Subfloor Ground Clearance and Ventilation Rules for STRUCTAflor®

Getting the subfloor right is one of the most important steps in achieving a high-performing and long-lasting floor system. Adequate ground clearance and ventilation not only ensure compliance with Australian building standards but also protect the structure, improve durability and contribute to a healthier indoor environment.

Why It Matters

Poor subfloor design can lead to a range of issues including dampness, mould, fungal decay and even structural damage. The National Construction Code (NCC) and industry standards such as AS 1884 and the Australasian Timber Flooring Association (ATFA) guidelines set clear expectations to:

• Allow adequate termite inspections.
• Minimise the risk of moisture build-up.
• Protect floor coverings from water and humidity damage.
• Safeguard occupants from potential health issues caused by mould.

Following these standards helps builders deliver safe, compliant and high-performing floors that stand the test of time.

Ground Clearance Guidelines

The correct ground clearance depends on the type of subfloor material and whether termite treatment has been applied.

For non-termite-treated subfloor materials :

• Maintain a minimum clearance of 400 mm between the ground and the underside of the bearer.
• On sloping sites, this clearance can reduce to 150 mm within 2 m of an external wall, provided access for inspection is maintained.

For termite-treated subfloor materials :

• A minimum clearance of 150 mm is acceptable.
  However, reduced airflow at this height can increase the risk of trapped moisture, so additional measures may be required such as:
• Installing mechanical ventilation systems.
• Laying builders’ plastic over the ground to act as a moisture barrier.
• Using STRUCTAflor® Ultimate for enhanced moisture protection.

Ventilation and Moisture Control

• Good subfloor ventilation is essential to maintain healthy airflow and prevent dampness. The NCC specifies different ventilation requirements depending on the climate zone, with humid and tropical regions needing more airflow than drier areas.

Best practice includes:

• Increasing ventilation when timber flooring will be installed above.
• Using mechanical ventilation where crossflow airflow is limited.
• Installing builders’ plastic or STRUCTAflor® Ultimate to limit ground moisture infiltration.

By maintaining adequate airflow, builders can avoid moisture accumulation and extend the life of both the subfloor and the finished flooring system.

Sloping and Drainage

Drainage is another critical factor in subfloor design. The ground beneath and around the subfloor should always slope away from the building to prevent water pooling. Poor drainage can cause serious problems such as:

• Decay and degradation in bearers and joists.
• Mould growth that affects indoor air quality.
• Foundation instability due to prolonged moisture exposure.

Ensuring proper site grading and directing downpipe runoff away from the structure are simple yet essential steps that prevent long-term structural and health issues.

Moisture and Floor Coverings

Before any floor coverings, whether timber, vinyl or carpet are installed, the subfloor’s moisture content should not exceed 14% , as recommended by AS 1884.

Steps for compliance:

• Test subfloor moisture using an approved meter.
• If levels are above 14%, improve ventilation, install ground plastic, and check drainage before proceeding.
  Ignoring this step risks adhesive failure, warping, and mould growth beneath the floor covering, compromising both aesthetics and performance.

The Bottom Line

Meeting subfloor ground clearance and ventilation requirements is fundamental when installing STRUCTAflor®. By following NCC, AS 1884 and ATFA guidelines, builders can:

• Prevent moisture-related damage.
• Protect flooring performance and longevity.
• Create safer, healthier living environments.

A well-ventilated, properly designed subfloor provides the foundation for lasting flooring performance. Getting it right from the start ensures that STRUCTAflor® not only looks great but performs with reliability for years to come.