From Greenhouse Gutter to Water Butt

A customer buys a downpipe diverter, a water butt and a length of hose, gets them home, and discovers the first problem within twenty minutes: the diverter does not fit the outlet on that greenhouse. The second problem arrives a week later, when the butt is full and water is quietly running back along the wall.

Rainwater collection looks like plumbing with three parts. It is actually a chain of matched interfaces, and the chain starts at a specific greenhouse outlet rather than at a generic pipe size.

Start at the gutter outlet

The correct starting point is the greenhouse model, and specifically its gutter and outlet arrangement.

Related reading: greenhouse models with integrated gutters and water outlets.

Greenhouses differ in ways that matter here. Some have a gutter on one side, some on both. The number of outlets can be one, two, or one per corner, and the position varies: end of the run, mid-run, or at a corner moulding. The outlet form varies too: a spigot designed for a downpipe, a rectangular or oval opening, a moulded lip, or a flat face that requires a specific connector. Diameter alone is a poor guide, because two outlets of similar bore can have completely different fittings around them.

This is where a catalogue habit fails. Buying a diverter by pipe diameter, then adding an adapter, works when the outlet is a plain circular spigot and the adapter is designed for it. It fails silently on moulded or non-circular outlets, where the adapter sits over the opening without sealing, and the leak appears only once water is running.

So the first instruction to any customer or buyer is to identify the model and photograph the outlet from two angles, with something for scale. That photograph, plus the model number, is what allows a supplier to confirm whether a given connector is appropriate.

One collection point or several?

A greenhouse with two gutters and four outlets can be plumbed in more than one way, and the choice has consequences beyond plumbing.

The simplest arrangement routes everything to a single collection point: gutters fall to one corner, one downpipe carries the water to the butt. Depending on the roof's catchment area, the number of outlets feeding that corner and the fall of the gutters, it can deliver the highest flow into one container and involve the shortest run of pipe. It can also concentrate the whole roof's output at one point, so whether a single blockage stops the system, and how much collected weight sits in one place, are questions about the specific layout and its support rather than fixed properties of the arrangement.

The alternative is parallel collection: each gutter or each pair of outlets feeds its own butt or its own section of the run. That can spread the storage around the garden, shorten individual pipe runs, and keep water available if one container is full or one outlet is blocked, though what each container actually receives depends on how the roof area is divided between them. It usually costs more containers and more pipe.

A third option is partial routing: one gutter collected, the other discharging to ground or to a soakaway. This is often the right answer for a customer who only wants water near one end of the greenhouse, and it tends to be cheaper. It should be chosen deliberately rather than arrived at by fitting what was available.

Which of those costs actually bite depends on the layout in front of you. How much roof area a given gutter actually drains, and therefore how much is lost under partial routing, is a function of the roof, the gutter runs and where the outlets sit, not of a general rule. The same applies to blockage: a single collection point with one outlet is more exposed than one with several, and the number of outlets is a property of the model.

RHS guidance on rainwater collection gives the practical requirements that apply regardless of layout. The butt should stand on a firm, level support, either a bought stand or bricks, high enough that a watering can fits under the tap. A filter should keep leaves and debris out of the butt. Multiple butts can be linked with a connection kit or a siphon. And the butt needs a tight-fitting lid, because otherwise spores of water-borne plant diseases can get in and algae will grow. RHS also recommends cleaning butts once a year with warm water and a little detergent.

That list reads like a set of small accessories. Read it as a set of constraints on layout instead. A butt that must stand on a raised, level support has a footprint and a height. A butt that must have a lid that closes has a top clearance requirement under the gutter. A butt that will be cleaned annually needs to be reachable and, in many designs, emptyable. Any of those can eliminate a position that looked fine on the plan.

Place the tank before choosing the connecting parts

The sequence that avoids most failures is to fix the container position first, then design the pipe run to it.

Position has four inputs. Support: level, firm, and raised if a watering can must fit under the tap. Access: room to reach the lid, the tap and the inlet, plus room to clean. Head: the butt must sit below the gutter outlet by enough for the water to reach it, and the pipe run should be planned so that it does not hold water and silt in a section that cannot drain. Discharge: where the overflow goes, which is the subject of the next section.

