Rainwater harvesting starts on the roof, but it is won or lost in the gutters. Every litre you store has already crossed the covering, run along a gutter and dropped down a downpipe, so a sagging run, a leaking joint or a blocked outlet loses water before it reaches a butt. This guide covers the plumbing side of collection: what the roof surface does to the water, how a diverter or tank connects to a downpipe, where to put storage, and what to fix first. The advice suits a standard semi and equally a period Bath house, where the downpipe may be cast iron, shared with a neighbour or covered by listing. It is about garden and utility use, not drinking water. For the whole rainwater system, start at the gutters, soffits and fascias hub.
What a harvesting system takes from the roof
A harvesting system is a chain, and each link has a job. The roof covering sheds the water. The gutter, fixed to the fascia board at the eaves, collects it. An outlet drops it into a downpipe, and somewhere on that downpipe a diverter, filter or tank connection takes a share. The store holds it, and an overflow sends the surplus to the drain or a soakaway when the store is full.
Most householders start with a water butt on a diverter kit, which is the simplest version of the chain. Larger systems use a buried or free-standing tank with a filter at the inlet and a pump to carry the water to a tap. The principle is the same. Roof water has to arrive clean enough, in enough quantity and with somewhere safe to go once the store is full.
The quantity depends on two things you can work out without a specialist: the plan area of roof that drains to the downpipe you are using, and how much rain falls. A roof collects over its footprint, not along its slope, so a steep roof and a shallow one with the same footprint deliver the same volume. Rainfall varies from year to year, and we will not quote a local total here because we have no sourced figure for Bath. A better habit is to look at how quickly an existing butt fills in a normal shower. If it overflows in a short spell, the butt is small for that roof, and the surplus is already being lost.
Think about timing as well. Rain arrives in bursts and the garden needs water in dry weeks. A small store fills and spills long before summer. Where the aim is real use rather than a token butt, the storage has to be large enough to bridge a dry spell, and that decision shapes everything else on this page, from the base to the overflow route.
Where the water should not go
Harvested roof water is for the garden, washing and cleaning jobs. It is not drinking water, and it should never be cross-connected with the mains supply. Any plumbing beyond a tap on a butt, such as supplying a toilet or washing machine, is a specialist job with its own rules, and building control is the right first call. Speak to your building control body first.
What the roof and gutters add to the water
Rain picks up whatever sits on the roof. Moss, lichen, bird droppings, leaf litter and grit all wash into the gutter in the first minutes of a shower, and so does any dust from nearby roads. Slate and clay are smooth and shed it quickly. A rough or heavily mossed covering holds more, and the first rain after a dry spell carries the most. That is why filtering and first-flush arrangements exist: they send the dirtiest opening part of each shower to waste before the rest goes to storage.
The metals matter too. Lead flashings and lead-lined valleys are normal on older Bath roofs, and a gutter system may be cast iron, aluminium or uPVC. If you plan to water edible crops from the store, ask a plumber or the supplier of your system about the materials the water crosses before you decide. Gardening use is a light demand and most owners treat roof water as fine for ornamentals and lawns, but choice of crop is your call.
A flat roof adds another point. A bituminous felt roof, or a roof coated with a treatment, can release residues and tints that are unwelcome in a garden store. A rubber sheet roof, known as EPDM, behaves differently. In each case the answer is to ask the installer, and to take a sample of what comes off before committing a large tank to it.
Gutter condition changes quality as well as volume. A gutter that holds standing water breeds algae and smells, and it feeds that water into the butt. Sediment builds in the bed of a run that falls the wrong way. Before connecting a store to any gutter, clear it, flush it and watch it drain. Our guide to blocked gutter damage explains what a neglected run does to the fascia and the wall below, and the same neglect pollutes your harvest.
Leaf guards and mesh at the outlet reduce the load on the filter. They are not a substitute for cleaning, and once the mesh clogs, rain spills past the gutter lip rather than reaching the pipe.
Connecting to the downpipe
A diverter is a fitting that replaces a short section of downpipe. Water flows down the pipe as normal until the butt is full, then the diverter closes the side branch and everything carries on to the drain. Simple kits are made for round and square uPVC downpipe in standard sizes. You cut a section out, push the fitting in, and run a flexible hose to the butt.
Cast iron is different. It is heavy, it is joined with sockets, and cutting it needs the right tools and some care, because a cracked socket or a pipe that loses its support can bring the rest of the run down. A fitting made for plastic does not simply clip on. Some owners fit a branch collar to cast iron, others have a section replaced with a purpose-made piece. On a listed building, cutting into original cast iron is not a casual change, so see our page on listed and conservation-area rainwater goods before drilling anything.
Choose the downpipe with care. It should drain a worthwhile area of roof, sit near the garden and have room around it for a butt or tank to stand. A pipe serving a small porch roof will never fill a large store. One serving the main rear slope will fill a butt quickly, so allow for the overflow.
