NCC 2022: ventilation and drainage requirements for Class 1 buildings

TL;DR

NCC 2022 splits ventilation and drainage across two instruments: Volume Two sets the performance requirements (H2 for drainage/weatherproofing, H4 for ventilation), and the ABCB Housing Provisions carry the Deemed-to-Satisfy numbers. For habitable rooms, openable area must be at least 5% of floor area (Housing Provisions 10.6.2). For drainage, stormwater from a 5% AEP storm must not damage neighbouring property and a 1% AEP storm must not enter the building (H2P1). Both failing are inspection defects that hold up your OC and can trigger re-work orders.

In plain English

The NCC 2022 ventilation and drainage rules for a Class 1 house sit across two instruments:

NCC 2022 Volume Two sets the performance objectives and requirements (what the building must achieve).
ABCB Housing Provisions Standard 2022 carries the Deemed-to-Satisfy (DTS) numbers (how you actually meet it in a standard build).

This is a structural change from NCC 2019, where the numbers lived directly in Volume Two. Builders and certifiers now need both documents open (verified 2026-05-08: ABCB, NCC 2022 Volume Two overview).

The ABCB Housing Provisions are normative: they are part of the NCC, not optional guidance.

What it requires: ventilation

Performance requirement (H4P5)

H4P5 requires that every occupied space in a Class 1 building has “means of ventilation with outdoor air which will maintain adequate air quality.” Mechanical systems must manage objectionable odours and prevent accumulation of harmful substances. The objective is to protect occupants from illness or loss of amenity due to lack of air freshness (H4O5) (verified 2026-05-08: NCC 2022 Part H4).

DTS: Housing Provisions Part 10.6

Part 10.6 of the Housing Provisions is the DTS path for H4P5 (verified 2026-05-08: NCC 2022 Housing Provisions Part 10.6).

Rooms covered: habitable rooms, sanitary compartments, bathrooms, shower rooms, laundries, and occupied spaces.

Three compliance options:

Method	Rule
Natural ventilation (direct)	Openable windows, doors or devices with ventilating area not less than 5% of the floor area of the room, opening directly to outside, a courtyard, verandah or sky-exposed space
Borrowed ventilation	Opening between the room and an adjoining room of not less than 5% of the floor area of the room being ventilated, plus the adjoining room’s own direct opening of not less than 5% of the combined floor areas of both rooms. Sanitary compartments cannot be the adjoining room
Mechanical ventilation	Exhaust fan or mechanical system. For sanitary compartments, laundries, kitchens, and bathrooms. Exhausted air must comply with condensation management under clause 10.8.2

Sanitary compartment location (10.6.3): A sanitary compartment cannot directly open into a kitchen or pantry unless separated by an airlock, hallway, or other room, or it has mechanical exhaust ventilation.

Subfloor ventilation: Housing Provisions Part 6.2

Subfloor ventilation is a separate obligation under H2D5. It applies to all suspended floors (timber and steel framing, suspended concrete slabs). Purpose: prevent moisture accumulation under the floor that leads to timber rot, fungal growth, and termite activity (verified 2026-05-08: NCC 2022 Housing Provisions Part 6.2).

Minimum aggregate openings by climatic zone (Table 6.2.1a):

Climate zone	No membrane (mm²/m of wall)	With impervious membrane (mm²/m of wall)
Zone A (low humidity)	2,000	1,000
Zone B (moderate humidity)	4,000	2,000
Zone C (high humidity)	6,000	3,000

Ground clearance (Table 6.2.1b):

Condition	Minimum clearance
Standard (without termite inspection)	150 mm
With termite inspection or management system	400 mm

Exception: on sloping sites, the 400 mm may reduce to 150 mm within 2 m of external walls.

Openings must be evenly distributed and positioned not more than 600 mm in from corners. Internal wall openings must match the adjacent external opening area.

Where excessive dampness or flooding is present: increase opening rates by 50%, or apply an impervious ground seal, or specify special timber durability (Class 1 to 2, or H3/H5 treated).

