Quick answer: A downpipe diverter sends rainwater from your downpipe into a water butt and returns the overflow to the drain once the butt is full. A downpipe filter does the same but also removes leaves, moss and debris, giving cleaner water and less maintenance. Check whether your downpipe is round or square before you buy.
If you have just bought a water butt, or you are in the process of choosing one, you will quickly come across the question of how to connect it to your downpipe, usually with a downpipe diverter or filter. And that is where things can start to feel a little confusing.
Downpipe filter or diverter? What is the difference? Do you need one or the other, or does it matter at all?
Here is a straightforward guide to help you choose the right one for your setup.
First things first: what does a downpipe accessory actually do?
Without any connection to your downpipe, a water butt just sits there waiting for rain to fall directly into it. Not ideal.
A downpipe accessory intercepts the flow of rainwater as it travels down your drainpipe and redirects it into the water butt instead. When the water butt is full, excess water is automatically sent back down the pipe and away to the drain, so there is no overflowing onto the ground.
That is the core job. Both filters and diverters do it. The difference is in what else they do along the way.
What is a downpipe diverter?
A downpipe diverter does exactly what the name suggests. It diverts rainwater from the downpipe into your water butt. Simple, reliable, and straightforward to install.
The water goes into the tank largely as it comes off the roof, which means any leaves, moss, grit, or debris that have made their way into the gutters can end up in the butt too. Over time this can lead to sediment building up at the bottom of the tank and, occasionally, a blocked tap.
If you are not too concerned about water quality and you are happy to give the water butt an occasional clean, a downpipe diverter does the job perfectly well.
GRAF offers two downpipe diverter options for round downpipes:
Avanti Downpipe Diverter A popular choice for its ease of installation. Rather than cutting the downpipe in half, you simply drill a hole in the side using the hole-saw provided, fit the rubber cup inside, and slot the diverter in. The whole thing takes a matter of minutes and works with plastic and aluminium, downpipes. Suitable for downpipes from 68 to 100mm in diameter and recommended for roof areas up to 80m².
Rapido Diverter Works in exactly the same way as the Avanti and installs just as quickly, but adds an overflow stop function for extra peace of mind. Also suitable for 68 to 100mm downpipes and roof areas up to 80m².
What is a downpipe filter?
A downpipe filter does everything a downpipe diverter does, but it also cleans the water before it enters the tank. As rainwater passes through the filter, leaves, moss, dirt, and other debris are removed, so the water going into your butt is significantly cleaner from the start.
Cleaner water means less sediment at the bottom of the tank, a tap that stays clear and free-flowing, and water that is more pleasant to use on your plants. It also means less maintenance over time, which for most people is reason enough to choose a filter over a basic diverter. The RHS also advises keeping gutters clear, as organic material can encourage bacteria to grow in stored rainwater.
GRAF offers filter options for both round and square downpipes:
Speedy Downpipe Filter (round downpipes) One of our most popular accessories. It filters debris before it reaches the tank, manages overflow automatically when the water butt is full, and includes a stop flow function so you can prevent water entering the water butt during a hard frost. Compatible with downpipes from 68 to 100mm in diameter and downpipes made of plastic and aluminium. Recommended for roof areas up to 80m². Like the diverters, it installs using the hole-saw provided without needing to remove the downpipe from the wall.
Square Downpipe Filter (square downpipes) If your downpipe is square, this is the one to go for. It fits downpipe sizes of 50x50mm, 65x65mm, and 100x73mm, achieves a water yield of around 90%, and includes a stop flow function for frost protection. Recommended for roof areas up to 50m².
What about a filling device?
There is a third option worth knowing about, which sits somewhere between a filter and a diverter in terms of what it does.
The Filling Device connects to a round downpipe and directs rainwater into your water butt while filtering out larger contaminants before they reach the tank. It also prevents the butt from overfilling and includes a flow stop function for frost protection.
Available in grey and brown to match the most common downpipe colours, it is the most discreet option of the three. If you want the connection to blend in as much as possible with your existing downpipe, the Filling Device is worth considering. It suits downpipes between 7 and 10cm in diameter and is recommended for roof areas up to 50m².
So which downpipe diverter or filter should you choose?
Here is a simple way to think about it:
Go for a downpipe diverter if:
- You want the simplest, most straightforward connection
- You are happy to clean your water butt occasionally
- Water quality is not a priority
Go for a filter if:
- You want cleaner water in your butt with less maintenance
- You are using the water on vegetables, fruit, or plants you care about
- You want to reduce the chance of the tap getting blocked over time
Go for a filling device if:
- You want filtration but prefer a more discreet finish
- Your downpipe is round and between 7 and 10cm in diameter
- You want something that blends with your existing downpipe colour
What should you check before ordering a downpipe diverter or filter?
Before you buy any downpipe accessory, check whether your downpipe is round or square. It sounds obvious, but it is easy to overlook and the accessories are designed specifically for each type. Most modern homes have round downpipes, but older properties and some contemporary designs use square ones. A quick look or a measure with a tape is all it takes.
Ready to connect your water butt?
Browse the full range of downpipe filters and accessories at grafuk.co.uk and find the right connection for your setup. If you are not sure which one suits your downpipe and water butt combination, get in touch and we will help you choose.
Posted by Callum Vallance-Poole, on September 29, 2026.