Waterproofing Basements

When DTS meets the real world 

NCC 2022 Volume One

Area of NCC Requirements:

• F1P2 – Preventing rainwater from entering buildings

• F1P4 – Rising damp

The Challenge

Below-ground masonry walls introduce a unique set of moisture and compliance risks—particularly when they also function as retaining elements. In this project, a masonry wall within a commercial laundry area was proposed to retain backfill, placing it in direct contact with soil and potential groundwater.

The NCC does not provide a clear Deemed-to-Satisfy (DTS) pathway for internal masonry walls that also act as retaining walls. Without an alternative compliance approach, the design raised legitimate concerns around rainwater ingress, rising damp, and the potential impact on internal amenity and long-term building durability.

To proceed, a performance-based solution was required to demonstrate that the wall would not adversely affect the building or its occupants.

What This Really Means

The NCC’s health and amenity provisions are outcome-focused. For below-ground construction, the intent is to ensure that rainwater and ground moisture are effectively managed so they do not enter the building or compromise internal spaces.

Where walls perform multiple roles—such as enclosure and retention—compliance depends on whether the system can reliably prevent water ingress and rising damp over time, not whether it matches a prescriptive construction detail.

The Solution

A performance-based assessment was undertaken to evaluate whether the proposed below-ground masonry wall achieved the intent of F1P2 and F1P4 under NCC 2022 Volume One.

The assessment considered:

• How rainwater and ground moisture are controlled at the wall interface
• Whether rising damp is effectively prevented under all anticipated conditions
• How the wall integrates with surrounding ground levels and floor construction
• Long-term durability and resistance to moisture-related deterioration
• Whether the overall performance is equivalent to, or better than, a non-retaining internal wall under DTS conditions

Rather than assessing the wall in isolation, the performance solution considered how the building performs as a complete system once the retaining condition is introduced.

Why This Matters

The final report confirmed that the proposed design satisfied the NCC performance requirements for preventing rainwater ingress and rising damp. Despite the absence of a DTS pathway, the wall was shown to deliver a dry, durable, and compliant outcome appropriate for a commercial building.

This project demonstrates how performance solutions provide certainty where prescriptive NCC provisions stop—allowing practical, site-driven design decisions without compromising compliance.

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