Roof Plumbing and Gutter Sizing Requirements

What This Requirement Covers

Roof plumbing and gutter sizing requirements set the minimum design, material and capacity standards for collecting and conveying rainwater from roofs to a lawful outfall. These requirements exist to protect people, buildings and surrounding infrastructure from water ingress, ponding and uncontrolled overflow during normal and extreme rainfall events. They apply to designers, builders, plumbers, certifiers and homeowners responsible for specifying, installing or maintaining roof drainage systems in Australia.

The rules sit in the Plumbing Code of Australia (NCC 2022 Volume Three), the ABCB Housing Provisions (for houses and small buildings) and referenced Australian Standards (notably AS/NZS 3500.3 for stormwater and product standards such as AS/NZS 2179.1 and AS 1273). They cover required rainfall design intensities, gutter and downpipe sizing tables, overflow requirements for eaves and box gutters, materials and fastening, and provision for access and maintenance.

Key Requirements

1. Design rainfall intensities - Eaves gutters are to be sized for a 5% annual exceedance probability rainfall intensity for normal operation; eaves gutter overflow measures and box/valley gutters must cater for a 1% annual exceedance probability event, as set out in the ABCB Housing Provisions and NCC Volume Three (Part E3P1 and Housing Provisions Part 7.4).
2. Capacity requirement - A roof drainage system must have capacity to transfer the volume of water anticipated in the applicable AEP event (see NCC Volume Three - E3P1 and jurisdictional variations in schedules).
3. Gutter sizing - Eaves gutter sizes are selected from tables in the ABCB Housing Provisions (Table 7.4.3a/7.4.3b/7.4.3c) which give required gutter cross-section or profile for a given roof catchment area (m2) and design rainfall intensity (mm/h). These tables assume guttering discharges into one downpipe unless otherwise noted (see ABCB Housing Provisions 7.4.3 Selection of guttering).
4. Downpipe sizing - Downpipe sizes are chosen to match the flow from the roof catchment and gutter. The Housing Provisions include tables for downpipe diameters (mm) related to roof catchment area and design rainfall intensity.
5. Box and valley gutters - Must have capacity to transfer the volume of water for a 1% AEP event (ABCB Housing Provisions and NCC Volume Three E3P1(2)). Special design and overflow measures are required where box/valley gutters discharge over internal areas.
6. Overflow provisions - A roof drainage system must provide overflow devices sized for a 1% AEP event and must direct overflow away from the building so it does not pond or enter/ damage the building (NCC Volume Three E3P2 and Housing Provisions explanatory text).
7. Material and product standards - Gutters, downpipes and flashings must be manufactured in accordance with AS/NZS 2179.1 for metal components and AS 1273 for uPVC components, and be compatible with upstream roofing materials (ABCB Housing Provisions 7.4.2).
8. Installation and watertightness - Internal roof drainage components must be watertight and installed to allow access for maintenance and clearing blockages (NCC Volume Three E3P3 and E3P4). Where applicable, design, construction and installation must comply with AS/NZS 3500.3 (NCC Volume Three E3D2 and state variations).
9. Fastening and support - Downpipes must be securely fastened to resist thermal expansion and weight from partial or total blockage as specified in jurisdictional deem-to-satisfy provisions where inserted (see VIC E3D2(2)
10. (a) and referenced insertion to AS/NZS 3500.3).

Relevant clauses and documents

1. NCC 2022 Volume Three - Plumbing Code of Australia: Part E3 (E3P1, E3P2, E3P3, E3P4, E3D1, E3D2).
2. ABCB Housing Provisions (NCC Housing Provisions) Part 7.4 Gutters and downpipes - Table 7.4.3a, 7.4.3b, 7.4.3c and Table 7.4.3d (design rainfall intensities), plus worked sizing formulae and overflow tables.
3. AS/NZS 3500.3 - Stormwater drainage (installation and performance) - referenced in NCC Volume Three E3D2 and state deem-to-satisfy provisions.
4. AS/NZS 2179.1 and AS 1273 - product manufacturing standards for metal and uPVC components (ABCB Housing Provisions 7.4.2).

