as a Tanking System.

What does an ETA actually certify?

A European Technical Assessment is a route to CE marking for construction products not fully covered by a harmonised European standard: an independent, pan-European assessment carried out by an approved body against an agreed assessment document — not a self-declaration, and not a marketing badge. It is the difference between a documented performance you can hand to a building control officer and a claim printed on a lid.

Poseidon holds ETA-23/0378, issued by ETA-Danmark under EAD 030352-00-0503, the assessment document for watertight covering kits for wet room floors and walls. It is assessed under AVCP system 2+, involving an independent notified body and ongoing factory production control, and assumes a 25-year working life when correctly installed and maintained. Declared characteristics include watertightness across the covering and around penetrations; crack-bridging category 1 at 0.4 mm; bond strength greater than 0.5 MPa, retained at ≥0.5 MPa after water, temperature and alkalinity durability testing; and an sd value of 33.65 m. Underlying testing is to EN 14891 at Buildwise in Belgium. Poseidon has also been tested against ANSI A118.10 and A118.12 at IPA Laboratories — detail in Poseidon Meets the US Tile Standards, and the full explanation of what an ETA covers is in What an ETA Means.

The GOAT holds ETA-25/0471, issued by ETA-Danmark on 16 May 2025 under EAD 030350-00-0402, the assessment document for liquid applied roof waterproofing kits. It confirms a 25-year assessed working life (W3), B_ROOF(t1) external fire performance, watertightness, wind load resistance above 50 kPa, and suitability for all roof slopes from flat to steep (S1–S4) at surface temperatures from −20 °C to +80 °C. Background in What Does an ETA Mean? The GOAT Roofing Membrane Explained.

Which product goes where in the building?

View the liquid waterproof membrane range

How much material should we allow?

Full figures are on the application and coverage rates page. Both Poseidon and The GOAT are two-coat systems and the published coverage already assumes both coats.

Has it been proved on real buildings?

At Reuben College, Oxford, Airtight White delivered basement airtightness on an award-winning scheme — a below-ground environment where sealing and waterproofing have to be considered together rather than as separate packages. Our best airtightness result on a school is 0.078 ACH at Wester Hailes High School, Edinburgh — a whole-building pre-test after our works, measured by EDP Building Sciences to ATTMA TSL4 in November 2025 against a 0.6 design target.

The certification record is the other half of the evidence, and it is deliberately specific: Poseidon's ETA-23/0378 with EN 14891 testing at Buildwise; The GOAT's ETA-25/0471 with its 25-year assessed working life. Those are documents that can be handed over, not claims taken on trust.

Reuben College — basement Full test data and certifications

Can we see it on site before we specify it?

Inspect a completed application before committing to it. The 360° tours let a client-side team walk finished work and look at kicker joints, upstands, corners and penetrations as they were left; the video channel shows the roller, brush and pour sequences in real site conditions.

360° site tours Application videos

Frequently asked questions