Heavy rainfall is a fact of life in Canterbury, and every litre of rain that falls on your roof needs to end up somewhere safe. The path that water takes after it leaves the roof edge is just as important as the spouting or downpipes that guide it there. We see homes where downpipes are well sized but discharge in ways that could potentially undermine foundations, saturate gardens or even breach council rules. In this guide, we look at the main options for downpipe discharge, explain why it matters and highlight common mistakes to avoid.
Why the Discharge Point Matters
Downpipes are the final leg of your rainwater system. They transfer water collected by the spouting to ground level or a stormwater connection. If the water isn’t directed away from the building properly, it will find its own path, often with damaging consequences. Discharging large volumes of roof water onto a driveway or next to the house footings can cause:
- Foundation damage: Continuous saturation of the soil next to the foundation can lead to settlement or heaving, particularly in Christchurch’s soft soils.
- Cladding and soffit staining: Water splashing up from hard surfaces can stain brick, plaster and weatherboards. Moisture also encourages moss and mould growth.
- Basement or sub‑floor flooding: Poorly directed downpipes can channel water towards vents or cracks in the foundation, causing dampness under floors.
- Overloaded internal gutters: When downpipes are undersized or incorrectly positioned, water backs up in the spouting. This risk is higher with internal gutters, where overflow can be hidden behind the parapet.
Where Can You Discharge Roof Water?
Christchurch City Council provides guidelines for connecting roof drainage to stormwater systems. The correct option will depend on your property’s location, soil type and available infrastructure. Here are the most common solutions:
1. Direct Connection to the Stormwater Network
In most urban areas a council stormwater main runs along the street. The preferred method is to connect your downpipe to this main via an approved lateral. This ensures that roof water is quickly conveyed away from the site. Connections require building consent and must be installed by a licensed drainlayer. Discharging directly into the kerb and channel is usually permitted only for minor roof areas or where no connection is available.
2. Soak Pits or Infiltration Trenches
Where a stormwater main is not available or the property is in a rural or semi‑rural area, you can discharge roof water into a soak pit (also known as a soakage hole or infiltration pit). This is a gravel‑filled pit or plastic crate lined with geotextile fabric that allows water to percolate into the surrounding soil. The size of the pit must be matched to the roof catchment and soil permeability. Soak pits are generally placed at least five metres from the house footing to protect foundations. In areas with high groundwater or heavy clay soils, soak pits may be ineffective.
3. Rain Gardens and Swales
For properties with ample space, a rain garden or grassed swale can capture and filter runoff before allowing it to soak away. These vegetated features slow the flow, trap debris and provide habitat for beneficial insects. They are particularly effective when combined with detention tanks that release water slowly to the stormwater network. Our team often integrates downpipes with rain gardens when upgrading to continuous spouting or adding flashings to protect cladding.
4. Rainwater Tanks
Harvesting rainwater is increasingly popular in Canterbury for garden irrigation and household use. Downpipes can be diverted into above‑ground tanks via first‑flush diverters. Tanks should still have an overflow that discharges safely to a soak pit or stormwater connection. Installing a tank doesn’t eliminate the need for well‑sized downpipes; you still need to convey peak flows during heavy rain.
5. Secondary Roofs or Lower Canopies
In some cases, downpipes from an upper roof are discharged onto a lower roof where the water is collected again. This requires careful design to avoid overloading the lower spouting and is generally used only on complex architectural roofs. Rainheads and spreaders can help dissipate the flow over a wider area.
Common Mistakes and How to Avoid Them
Discharging Too Close to the House
One of the most common errors we see is downpipes that end only a few centimetres from the wall. Water hits the ground and runs back towards the foundation. Always extend downpipes away from the building using a drainpipe, flexible hose or a properly graded channel. A minimum distance of one metre is a good rule of thumb, but five metres is better when using soak pits.
Connecting to the Sewer
It may seem convenient to tie a downpipe into a nearby waste‑water pipe, but it’s illegal and creates problems during heavy rain. Stormwater can overwhelm the sewer network and lead to sewage overflows. Always connect to the stormwater system or a properly designed soakage solution.
Putting Water on Impermeable Surfaces
Directing downpipes onto driveways, decks or paved areas causes puddling and increases slip hazards. Hard surfaces also speed up runoff, contributing to downstream flooding. Wherever possible, discharge water onto permeable ground or into a detention system that slows and filters the flow.
Ignoring Council Requirements
Christchurch City Council may require building consent for new stormwater connections or soak pits. In flood management areas there may be rules about attenuating runoff. Before changing your downpipe discharge, check the local regulations or talk to us. We’ll guide you through compliance.
Maintenance Tips
Once your downpipes are correctly sized and discharged, keep them working by inspecting them twice a year. Look for:
- Blockages: Clear leaves, seed pods and debris from the top of the downpipe and rainhead. Consider installing gutter guards or screens to reduce clogging.
- Leaks and rust: Check joints and brackets for corrosion or loose screws.
- Movement: Ensure downpipes remain secured to the wall and haven’t been knocked out of alignment by garden tools or children’s bikes.
- Erosion at the discharge: If water is scouring soil where the downpipe ends, add a splash block or extended pipe to dissipate the flow.
Talk to the Christchurch Downpipe Experts
Understanding where your downpipes discharge and ensuring they are properly sized can save thousands of dollars in future repairs. We design, fabricate and install complete rainwater systems that include downpipes, rainheads, continuous spouting and flashings. Get in touch today to arrange a free site assessment.
