Downpipes should be positioned as part of the roof drainage design, using the roof catchment, gutter type, valley locations, local rainfall and overflow path. They do not all have to disappear underground. A downpipe may connect to compliant below-ground stormwater drainage or discharge through another council-approved arrangement.
The final route must reach the property's lawful point of discharge without sending water into the sewer, against the building or onto neighbouring land. Check the approved plans and relevant council requirements before changing an existing system.
How are downpipe positions decided?
Downpipe number, size and position work together with the gutter and overflow design. A convenient corner is not necessarily the correct hydraulic position, particularly where a valley gutter concentrates roofwater.
The NCC Housing Provisions Part 7.4 includes Deemed-to-Satisfy rules that limit one downpipe to serving 12 metres of gutter and place downpipes as close as possible to valley gutters. Keep that figure in its proper context: another building class or an approved Performance Solution may use a different engineered design.
Overflow also matters. The roof system should manage water when heavy rain or a blockage exceeds its normal capacity so it does not pond against, enter or damage the building. Extra downpipes cannot compensate for an unsuitable outlet or an undersized, damaged buried drain.
Do downpipes need to run underground?
No single underground rule applies to every Queensland property. The ABCB's gutter and downpipe guidance defines a downpipe as carrying rainwater from the gutter to a subsurface drainage system or to ground level. The lawful discharge arrangement determines which option is acceptable at the site.
| Outlet arrangement | What must be confirmed |
|---|---|
| Buried stormwater drain | The pipe is correctly connected, serviceable, adequately sized and routed to the approved discharge point. |
| Rainwater tank | The inlet suits the roof system and the full tank has a safe, lawful overflow route. |
| Approved surface outlet | Runoff will not pond against the building, erode the site, cross a public path or create nuisance beyond the property. |
| Approved soakage system | Soil, groundwater, setbacks, available area and council acceptance support infiltration at that location. |
A nearby kerb, street pit, easement, gully or buried pipe is not automatically available for connection. Do not assume an outlet is lawful from appearance alone.
Find the lawful point of discharge first
The lawful point of discharge is the authorised destination for stormwater from the property. It may be a council drain, street system or another approved arrangement. The applicable council and the property's approval history control the answer.
Ipswich City Council, for example, requires owners to maintain approved roofwater drainage and describes authorised council drainage, roads and accepted soakage as possible lawful outlets. Brisbane and Logan publish their own requirements. Use the council for the property rather than applying a rule from a neighbouring area.
Where a proposed pipe must cross downstream private land, written owner consent, an easement or development approval may be involved. Neither the visible direction of fall nor an informal agreement authorises excavation through another property.
Never connect roofwater to sewer
Stormwater and sanitary sewerage are separate systems. Do not connect a downpipe or rainwater-tank overflow to a sewer opening or overflow relief gully.
Urban Utilities tells property buyers to check that stormwater pipes are not connected to sewer. Logan City Council's overflow relief gully guidance also says roofwater downpipes and tank runoff must not enter the gully. Incorrect connections can overload sewerage during wet weather and cause overflows.
Our guide to where sewer and stormwater drains lead explains how to keep the two networks distinct.
Rainwater tanks still need an overflow route
A tank can intercept roofwater for permitted reuse, but it does not remove the need for stormwater disposal. Once the tank is full, its overflow must carry incoming rain to the approved destination without flooding the building, neighbouring land or a public area.
Record which downpipes feed the tank and trace the overflow separately. A disconnected, undersized or poorly graded overflow can create the same ponding and erosion problems as a faulty downpipe.
Check an existing buried route before adding to it
Existing stormwater lines can be blocked, broken, undersized, abandoned or incorrectly connected. Trace the system before adding roof area, moving a downpipe or assuming an old pipe reaches the street.
Start with approved plans, visible pits and safe ground-level observations during rain. Our guide to finding stormwater drains on a property covers the evidence to collect. Where the route changes direction, check which bend or offset suits a downpipe before treating every overflow as a blockage.
Excavation and licensed-work boundaries
Do not excavate blindly beside footings, retaining walls, easements or buried services. Lodge a Before You Dig Australia enquiry and follow each asset owner's instructions. BYDA also warns that plans are indicative and may not show private domestic services or exact depths, so they are one safety input rather than a complete map.
QBCC's drainage licence scope includes installation, commissioning, maintenance and testing of above- and below-ground stormwater drainage. A licensed drainer or plumber can trace the existing route, assess defects and cross-connections, and confirm the approval or notification pathway for proposed work.
What to check before changing a downpipe
- the roof catchments, gutter profiles, valleys and current overflow points
- the lawful point of discharge shown on approved plans
- where every existing downpipe and tank overflow leads
- whether the buried line is connected, clear, intact and adequately sized
- site fall, soil, groundwater, boundaries, easements and neighbouring impacts
- footings, retaining walls and underground utility locations
- the relevant council, building and licensed-drainage requirements
If an existing downpipe is backing up or not discharging, arrange an assessment through our blocked downpipe service before connecting more roofwater to it. New drainage design, lawful-discharge approval or major excavation may also require council, certifier or engineering input beyond clearing the blockage.