What it requires: drainage

Performance requirement (H2P1)

H2P1 sets two stormwater thresholds for Class 1 buildings (verified 2026-05-08: NCC 2022 Part H2):

AEP (annual exceedance probability)	Requirement
5% AEP	Surface water collected or concentrated by the building or sitework must be disposed of in a way that avoids the likelihood of damage or nuisance to any other property
1% AEP	Surface water must not enter the building

AEP explained: 5% AEP means a 5% chance of that rainfall intensity being exceeded in any given year (roughly a 1-in-20 year storm). 1% AEP is a 1-in-100 year storm. NCC 2022 replaced the older “average recurrence interval” terminology with AEP to align with AS/NZS 3500.3.

The DTS path for H2P1 is either AS/NZS 3500.3:2018 or ABCB Housing Provisions Part 3.3 (H2D2).

DTS: Housing Provisions Part 3.3 (site drainage)

Part 3.3 covers four drainage categories (verified 2026-05-08: NCC 2022 Housing Provisions Part 3.3):

1. Surface water drainage (3.3.3)

Slab-on-ground buildings: ground around the slab must be graded away from the building.

Area condition	Minimum fall
Low rainfall / impermeable surfaces (concrete, clay paving)	25 mm over 1 m from building
All other cases	50 mm over 1 m from building

Minimum slab height above finished ground level:

Condition	Minimum slab height
Low rainfall / sandy areas	100 mm
Paved areas sloping away from building	50 mm
Other conditions	150 mm

Suspended floors: ground beneath must be graded so that the area under the building is above adjacent external finished ground level.

2. Subsoil drainage (3.3.4)

Required where site conditions warrant (high water table, clay soils, significant upslope catchment). Key specs:

Minimum grade: 1:300 uniform fall
Discharge into external silt pit or sump
Invert of outlet at least 50 mm below the inlet level
Must include cleaning and maintenance provisions

References AS/NZS 3500.3 for design guidance.

3. Underground stormwater drainage (3.3.5)

Discharge position approved by the appropriate authority (usually council or the local water utility)
System must prevent any overflow during heavy rain from entering the building
Minimum cover depths for 90 mm Class 6 uPVC pipe: 100 mm (in soil), 50 mm (under paved areas), 75 to 100 mm (under vehicle traffic areas)

4. Excavation safeguards

Drainage excavation must remain within the safe-for-excavation zone described in Figure 3.3.2 when near footings.

DTS: Housing Provisions Part 7.4 (gutters and downpipes)

Part 7.4 is the DTS path for H2D6 (roof and wall cladding drainage). It moved from Part 3.5.3 in NCC 2019 and contains significant changes (verified 2026-05-08: ABCB, NCC 2022 gutter and downpipe requirements).

Sizing thresholds:

Eaves gutters: sized to remove rainwater at 5% AEP, 5-minute duration rainfall intensity for the location (Table 7.4.3d)
Overflow measures: sized for 1% AEP, 5-minute duration rainfall intensity

Gutter types: Part 7.4 defines six gutter types (A to F) by cross-sectional area. Once rainfall intensity and catchment area are known, the correct type is selected from Table 7.4.3a.

Key dimensional requirements:

Minimum gutter fall: 1:500
Maximum gutter length per downpipe: 12 m
Bracket spacing: maximum 1.2 m intervals
Valley gutters: minimum 15 mm freeboard (new requirement in NCC 2022)

Overflow options (continuous):

Method	Capacity
Front-face slotted gutter	0.5 L/s per metre
Controlled back gap	1.5 L/s per metre
Controlled front bead	1.5 L/s per metre

Box gutters: excluded from Part 7.4 in NCC 2022. The only DTS path for box gutters is AS/NZS 3500.3 or a Performance Solution.

