NCC 2019 Volume Two
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Part 3.4.1 Subfloor ventilation

Part 3.4.1 Subfloor ventilation

Appropriate Performance Requirements

Appropriate Performance Requirements

Where an alternative subfloor ventilation system is proposed as a Performance Solution to that described in Part 3.4.1, that proposal must comply with—

  1. Performance RequirementP2.2.3; and
  2. the relevant Performance Requirements determined in accordance with A2.2(3) and A2.4(3) as applicable.

Explanatory information:

Explanatory information:

Part 3.4.1 applies to the subfloor space of all suspended floors of a building or deck, including but not limited to, timber and steel-framed subfloors and suspended concrete slabs.

3.4.1.1 Application

Compliance with this acceptable construction practice satisfies Performance Requirement P2.2.3 for subfloor ventilation.

3.4.1.2 Subfloor ventilation

(a)

Subfloor spaces must—

(i)

be provided with openings in external walls and internal subfloor walls in accordance with Table 3.4.1.1 for the climatic zones given in Figure 3.4.1; and

(ii)

have clearance between the ground surface and the underside of the lowest horizontal member in the subfloor in accordance with Table 3.4.1.1 (see Figure 3.4.3a and Figure 3.4.3b).

Table 3.4.1.1 Subfloor openings

Climatic zone (see Figure 3.4.1 )

Minimum aggregate subfloor ventilation openings with no membrane (mm 2 /m of wall)

Minimum aggregate subfloor ventilation openings with ground sealed with impervious membrane (mm 2 /m of wall)

A

2000

1000

B

4000

2000

C

6000

3000

Note to Table 3.4.1.1: In situations where openings in external walls and internal subfloor walls, including separating walls, are not able to be provided, additional measures must be provided to ensure that the overall level of ventilation of the subfloor space is maintained. This may include measures similar to those in 3.4.1.2(e) i.e. providing durability class timbers, or having the ground sealed in the subfloor space with an impervious membrane.

Table 3.4.1.2 Ground clearance

Climatic zone (see Figure 3.4.1 )

Minimum ground clearance height where termite inspection or management system is not required (mm)

Minimum ground clearance height where termite inspection is required (mm)

A, B and C

150

400

Notes to Table 3.4.1.2 :

1.

400 mm clearance required only where termite management systems are installed that need to be inspected (see Part 3.1.4).

2.

On sloping sites the 400 mm clearance required by 1. may be reduced to 150 mm within 2 m of external walls in accordance with Figure 3.4.3a.

Figure 3.4.1 Climatic zones based on relative humidity
v2_Fig341_2016.svg
(b)

In addition to (a), a subfloor space must—

(i)

be cleared of all building debris and vegetation; and

(ii)

have the ground beneath the suspended floor graded in accordance with 3.1.3.3; and

(iii)

contain no dead air spaces; and

(iv)

have openings evenly spaced as far as practicable (see Figure 3.4.2); and

(v)

have openings placed not more than 600 mm in from corners.

(c)

In double leaf masonry walls, openings specified in (a) must be provided in both leaves of the masonry, with openings being aligned to allow an unobstructed flow of air (see Figure 3.4.2).

(d)

Openings in internal subfloor walls specified in (a) must have an unobstructed area equivalent to that required for the adjacent external openings (see Figure 3.4.2).

(e)

Where the ground or subfloor space is excessively damp or subject to frequent flooding, in addition to the requirements of (a) to (d)—

(i)

the subfloor ventilation required in (a) must be increased by 50%; or

(ii)

the ground within the subfloor space must be sealed with an impervious membrane; or

(iii)

subfloor framing must be—

(A)

where above ground — above ground durability Class 1 or 2 timbers or H3 preservative treated timbers in accordance with AS 1684.2, AS 1684.3 or AS 1684.4; or

(B)

where in-ground — in-ground durability Class 1 or 2 timbers or H5 preservative treated timbers in accordance with AS 1684.2, AS 1684.3 or AS 1684.4; or

(C)

steel in accordance with NASH Standard 'Residential and Low-Rise Steel Framing' Part 2.

Explanatory information:

Explanatory information:

3.4.1.2(e) specifies additional requirements for preventing deterioration of subfloor members where the ground or subfloor space is excessively damp, as would occur in areas with high water tables, poor drainage or in areas frequently affected by flooding or water inundation.

Figure 3.4.2 Typical subfloor ventilation details—Typical cross ventilation of subfloor area
v2_Fig342_2016.svg
Figure 3.4.3a Subfloor clearance requirements
v2_Fig343a_2019.svg

(see Notes to Table 3.4.1.1 and Table 3.4.1.2)

Figure 3.4.3b Subfloor clearance requirements
v2_Fig343b_2016.svg

Explanatory information:

Explanatory information:

Subfloor ventilation is cross ventilation of the subfloor space between the underside of the subfloor and the ground surface under a building.

Ground moisture rising into or entering the subfloor space can create a damp environment which encourages timber rot, fungus growth and the potential for termite activity. Subfloor ventilation increases air flow, reducing any damaging water vapour in the subfloor space.

Factors that can affect achieving satisfactory levels of subfloor ventilation include height above ground, prevailing breezes (air transfer), differential temperature and humidity between the subfloor and the external environment and good building practice.

The amount of subfloor ventilation required for a building is related to the relative humidity likely to be encountered in that location.

Figure 3.4.1 shows three broad climatic zones based on the prevailing relative humidity and includes a description of the relative humidity conditions which define each zone. If reliable weather data is available, these descriptions may be useful in determining which zone a particular location is in.

The zones shown in Figure 3.4.1 were determined by analysis of the average relative humidity at 9 am and 3 pm in January and July. The season with the highest relative humidity is used. Generally this will be July for southern Australia and January for northern Australia.

Table 3.4.1.1 and Table 3.4.1.2 specify the minimum amount of subfloor ventilation openings and height of subfloor framing members above ground level for the three climatic zones illustrated in Figure 3.4.1. The table allows subfloor ventilation rates to be halved if the ground within the subfloor space is sealed by an impervious membrane because humidity levels in the space will not be affected by moisture from the soil.