Residential vs Commercial

1. Residential (Class 1/10)
2. The ABCB Housing Provisions supply specific deemed-to-satisfy tables (Part 7.4) for eaves gutter and downpipe sizing aimed at houses and small residential buildings. These tables use simplified catchment area-to-gutter/downpipe mappings and specify use of 5% AEP for normal gutter sizing and 1% AEP for overflow measures. Designers of Class 1/10 buildings commonly follow these tables in conjunction with AS/NZS 3500.3.
3. Commercial and larger buildings (Class 2-9)
4. For Class 2-9 buildings, NCC Volume Three Part E3 and AS/NZS 3500.3 apply directly. Larger or higher-importance buildings often require bespoke hydraulic calculations for gutter and downpipe sizing using the relevant design rainfall intensity and may be required to adopt higher safety/overflow standards (for example choosing 1% AEP for critical roof drainage or providing additional redundancy). Box gutters, valley gutters and internal roof drainage above internal spaces require more conservative design and overflow capacity in commercial buildings.

In short, residential guidance provides prescriptive tables suitable for most houses, while commercial work generally requires project-specific design and strict compliance with AS/NZS 3500.3 and NCC Volume Three performance requirements.

Exceptions and Exemptions

1. Acceptable Alternative Solutions - Where deemed-to-satisfy provisions are not followed, an acceptable alternative solution demonstrating at least equivalent performance may be used; this typically requires engineering substantiation and certifier approval (NCC Volume Three E3D1 and general NCC performance-based pathways).
2. Local amendments or deemed-to-satisfy inclusions - Some states insert additional text into NCC Volume Three (for example fastenings or additional requirements). Where a state schedule requires compliance with a particular method or standard, the local provision takes effect for that jurisdiction.
3. Roofs not connected to stormwater systems - Special rules apply where stormwater cannot be discharged to a network utility operator’s system; system must transfer stormwater to an approved disposal system and provide maintenance access (NCC Volume Three E3D2(1)).
4. Minor buildings - Very small incidental structures may be exempt from full-size calculations if the roof catchment is below thresholds specified in the Housing Provisions tables; check Table 7.4.3 series for minimum catchment thresholds.

State and Territory Variations

1. The NCC allows state and territory schedules to modify or add provisions. Examples from the NCC Volume Three and schedules:
2. Tasmania - NCC Volume Three includes Tasmanian specific performance and deemed-to-satisfy wording for roof drainage (see TAS E3P1 to E3D3) and references Housing Provisions or AS/NZS 3500.3 as options.
3. Victoria - Victoria inserts additional deem-to-satisfy requirements mandating that design, construction and installation be in accordance with AS/NZS 3500.3 and SA HB 39, and inserts a fastening requirement for downpipes to resist thermal expansion and blockage weight (see VIC E3D2).
4. Other jurisdictions - Check the relevant state schedule in NCC 2022 Volume Three Schedules 4-12 and the state plumbing regulations for variations; the NCC Housing Provisions note the need to verify specific state schedules.

Always verify the relevant state schedule in NCC Volume Three and any state plumbing regulation or handbook before finalising design or installation.

Practical Compliance Tips

1. Use the correct AEP - Size normal eaves gutters to the 5% AEP intensity and design overflow/box/valley gutters for 1% AEP as required by the ABCB Housing Provisions and NCC Volume Three.
2. Follow the Housing Provisions tables for houses - For Class 1/10 buildings, start with Table 7.4.3a/3b/3c and Table 7.4.3d for rainfall intensities; only depart with documented engineering justification.
3. Match downpipes to gutter discharge - Install downpipes of the diameter specified for the roof catchment in the table; undersized or too few downpipes are a common cause of overflow and water ingress.
4. Provide overflow paths - Always specify overflow measures (slots, rainheads or additional downpipes) to handle the 1% AEP event and ensure overflow is discharged away from walls, openings and lower roofs.
5. Use certified products and compatible materials - Select gutters/downpipes and flashings manufactured to AS/NZS 2179.1 (metal) or AS 1273 (uPVC), and ensure compatibility with roof cladding to avoid corrosion or premature failure.
6. Allow for maintenance and access - Design access for clearing debris from gutters, rainheads and downpipes; include leaf guards where appropriate but do not rely on them to substitute sized overflow capacity.
7. Check state schedules early - Confirm any state or territory variations in the applicable NCC Volume Three schedule (e.g., VIC, TAS) and adopt additional fastening or installation requirements where inserted.

Citations:

1. NCC 2022 Volume Three - Plumbing Code of Australia, Part E3 (E3P1, E3P2, E3P3, E3P4, E3D1, E3D2).
2. ABCB Housing Provisions 2022, Part 7.4 Gutters and downpipes, Table 7.4.3a/7.4.3b/7.4.3c and Table 7.4.3d.
3. AS/NZS 3500.3 - Stormwater drainage (installation and design requirements).
4. AS/NZS 2179.1 and AS 1273 - product manufacturing standards for guttering components.

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