What it doesn’t cover

Sanitary (foul) plumbing and drainage (toilets, showers, sinks): that is NCC Volume Three (PCA) and AS/NZS 3500.2, administered via the Certificate of Compliance (Plumbing).
Waterproofing wet areas (showers, wet rooms): covered under AS 3740 and NCC H2P2, not this Part.
Swimming pool drainage: H2P4 applies but has no DTS provisions; requires a Performance Solution or engagement of a licensed hydraulic engineer.
Sub-surface drainage systems for flood-prone or high-water-table sites: AS/NZS 3500.3 or a hydraulic engineer is the path; Part 3.3 alone is insufficient.
Commercial or apartment buildings: Volume Two applies to Class 1 and Class 10 only. Class 2 to 9 buildings use Volume One (Part F1 for surface water, Part F6 for ventilation).

Practical implications

On-site grading is your first line of defence. Getting the slab height and ground grading right at the concrete stage is far cheaper than rectifying drainage complaints after lock-up. 150 mm slab height is the default unless you are on a paved area sloping away.

Certifiers check gutter sizing. Under NCC 2022, certifiers now see a documented sizing methodology required for gutters. Have the Table 7.4.3d rainfall intensity for your location and the catchment area calculation available.

Overflow measures are mandatory, not optional. NCC 2022 made overflow measures a hard requirement across all eaves gutter designs, not just high-rainfall areas. Valley gutters need 15 mm freeboard as a minimum.

Box gutters need engineer involvement. If the design includes box gutters, Part 7.4 no longer applies. Use AS/NZS 3500.3 sizing or engage a hydraulic engineer for a Performance Solution.

Subfloor ventilation is tied to your climate zone. Builders working across multiple states need the correct Table 6.2.1a opening rates for each zone. Zone C (tropical and high-humidity coastal areas) requires three times the opening of Zone A.

Borrowed ventilation has limits. It is not allowed from a sanitary compartment, and the combined floor area calculation means borrowed ventilation from a large room to a small room is easy to meet, while the reverse often fails.

State variations

NSW

NSW adopts NCC 2022 (Amendment 2 in force from 29 July 2025) including H2 stormwater and H4 ventilation without amendment to the Class 1 ventilation 5% rule or the 1%/5% AEP stormwater thresholds. Plumbing and drainage work must comply with the Plumbing Code of Australia (NCC Volume Three) and is regulated by NSW Fair Trading, which inspects work and processes the plumber’s Certificate of Compliance via the Notice of Work / Certificate of Compliance system. Stormwater discharge points are approved by the local council; in subdivision and infill development zones, Sydney Water (or Hunter Water in the Hunter region) issues a Section 73 Compliance Certificate before council releases the OC (verified 2026-05-09).

VIC

Victoria adopts NCC 2022 with state variations published as Schedule 10. Volume Two H2 and H4 apply unchanged for Class 1 buildings (1%/5% AEP and 5% openable area), but Volume Three Vic Part E4 stormwater calls up AS/NZS 3500.3 design and adds VBA-published guidance on bedding and discharge to a legal point of discharge. Drainage and stormwater work is licensable plumbing work under the Plumbing Regulations 2018 and must be certified by a licensed plumber via a Certificate of Compliance lodged with the VBA. Water authorities (Yarra Valley Water, City West Water/Greater Western Water, South East Water) approve service connections; stormwater discharge to a legal point is council-approved (verified 2026-05-09).

QLD

Queensland confirmed in 2025 that NCC 2022 remains the mandatory standard for all building and plumbing work until 30 April 2027, with NCC 2025 deferred to 1 May 2027. The Queensland Plumbing and Wastewater Code (QPW Code) operates alongside the PCA and in some clauses imposes higher standards or QLD-specific requirements (notably backflow, on-site wastewater and rainwater tank plumbing), administered with the Queensland Development Code. Plumbing and drainage work is licensed by the QBCC and the relevant local government inspects on-property work; the Form 4 (Certificate of Compliance) is the plumber’s sign-off. Stormwater connection points are approved by the local council, with the SEQ water retailers (Urban Utilities, Unitywater) handling service connections in their footprints (verified 2026-05-09).

