Bedroom Room Ceiling Heights

Performance based alternatives 

NCC 2022, Volume 2

Area of NCC Requirements:

• H2P2 – Weatherproofing

The Challenge

Rooftop gardens are increasingly being incorporated into residential design, offering environmental, aesthetic, and amenity benefits. However, from a regulatory perspective, they introduce a unique challenge: the NCC does not provide Deemed-to-Satisfy (DTS) provisions specifically addressing rooftop garden construction or waterproofing.

In this project, a rooftop garden was proposed above a two-storey dwelling. Without a prescriptive pathway, the design raised legitimate concerns about long-term weatherproofing performance, potential water penetration, and the risk of structural deterioration if moisture was not effectively managed.

To proceed, a performance-based approach was required to demonstrate that the roof assembly would achieve the NCC’s weatherproofing intent.

What This Really Means

The NCC’s weatherproofing requirements are outcome-focused. Roofs must prevent water ingress, manage moisture safely, and protect the structure and internal spaces below.

When a design element—such as a rooftop garden—falls outside the scope of DTS provisions, it does not mean it cannot be built. It means the design must clearly demonstrate that it performs at least as well as a conventional roof in resisting water penetration and maintaining durability over time.

The Solution

A performance-based assessment was undertaken to evaluate whether the proposed rooftop garden achieved the intent of Performance Requirement H2P2.

The assessment considered:

• How water is managed across the rooftop surface under normal and extreme conditions
• Whether the waterproofing strategy provides continuous and durable protection
• How drainage, build-up layers, and detailing work together as a system
• The potential impact of moisture on the structure below the roof
• Whether the overall performance is equivalent to, or better than, a traditional DTS-compliant roof assembly

Rather than focusing on prescriptive rules, the assessment examined how the rooftop garden functions as a complete weatherproofing system.

Why This Matters

The final report confirmed that the proposed rooftop garden satisfies NCC Performance Requirement H2P2. Despite the absence of DTS guidance, the design was shown to provide effective weatherproofing and protect the building’s structure and amenity.

This project demonstrates how performance solutions enable innovative residential features—like rooftop gardens—to be delivered safely and compliantly, even where the NCC is silent.

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