Overflow routes
A full store needs somewhere to send surplus. The usual choices are back into the downpipe below the diverter, to a gully, or to a soakaway. A soakaway or any new drain connection can be a building control matter and depends on ground conditions, so ask your building control body before you dig. The overflow must never run towards the house wall or the foundations. We see butts with their overflow pipe discharging against masonry, which keeps the base of the wall damp and works against the point of the gutter in the first place.
If you share a downpipe with a neighbour, as on many terraces, a diverter on it takes water from both properties, and a conversation first is the sensible course. Our page on the shared gutter on a terrace covers who owns what.
Storage: butts, tanks and where to stand them
A litre of water weighs a kilogram, so a full store is heavy. A butt on a thin plastic base or a few loose paving slabs will lean, and a leaning butt on a diverter pulls the hose and then the fitting out of line. Give any store a firm, level base: a solid slab or a purpose-made stand, not soil. Raise it high enough for a watering can to fit under the tap. The extra height also gives you a little more pressure for a hose.
A lid is not optional. It keeps out leaves, light and insects, and it makes the water safer around children and animals. Fit a fine mesh at the inlet, a screened overflow, and a tap placed above the sediment layer at the bottom. Twice a year, empty the store and scrub it out. Sunlight on a clear container grows algae, so use an opaque butt in a sunny spot.
Frost damages stores. A full butt that freezes can split, and a frozen diverter can stop working or crack. In cold weather, disconnect the hose, leave the butt a little below full and open the tap. For a larger buried tank, frost is less of a worry because the ground insulates it, but the access cover and inlet still need a fit that keeps out surface water and debris.
Bath makes siting harder than it sounds. Many houses have basement areas to the front, narrow rear yards or garden walls built against the house, and some terraces share a passage. If there is no room beside a downpipe, a butt can be placed in a less obvious position and fed through a hidden hose, but a long run of flexible pipe is a trip hazard and a place for water to collect. Where space is tight, one larger tank placed where it fits beats several small ones that never fill.
Do not place a store against a wall that is already damp. Damp walls in older houses are common, and a base of standing water against them makes the problem worse. Keep a gap and let air circulate.
Fix the gutters before you collect from them
The most common mistake is to bolt a harvesting system onto a gutter that was already failing. A gutter that brims in a downpour sends the water you wanted down the face of the wall. If the joints weep, you lose some at every union. If the fall is wrong, water pools and spills over the back, which can reach the roof timbers.
Gutters need a slight fall towards the outlet. One manufacturer's guidance for a small gutter range recommends a fall of about 1 in 350, and the exact figure varies by product, so follow the maker's instructions for the system you have. If the run has sagged between brackets, water stays in the dip. That is a repair, not a harvesting problem, and our page on the sagging gutter explains the usual cause, which is failed brackets or a rotten fascia behind them.
Brackets on that kind of gutter should be no more than 500 mm apart, again according to the same installation guide, and cast iron needs closer support because it is heavy when full. Bear in mind the extra weight too. Once water is held in a gutter or backs up from a full tank, the load on old fixings rises.
Joints are the next check. A uPVC union with a perished seal drips onto the wall, and a cast iron joint sealed with putty or mastic years ago can open when the iron moves. Inspect the joints over the outlet and over any diverter, where a leak does the most harm. See our page on a leaking gutter joint for the repair options.
Finally, think about capacity. Water arriving faster than the outlet can take it overflows, however good the butt. A larger outlet, a second downpipe or a deeper gutter may be needed on a big roof section. When we survey a house for gutter work, we look at how much roof drains to each outlet, and a harvesting system makes that question more pressing.
Bath houses, listed buildings and neighbours
On an older building the rainwater goods help define the elevation, which counts for a lot in Bath. Around 5,000 buildings in the city are listed, while the designated conservation zone spreads across about a half of the city. On a listed house, tapping into original cast iron, drilling it or relocating it could need a consent granted by B&NES, and starting before you hold one breaks the law. Consent of that kind is separate from planning permission. Ring the council before anything is touched, and read our page on listed and conservation-area rainwater goods.
An unlisted house inside the designated area faces less, yet a visible tank at the front or a swap of gutter material deserves an early word with B&NES planners before you buy equipment. A butt at the rear, tucked behind a pipe, rarely changes how the house looks from the street.
Some buildings suit a different approach. A rear extension with a flat roof, a garage or an outbuilding gives you a gutter and downpipe with no heritage constraint, and it is often the most practical place to collect from. A garden room with its own roof can feed a butt with no work on the main house.
Think about the neighbours too. On a terrace the downpipe at the party line is often shared or sits against the boundary. Overflow from a store must not run onto the next property, and any change at the shared pipe is a matter for both owners.
If you are also considering new gutters or a refurbished roofline, collecting rainwater can be designed in. Our page on roofline upgrades sets out how the work fits together, and if conservation rules stop you changing the material, our note on plastic guttering where conservation rules apply shows what a change involves. Our survey covers the whole run, with pictures of your own eaves, and the written fixed price that follows lets you plan collection around the repairs the gutters need.