Downpipe sizes: what the common sizes are and how to pick the right one
Most houses use one of a few standard sizes. The right one depends on the roof area it drains and how hard it rains where you live.
Downpipes come in a handful of standard sizes, and the right one depends on how much roof drains into it. A downpipe that is too small overflows the gutter in heavy rain. Bigger is rarely a problem, but it is not always needed.
Start with the sizes you are likely to see.
The common sizes
On Australian homes you will mostly find round and rectangular steel or PVC pipes. Round pipes are often around 90 mm across, with smaller 65 mm ones on small roofs. Rectangular pipes of roughly 100 by 50 mm are also common on older houses. Check the actual figure on your own pipe, because makers differ a little.
| Shape | Where you see it | Note |
|---|---|---|
| Round, smaller | Small roofs, patios | Easy to block with leaves |
| Round, larger | Most modern houses | Carries more water per pipe |
| Rectangular | Older and some modern homes | Needs to match the gutter outlet |
What decides the size you need
Three things do. The roof area draining to that pipe, the rainfall intensity in your region, and how many downpipes share the load. A tropical north needs far more capacity than a dry inland town.
The sizing rules are set out in the plumbing standard for stormwater, and your local authority can ask for them to be followed. We do not give a formula here because it depends on local data.
Matching downpipes and gutters
The gutter outlet and the downpipe have to fit each other. A small pipe on a large gutter creates a bottleneck. If you are replacing only one part, take the old one to the supplier and match it.
Leaf guards help, but they do not make a small pipe cope with heavy rain.
Downpipes and a rainwater tank
If you plan to collect roof water, the downpipe feeds the tank, so its capacity matters twice. It also needs to deal with overflow. See how a tank is plumbed and what happens when a tank overflows.
A first flush device can sit on the downpipe too: our guide to the first flush diverter explains where it goes.
How to check whether your downpipe copes
You do not need a formula to spot a struggling downpipe. A few observations tell you most of it.
- Watch the gutter during heavy rain. Water spilling over the front means the pipe or the gutter cannot keep up.
- Clear the leaves first. A blocked outlet looks like an undersized pipe.
- Count the downpipes. Roof area is shared between them, so one pipe for a big roof is a warning sign.
- Measure your pipe. Check the actual size, because makers differ a little.
- Match the gutter outlet. A small pipe on a large gutter makes a bottleneck.
- Ask the local authority. Sizing rules sit in the plumbing standard for stormwater, and local data decides the answer.
If the problem persists after cleaning, ask a licensed plumber to size it properly.
Replacing a downpipe: what you can do and what needs a licence
| Task | DIY? | Notes |
|---|---|---|
| Measuring the existing pipe | Yes | Take the old part to the supplier and match it |
| Replacing a plain section with the same size | Often yes | Check the gutter outlet and fittings fit |
| Swapping round for rectangular | Only with matching fittings | Match the outlet or change the fittings as well |
| Adding a leaf guard | Yes | It helps, but it does not make a small pipe cope with heavy rain |
| Connecting to a tank | Usually no | Licensed plumbing work in most states and territories |
| Connecting to the stormwater system | Usually no | Check with your state or territory plumbing regulator |
If the downpipe feeds a tank, read how a rainwater tank is plumbed first.
Frequently asked questions
What is the most common downpipe size?
Round pipes of around 90 mm are common on modern houses. Smaller round pipes and rectangular profiles are also used. Check your own pipe.
How do I know if my downpipe is too small?
If gutters overflow in heavy rain even when they are clean, the downpipe or the number of downpipes may not be enough.
Can I swap a round downpipe for a rectangular one?
Only if the gutter outlet and fittings suit it. Match the outlet, or change the fittings as well.
Is connecting a downpipe to a tank DIY?
Not usually. Connections to a tank or the stormwater system are licensed work in most states and territories.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NHMRC. Australian Drinking Water Guidelines
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)