Roof Drainage Plans for Single-Storey Houses in NSW: CC & CDC

Why a Roof Drainage Plan Matters for a Single-Storey Build

A single-storey house looks simple on paper, but your certifier still needs to see how every square metre of roof gets drained to a lawful discharge point before they issue a Construction Certificate (CC) or Complying Development Certificate (CDC). If the drainage documentation is missing or vague, the approval stalls – and on a CDC job, the private certifier has very little discretion to wave things through.

This post covers what a council-ready roof drainage plan for a single-storey dwelling in NSW should contain, and the sizing basics that sit behind it.

What Certifiers Expect to See on the Plan

Whether you are going through council with a DA and CC, or straight to a private certifier under the State Environmental Planning Policy (Exempt and Complying Development Codes) 2008, the drainage plan generally needs to show:

• Roof catchment areas – each section of roof, its plan area and which gutter and downpipe serves it.
• Gutter type and size – eaves gutter profile and nominal size, with falls indicated.
• Downpipe locations and sizes – typically 90 mm or 100 mm round (or equivalent rectangular), spaced so no downpipe is overloaded.
• The discharge point – street kerb and gutter, an inter-allotment drainage easement, or the council street drainage system, depending on the lot.
• Surface drainage – how hardstand and overland flow around the house is managed so water is not directed onto neighbours.
• Rainwater tank connection where BASIX commitments require one, including the overflow path back to the stormwater system.

Sizing Gutters and Downpipes to AS/NZS 3500.3

The technical benchmark is AS/NZS 3500.3:2021 (Plumbing and drainage – Stormwater drainage), which the National Construction Code (NCC) references for roof drainage. The standard works on a straightforward logic:

• Rainfall intensity – design rainfall for your site is determined from the standard’s rainfall data, based on location and the design storm the element must handle. Eaves gutters are designed for a more frequent storm, while any overflow provisions deal with rarer, more intense events.
• Catchment area – the plan area of roof draining to each gutter length, adjusted for pitch.
• Gutter capacity – a given gutter profile at a given fall can carry a known flow. If the catchment produces more, you need a bigger gutter, more fall, or another downpipe to split the load.
• Downpipe capacity – each downpipe serves a maximum catchment depending on its size and the rainfall intensity. On most single-storey homes in Sydney this works out to one downpipe per modest roof section, but large open-plan roofs and low-pitch designs catch builders out.

The standard also expects overflow measures – slotted gutters, overflow weirs or similar – so that in a storm exceeding the gutter design capacity, water spills outside the building rather than back into the eaves and wall frame. Certifiers and plumbing inspectors increasingly look for this detail on the drawings, not just on site.

Common Mistakes on Single-Storey Jobs

• Copying downpipe positions from the architect’s elevations without checking catchment areas. Elevations are drawn for looks, not hydraulics.
• No nominated discharge point. “To stormwater” is not enough – the plan must show where the water lawfully leaves the site.
• Ignoring the rear of a sloping block. If the lot falls away from the street, you may need an inter-allotment easement or a council-accepted alternative, and that should be resolved before the slab is poured.
• Forgetting the BASIX tank. If the BASIX certificate commits to a rainwater tank, the drainage plan must show the connected roof area and tank overflow.
• Valley and box gutter sections treated like eaves gutters. These carry concentrated flows and have stricter design requirements under AS/NZS 3500.3.

DA/CC vs CDC: Does It Change the Drainage Plan?

Under a DA, the council’s Development Control Plan (DCP) may impose extra requirements – on-site detention in some catchments, specific discharge rules, or easement requirements. Under CDC, the Codes SEPP sets the framework, but the drainage still has to comply with the NCC and AS/NZS 3500.3, and the certifier must be satisfied stormwater is disposed of legally. Either way, a clear, dimensioned drainage plan prepared by someone who understands the standard is the fastest path through approval. Always confirm the current requirements of the local council’s DCP for your specific lot.

Get a Council-Ready Plan Before You Lodge

Contrive Consultants prepares council-ready stormwater and roof drainage plans for builders across Sydney and NSW, sized to AS/NZS 3500.3 and documented the way certifiers want to see them. See our services, or contact us on +61 497 848 111 or info@contriveconsultants.com.au to get your single-storey project moving.