So you’ve got a water butt. Or you’re about to get one because the water bill went up and someone on a forum pointed out that your roof is currently throwing away thousands of litres a year for free.
Either way, here’s the thing nobody puts on the box: the actual installation is easy, but almost every failed water butt fails for the same three reasons — a garbage base, a missing overflow, and a downpipe cut that can’t be undone. Get those right and the rest is a couple of hours with a hacksaw and a screwdriver.
This is the version of the instructions you’d get from a mate who’s done four of these and learned the hard way.
Why bother (the honest version)
A roof is a massive water collector. A modest one dumps tens of thousands of litres a year down the drain. A single butt catches a chunk of that, and garden watering is one of the few household water uses where rainwater is genuinely better than tap water — it’s soft, it’s room temperature, and it doesn’t have chlorine in it.
The uncomfortable bit: a butt on its own saves you very little money. The savings come from the fact that you stop dragging a hose around, so you water more, so your plants stop dying, so you stop buying replacements. The water saving is a bonus.
What you actually need
- The butt itself (opaque or dark — clear ones turn into a swamp)
- A solid base: paving slabs, or a proper stand, or both
- A downpipe diverter kit sized for your downpipe
- A tap or spigot, plus a washer
- An overflow pipe or flexi hose long enough to reach somewhere useful
- Hacksaw, file, drill, tape measure, spirit level
- PTFE tape, because threaded fittings always weep
Step 1: Pick the spot for the right reasons
It has to be next to a downpipe — that’s the only non-negotiable. Everything else is preference, but preferences matter:
- Close to where you’ll use it. A butt at the back corner of the garden only gets used twice a year because hauling a can that far is miserable.
- On flat, firm ground. Not decking, not gravel, not bare soil, not a slope you “think” is flat.
- Somewhere the overflow can escape to. More on this later — this is where people wreck things.
- Not blocking a path or a vent or a door. Obvious until you’re carrying a full watering can in the dark.
Step 2: Build a base that won’t betray you
This is the step everyone skips and everyone regrets. A full butt of water weighs roughly the same as a small car. Put that on soil and it sinks, tilts, and either splits or dumps its contents against your wall.
Do it properly:
- Clear the area and compact the ground.
- Lay a couple of paving slabs — or a small pad of compacted hardcore topped with slabs.
- Check the slabs are level in both directions with a spirit level. Not “looks level.” Level.
- If you’re using a stand or riser, bolt it down or weight it, and put it on the slabs, not the mud.
Why bother with a stand? Because a butt sitting on the ground gives you a pathetic trickle from the tap. Raise the outlet even 30–50 cm and you get real, usable flow — enough to fill a can in a reasonable time or run a low-pressure hose. It’s the single biggest quality-of-life upgrade and it costs almost nothing.
A note on stands: they must be rated for the weight. Those flimsy plastic ones that come bundled with cheap kits are fine for an empty butt and terrifying for a full one. If it flexes when you push it, it’s not going to hold.
Step 3: Decide how you’re feeding it — diverter vs direct
Two options, and only one of them is a good idea.
The direct feed (don’t)
You cut the downpipe clean through and point the whole thing into the butt. Simple, and it works perfectly — right up until the butt is full. Then the downpipe has nowhere to go, water backs up the pipe, and it either overflows at a joint or starts pushing back into the gutter and down your wall. Avoid.
The diverter (do this)
A diverter is a small unit that slots into a section you cut out of the downpipe. It has a chute that sends a portion of the water sideways into the butt while the rest continues down the pipe as normal.
Here’s the bit that isn’t explained well anywhere: when the butt fills up, the water simply backs up inside the diverter and carries on down the downpipe like the butt isn’t there. No flooding, no drama, no float valve to jam. It’s a passive, self-regulating system and it’s honestly elegant.
The other advantage is reversibility. Keep the offcut of downpipe. If you ever need to remove the butt, you slot the offcut back in and nobody can tell you were ever there. Which matters a lot if you rent, or if your building has opinions about pipework.
Step 4: Cut the downpipe without wrecking it
- Offer the diverter up to the downpipe and mark the top and bottom of the section it replaces.
- Measure twice. The diverter outlet needs to sit above the butt’s inlet, and the butt’s inlet is fixed by the height of the butt — so sort the base and stand first, then cut.
- Cut with a hacksaw, square, with slow strokes. Downpipe is thin and kinks if you rush.
- File the cut edges smooth. Burrs catch debris and create leaks.
- Fit the diverter with the supplied clips or brackets, and check the pipe still runs straight and secure above and below it.
Drill the butt’s inlet hole to the size the kit specifies, fit the seal from the outside in the orientation the instructions show, and tighten until snug — not until you strip the threads.
Step 5: The overflow. Do not skip the overflow.
Every butt has a high-level overflow outlet. Connect it to something. If you don’t, a full butt leaks from the lid, runs down the side, and soaks the ground and wall right where it stands — which is exactly where you don’t want standing water.
Options, in order of preference:
- Route it back into the drain the downpipe originally fed. Cleanest option, no mess.
- Route it to a soakaway or a planted bed a couple of metres from the wall.
- Route it into another butt if you’re building a chain. Link them low, overflow high.
Point it away from the building, with a slight fall. A flat overflow pipe holds water and grows things.
Step 6: Fit the tap and test
Wrap PTFE tape around the tap thread, screw it in, and tighten by hand plus a quarter turn. Fill the butt from a hose first — it’s faster to spot a leaking seal than waiting for rain. Check the tap, the inlet, and the diverter.
The awkward realities nobody mentions
- Roof water is not drinking water. It’s fine for watering, washing cars, and flushing. Don’t drink it, don’t use it for anything that goes in your mouth unwashed.
- The first rain after a dry spell is filthy. It’s washing bird mess and roof grit off. Dump or divert the first flush if you’re precious about your plants, or just accept that the butt will have sludge in the bottom.
- Algae and midges are a lid problem, not a water problem. Opaque body, lid on, and a bit of mesh over the inlet kills both.
- Winter kills cheap butts. A full butt in a hard freeze can split. Many are fine, but if yours is thin-walled, drain it down before the cold sets in.
- Clean it once a year. Empty it, scoop the sludge out, rinse, refit. Ten minutes, and it stops the smell.
Conclusion
A water butt install is one of those jobs that looks like plumbing and is actually just carpentry with water — the hard part is the base, the cutting is easy, and the whole thing lives or dies on whether you gave the overflow somewhere to go.
Do the slabs, level them properly, use a diverter instead of a hard cut, raise the thing off the ground, and give the overflow a route that isn’t your wall. It’s an afternoon’s work, it’s completely reversible, and then you get to spend the rest of the summer feeling quietly smug every time it rains.