Water Leaks & Structural Leaking

The cost of ignoring it

Water entering a building rarely stays a leak. Left to continue, it carries moisture and chlorides to the reinforcement, decays finishes and linings, and damages the rooms below, so a leak that could have been repaired at its source becomes a concrete repair and reinstatement scope.

External membrane build-up over a structural slab

Where the water is coming from

Most leaks are not where they appear. Water enters at a detail — an upturn terminated short, an unsealed penetration, a bridged movement joint, a crack through a slab, a cold joint in a basement wall — and travels before it shows as a stained ceiling, efflorescence, drummy tiles or water on a carpark floor. Repairing where the water appears, rather than where it enters, is the most common reason a leak returns.

Structural leaking

Structural leaking is water passing through the concrete itself. It occurs through shrinkage and movement cracks in suspended slabs and walls, at construction and cold joints where one pour meets another, around penetrations cast or cored through the structure, and through basement, lift pit and retaining walls where groundwater is present.

It is repaired at the concrete. Leaking cracks and joints are injected with polyurethane resins, which react with water and expand to seal the path, and which can be injected while the crack is actively leaking. Where a crack also affects structural capacity, it is injected with structural epoxy to the engineer’s requirements. Below ground, where the outer face of a wall cannot be reached, crystalline or negative-side systems are applied to the internal face. Water that reaches reinforcement also starts the corrosion process, so structural leaks are investigated for concrete damage at the same time; see concrete repair.

Diagnostic approach

The breach is located before the repair is scoped:

Moisture mapping of affected and adjacent areas to establish the wet footprint
Flood testing of suspect zones in isolation, testing one variable at a time
Electronic leak detection on accessible membranes to localise breaches without demolition
Thermal imaging to trace water paths behind finishes
Crack and joint survey of slabs and walls, recording which are leaking, which are moving, and which are dry
Detail inspection at upturns, penetrations, drainage and junctions, where most membrane failures originate

The report identifies entry points, the water path, consequential damage, and repair options ranked by level of intervention, from sealing a single crack or detail through to full membrane replacement.

Repair methodology

Injection repairs follow the crack and joint survey: ports are placed along the crack, the resin selected for the condition — polyurethane for water, epoxy for structure — is injected until refusal, and the repair is checked against water once cured.

Membrane repairs to external above-ground surfaces are governed by AS 4654.1-2012 (materials) and AS 4654.2-2012 (design and installation), both called up by the National Construction Code for roofs, balconies and similar surfaces; internal wet areas fall under AS 3740:2021. The system is matched to exposure and trafficability: liquid-applied polyurethane or PMMA for complex details and falls correction, and sheet systems for large trafficable decks. Substrate preparation, primer, detail reinforcement, membrane and protection are each hold-pointed with photographs, as manufacturer warranties are conditional on documented preparation and detailing.

Typical locations

Leaks occur through carpark and podium slabs, basement and lift pit walls, planters and roof decks, balconies and terraces, window and door junctions, and bathrooms over habitable rooms in strata buildings. Balcony membranes, drainage and balustrade anchorage are covered in more detail under balcony and balustrade remediation.

Water leaks — common questions

01 Water is appearing in one spot. Is the leak directly above it?

Rarely. Water tracks along membranes, slab folds, conduits, cracks and bond breaks, and can surface metres from where it entered. Testing is therefore carried out before any demolition is recommended. Flood testing, moisture mapping and electronic leak detection localise the breach, so that a small area is opened up rather than an entire terrace or ceiling.

02 What is structural leaking?

Water passing through the structure itself rather than through a failed membrane — through cracks in a slab or wall, through construction and cold joints, around penetrations, or through basement and lift pit walls below ground. It is common in carparks, podiums and basements, and is repaired at the concrete, typically by injection, rather than by adding a new surface coating over it.

03 Can a leaking crack be sealed while water is coming through it?

Often, yes. Polyurethane injection resins react with water and expand to fill the crack, so they can be injected into a crack that is actively leaking. Where the crack also affects the load-carrying capacity of the element, structural epoxy is used instead, once the crack is dry enough to accept it; the engineer determines which applies.

04 Can a new membrane be applied over the existing one?

Sometimes, provided the existing membrane is sound, compatible and correctly detailed at upturns and penetrations. Adhesion and compatibility are tested first. Where the failure is systemic — the wrong system, no falls, or failed details — an overlay is expenditure on a repair that will fail. The testing establishes which of the two conditions applies, and the report sets out the evidence for that conclusion.

05 What warranty applies?

Installed systems carry the manufacturer's material warranty, for the period stated for that system, together with our workmanship warranty. Both are conditional on the substrate and the details being correct, and the documented methodology and hold-point records provide the evidence of that.