Only when the position is fixed does the connecting hardware become a defined requirement. Then the questions are concrete: what connector matches this outlet, what pipe size carries the flow from this run, how many bends and what fall the run needs, and what happens at the butt inlet.

The specific question worth answering here is whether a given manufacturer's rainwater kit is suitable for a given greenhouse series. Buyers and dealers often look for an explicit compatibility list, and often assume one exists. Compatibility lists of this kind do exist. Some manufacturers of rainwater kits publish a list of the greenhouse series a given kit is not suitable for, which is the clearest possible demonstration that fit is a per-model question. What matters for the customer in front of you is not that list but this greenhouse: the outlet form, the connector, and the tank position on this site. What that means for a dealer is not that no such list exists anywhere, but that a compatibility list should not be assumed, and that suitability has to be confirmed per greenhouse model, per outlet number and per outlet type. Writing “greenhouse rainwater connector” on an order as if it were a universal part is the specific mistake to avoid.

Follow the water when the tank is full

The overflow is the part most easily left out of a rainwater plan, and where it is missing it is often what produces the visible damage.

When the butt fills, the water has to go somewhere. There are three acceptable destinations and one unacceptable default. The acceptable ones are: back along the original downpipe route, if the diverter is designed to pass surplus that way; out of a dedicated overflow outlet to a defined place, such as a soakaway, a drain, a planted area, or a lower butt; or into a linked container as part of a chain. The unacceptable default is “it will run down the side of the butt and find its way,” which in practice means it runs along the wall, erodes the base, or pools where the customer walks.

So the overflow question is a site question, not a parts question. Where does the surplus go when the ground is already wet, which is exactly the weather in which the butt is full? An overflow that discharges onto a patio the customer crosses in the rain has solved one problem and created another. Answering it means looking at the ground the overflow will reach, at how that ground drains in wet weather, and at any local rule that governs where rainwater may be discharged on the property. None of those three can be settled from a product catalogue.

The overflow also sets a requirement on the butt's position. It has to be able to reach its destination, which may mean the butt sits slightly higher, or the overflow runs to a lower point, and it may rule out the neat corner where the customer wanted it. This is the same problem as the door that fits and will not open, discussed in the greenhouse site check: the space the product needs is larger than the space the product occupies.

RHS's guidance on the lid belongs in this section too. A tight lid is not only about disease and algae, it also keeps the butt from becoming a mosquito habitat and from filling with leaf litter that will eventually block the outlet. The lid requirement has to survive the pipe and overflow arrangements, which is why the lid needs to be checked after the plumbing is planned, not before.

Sell a documented combination

The final step is to turn a pile of parts into a documented combination, and that document is the actual product a dealer is selling.

It should list what has been confirmed to match this greenhouse: the connector at the outlet, identified by what it fits rather than by diameter; the pipe and its route; the butt and its support; the inlet fitting; the overflow arrangement and its destination. Every item on that list is confirmed against a specific greenhouse model and the actual layout on site, not against the category in general, because outlet form, connector fit, container position and overflow route all change with the model and the siting. It should also list what the customer must supply or prepare: the level base, the support height, the route preparation, and any cutting. And it should state what remains unconfirmed, because there is usually something.

Dealers who can produce that page are selling a working water path rather than a bag of fittings. The practical difference appears in returns: a customer who knows the overflow was designed, and knows what to check each autumn, does not come back in November with a flooded patio.

Supplying this properly takes both routes. A garden centre selling ten greenhouses a season needs a handful of each connector rather than a carton of one, which is a mixed order across a small range. An installer fitting one model repeatedly needs the opposite: single-model bulk quantities of the one connector that model takes. Treating either as the only way to buy is asking the customer to fit the order to the packing.

Written once per greenhouse model and kept, that page stops the same compatibility question being answered from scratch by a different person every season.

Planning rainwater collection accessories alongside a greenhouse wholesale range? Send the greenhouse model, gutter outlet photos and required connection details. We can review sourcing options and request model-specific compatibility information before quoting.