WA

WA adopts NCC 2022 Volume Two for Class 1 ventilation and stormwater without amendment to the 5% openable area rule or the 1%/5% AEP thresholds. Plumbing and drainage work is regulated under the Plumbers Licensing and Plumbing Standards Regulations 2000, which adopt and modify the PCA; licensing and post-work Notice of Intention/Certificate of Compliance lodgement runs through Building and Energy (DEMIRS). Site stormwater disposal is largely an on-property soakwell solution under local government rules rather than a piped council connection in most metro lots. Water Corporation approves water and sewer service connections; stormwater discharge is approved by the relevant local government (verified 2026-05-09).

SA

SA adopts NCC 2022 with Schedule 8 South Australia variations, including SA H9 water efficiency (rainwater tank plumbed to WC, water heater or laundry cold outlets) but no divergence from the H2 1%/5% AEP stormwater thresholds or H4 5% ventilation rule for Class 1. Plumbing work is regulated by the Office of the Technical Regulator under the Water Industry Act 2012, with contractor licensing administered by Consumer and Business Services (CBS); a licensed plumber lodges the Certificate of Compliance after work. Stormwater connection and overland flow are approved by the local council; SA Water handles water and sewer service connections (verified 2026-05-09).

TAS

Tasmania adopts NCC 2022 with Schedule 9 variations, including TAS Part E4 stormwater drainage, which requires discharge to a Network Utility Operator stormwater system or an approved disposal system; H2 1%/5% AEP and H4 5% openable area apply unchanged. Plumbing work is licensed and regulated by Consumer, Building and Occupational Services (CBOS) and classified into four categories under the Director’s Determination, with Category 3 (Notifiable Work) and Category 4 (Permit Work) requiring council notification or permit. A TasWater Certificate of Water and Sewer Compliance and a Form 21 Certificate of Completion (Plumbing Work) are needed before practical completion (verified 2026-05-09).

NT

NT adopts NCC 2022 with Schedule 6 variations; the NT Appendix to the PCA modifies sanitary drainage (Part C2) and excessive noise (Part D1), with H2 1%/5% AEP and H4 5% ventilation rules applying as drafted (cyclonic-region site grading is the practical pinch point). Only a plumber holding an advanced tradesman licence from the Plumbers and Drainers Licensing Board can contract for and certify plumbing or drainage work; certifying plumbers must also be registered with the NT Building Practitioners Board. Power and Water Corporation approves water and sewer connections; stormwater connections are approved by the relevant municipal council (Darwin, Palmerston, Litchfield, Alice Springs) (verified 2026-05-09).

ACT

ACT adopts NCC 2022 with the ACT Appendix to the PCA (variations to Volume Three under the Water and Sewerage Act 2000); H2 1%/5% AEP stormwater thresholds and H4 5% openable area apply unchanged for Class 1. Plumbing licensing and the Certificate of Compliance for plumbing or sanitary drainage work are administered by Access Canberra with assessable work requiring inspection. Icon Water operates and approves connections to water and sewer mains under its Water and Sewerage Service and Installation Rules; stormwater tie points and overland flow are managed by ACT City Services / Transport Canberra and identified on plumbing plans (verified 2026-05-09).

Source link

NCC 2022 Volume Two, Part H2 and Part H4 (ABCB, verified 2026-05-08)
ABCB Housing Provisions Standard 2022 (ABCB, verified 2026-05-08)
Part 10.6 Ventilation (ABCB Housing Provisions, verified 2026-05-08)
Part 3.3 Drainage (ABCB Housing Provisions, verified 2026-05-08)
Part 6.2 Subfloor ventilation (ABCB Housing Provisions, verified 2026-05-08)
Part 7.4 Gutters and downpipes (ABCB Housing Provisions, verified 2026-05-08)
AS/NZS 3500.3:2018 scope (Standards Australia, verified 2026-05-08)

References

NCC 2022 Volume Two, Building Code of Australia Class 1 and 10 buildings (ABCB, verified 2026-05-08)
ABCB Housing Provisions Standard 2022 (ABCB, verified 2026-05-08)
New NCC 2022 requirements for gutters and downpipes (ABCB, verified 2026-05-08)
AS/NZS 3500.3:2018, Plumbing and drainage Part 3: Stormwater drainage (Standards Australia, verified 2026-05-08)