Why Is Water Ingress a Major Issue in Residential Buildings?

Water ingress is a leading cause of building defects in Australia. A 2019 study by Deakin University found that moisture-related issues were the most commonly reported defects in residential buildings. The rise of townhouses and apartments sharing common walls and floors has increased the complexity of waterproofing, making effective design and construction practices more critical than ever.

• Poor waterproofing can lead to:
• Structural degradation of timber, steel, and concrete elements
• Mould and mildew growth, posing health risks
• Damage to finishes, flooring, and internal linings
• Legal disputes between neighbours over shared defects

Is Water Ingress Caused by Design, Construction or Maintenance?

Water ingress typically results from a combination of design flaws, poor installation, and inadequate maintenance. The building envelope—including roofs, walls, balconies, and wet areas—must be carefully detailed and constructed to prevent water penetration. Common issues include:

• Insufficient falls on balconies and roofs
• Inadequate sealing around windows and doors
• Improperly installed or deteriorated waterproofing membranes
• Blocked or poorly designed drainage systems

Regular maintenance is essential to identify and address potential problems before they escalate.

Who Is Responsible for Waterproofing and What Standards Apply?

Waterproofing is a shared responsibility. Architects must design compliant systems with the correct detailing, and builders must install materials according to the manufacturer specifications.

The most common failures are due to poor workmanship, incompatible products or insufficient detailing during construction.

Australian Standards play a critical role in ensuring waterproofing systems are reliable and fit for purpose. These include:

• AS 3740 2021 – Waterproofing of internal wet areas
• AS 4654.1 and AS 4654.2 – External above ground waterproofing
• AS 4858 2004 – Wet area membranes

Where Does Water Ingress Commonly Occur?

Roofs

Roof structures, particularly flat roofs and internal box gutters, are among the most common sources of water ingress in Australian dwellings. Inadequate falls, poor drainage design, and lack of compliant overflow provisions often lead to ponding and eventual leakage into the internal building envelope.

According to NCC Volume Two – Part 3.5.1 and AS/NZS 3500.3, roof drainage systems must be designed with sufficient fall and overflow capacity to cope with local rainfall intensities.

• Metal and tiled roofing systems typically offer good weatherproofing performance when installed per manufacturer guidelines and AS 1562.1 (Design and Installation of Sheet Roof and Wall Cladding).
• Concrete roofs require special consideration, as concrete is only water-resistant. Waterproofing membranes must be applied in accordance with AS 4654.2 for external above-ground use, ensuring the membrane is UV-resistant, trafficable (if accessible), and securely bonded to the substrate.

Walls and Cladding

Water ingress at walls is frequently observed around window and door openings, typically due to inadequate flashing, sealing, or poor workmanship. Wind-driven rain can penetrate if systems are not correctly detailed or constructed.

Per NCC Volume Two – Part 3.5.3 and AS 4284 (Testing of Building Facades), windows and external wall systems must be appropriately flashed and sealed to prevent moisture penetration under pressure.

• Brickwork is inherently porous and requires appropriate damp-proofing strategies, including damp proof courses, cavity flashings, and weep holes, in line with AS 3700 (Masonry Structures).
• Lightweight cladding systems must be ventilated, with provision for drainage and pressure equalisation, to comply with both manufacturer instructions and the performance requirements of the NCC.

Bathrooms and Showers

Wet areas, particularly bathrooms, are high-risk zones for water ingress due to the frequent presence of moisture and multiple trade interfaces. Common failures include poorly prepared substrates, membrane discontinuity, and insufficient falls.

Per AS 3740:2021, all internal wet areas must incorporate a compliant waterproofing membrane system. Key requirements include:

• A minimum fall of 1:100 in shower floors towards the waste.
• Waterproofing of the entire floor area and upturns at walls in shower enclosures.
• Use of water-resistant substrate materials.

Ongoing maintenance, such as tile regrouting every five to six years, is essential to prevent moisture infiltration beneath surface finishes.

Balconies

Balconies are exposed to consistent rainfall and weathering, requiring durable and well-detailed waterproofing systems.

• Key design elements include minimum falls (generally 1:100) to drainage outlets, step-down thresholds, compatible finishes, and correct termination of membranes at junctions with walls, doors, and drains.
• Waterproofing membranes for balconies must comply with AS 4654.1 & AS 4654.2, ensuring they are trafficable, UV-stable, and properly bonded.

Even minor detailing oversights, such as inadequate slope or poorly terminated joints, can lead to long-term deterioration.

Foundations and Basements

Sub-grade structures, including foundations and basements, are highly susceptible to water ingress due to hydrostatic pressure, capillary action, and soil reactivity.

• Water can infiltrate through construction joints, cracks, or movement-induced fissures unless adequately managed.
• Polyethylene vapour barriers provide partial protection, but a complete waterproofing strategy often requires:
• Perimeter drainage systems
• Waterproof tanking (for habitable spaces)
• Ventilated wet systems (for non-habitable basements)

Basement design should align with AS 2870 (Residential Slabs and Footings) and be informed by geotechnical and hydraulic site conditions. Additionally, local council regulations around groundwater discharge must be reviewed and incorporated into the design.