How to choose a waterproofing system: the questions that decide it

01Start from the water

Rain running off a roof is not the same load as groundwater pressing on a basement wall, or water held in a tank. Three facts set the starting point: where the water comes from, whether it acts under pressure, and which face of the element it reaches first.

• Rain and runoffRoofs, terraces and balconies: water leaves if the falls and drains work.
• Groundwater and wet soilBasements, foundations and retaining walls: possible pressure, salts and sulphates.
• Retained waterTanks, pools and fountains: constant pressure from inside, and drinking-water approval where it applies.
• Service waterBathrooms, kitchens and plant rooms: intermittent wetting under finishes.

02Which side can you reach?

A membrane works best on the water (positive) side, where pressure pushes it onto the structure. When that face cannot be reached — an existing basement, a tank in use from outside — the options narrow to systems made for the negative side, such as some cementitious and crystalline products, usually after the cracks are injected.

A new building allows the choice; an existing one usually imposes it.

03What will sit on top of it?

• Exposed to the sunUV-stable membranes or a UV-resistant top coat, with daily heat cycles and thermal movement.
• CoveredTiles, screed, ballast or soil: a protection layer and compatibility with the finish above.
• TrafficPeople, maintenance or vehicles: a wearing course or a protected system.
• PlantingRoot resistance, drainage and filter layers.

04Movement, cracks and the substrate

Concrete moves with temperature and shrinkage, and buildings move at joints. Rigid systems such as rigid cementitious coats suit stable structures, elastic membranes follow movement within their limits, and joints need a joint system rather than a coating.

The substrate decides adhesion: concrete, block, screed, steel, old membranes and tiles each need their own preparation. Moisture in the substrate rules out some systems, such as resins, while others — cementitious and crystalline — need it.

05Details decide the result

Most leaks start at details rather than in the open area: parapets and upstands, drains, internal and external corners, pipe penetrations, construction and expansion joints, and transitions between two systems. A system that details well at your critical points is worth more than one that looks better on paper.

• Count the penetrations and drains before choosing the system.
• Check the upstand heights and how the waterproofing ends at the top.
• Plan the transitions where two systems meet: floor to wall, roof to parapet.

06A quick map of the families

07From family to product: what to ask for

• The approved specification and drawingsThey set the system and the details.
• The manufacturer’s TDSLimits, consumption, curing and compatibility.
• Approvals where they matterDrinking-water contact, fire classification for roofs, consultant approval.
• A method statement and an ITPHow the work is executed, checked and recorded.
• The warranty termsWarranty according to the approved system, manufacturer recommendations and the project agreement.

The System Finder gives a preliminary shortlist from these questions; an engineering review confirms it.