Windows in Waterproof Walls

Addressing AS 3740 shortfalls

NCC 2022 Volume Two

Area of NCC Requirements:

• Section H: Damp and weatherproofing
• Relevant Performance Requirements:
• • H2P1: Rainwater management
  • H2P2: Weatherproofing
  • H2P3: Rising damp

The Challenge

Planter boxes integrated into external walls can look great and add real amenity, but they also introduce a concentrated moisture risk right at the building envelope. In this project, a first-floor planter box included a concrete upstand constructed without the typical minimum set-down that is commonly relied upon to reduce the risk of surface water entering the wall and floor junction.

Without that set-down, the likelihood of water sitting against the wall junction increases. If not properly managed, this can lead to moisture ingress, deterioration of adjoining building elements, and conditions that support rising damp and mould growth—ultimately compromising durability and occupant health.

Because the arrangement fell outside the usual prescriptive expectations, a performance-based pathway was required to demonstrate compliance with the NCC’s damp and weatherproofing intent.

What This Really Means

The NCC is outcome-focused: external walls and junctions must manage water so it does not penetrate the building fabric, cause dampness, or reduce amenity over time. Where standard geometric protections (like a set-down) are not provided, the design must clearly demonstrate that water will still be controlled through the overall system particularly at transitions, terminations, and drainage points.

The Solution

A performance-based assessment was undertaken to verify that the planter box and wall junction achieves the intent of H2P1, H2P2 and H2P3.

The assessment considered:

• How surface water is directed away from the wall junction and prevented from ponding
• Whether the waterproofing strategy provides continuous protection at critical interfaces
• How drainage is managed so water cannot build up behind finishes or within concealed spaces
• How moisture risk is controlled over time, including durability and maintenance expectations
• Whether the overall outcome is equivalent to, or better than, a conventional set-down arrangement

Rather than relying on a single prescriptive detail, the performance pathway evaluated how the planter box assembly functions as a complete water-management system.

Why This Matters

The final report concluded that the proposed planter box waterproofing and drainage design meets the relevant NCC performance requirements when installed in accordance with applicable Australian Standards and manufacturer specifications. Despite departing from typical set-down geometry, the system was shown to effectively manage water and prevent moisture ingress in a way that protects both the building and its occupants.

This project is a strong example of where performance solutions unlock modern architectural features while still maintaining robust, certifiable weatherproofing outcomes.

Got planter boxes that don’t fit the DTS playbook? 

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