Water and external systems · Drainage & Rainwater
Rainwater Harvesting System
A reference on how a harvesting arrangement joins a drainage system to a supply system at a storage vessel, and why the separation from the mains and the overflow back to drainage are the roles that define it.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What rainwater harvesting is
A rainwater harvesting system takes water a roof was already shedding and gives it a use before it reaches the drain. Water leaving the roof is filtered, the earliest and dirtiest part of an event is often diverted away, and what remains is held in a vessel and drawn on for garden watering, washing down and other uses that do not call for drinking water.
Read as a whole it is a drainage system and a supply system joined at a tank, and the interesting behaviour is at that joint. On the drainage side it must always be able to let go of water it cannot hold; on the supply side it must never be able to reach the mains. Those are opposite instincts, and the components serving them, the overflow and the separation from the potable supply, are what define the system rather than decorate it.
Its nearest sibling is the stormwater attenuation system, and the difference is what a full vessel means. Harvesting wants the vessel full and treats stored water as an asset; attenuation wants its store empty and ready, and treats a full store as the condition it was arranged to avoid. Only water collected outside the building is in scope here, since what happens once it is inside is plumbing and building services, and preventing any connection between the networks is a matter for qualified professionals.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- rainwater collection and reuse system
- rainwater recycling system
- stored rainwater supply
- rainwater catchment system
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Capture water a roof is already shedding and hold it for later use rather than sending all of it to drain.
- Supply uses that do not require drinking water, from a store refilled by weather rather than by demand.
- Intercept debris and the first, dirtiest part of an event before storage, leaving fitness for any particular use to the designer and the relevant authority.
- Return to behaving as drainage whenever the store is full, without ever offering a path towards the mains supply.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. It is distributed rather than stacked: what matters is that the path connecting these roles stays continuous and unobstructed, not the order they are listed in.
- SubstrateCollection surface and its conveyance
The roof and the rainwater goods serving it. What the surface is made of, what grows on it and what lands on it decides what arrives at the filter, so the collection surface belongs to the water quality arrangement rather than being merely a catchment.
- Control layerPre-storage filtration and first-flush diversion
A filter removing leaves and grit, often with an arrangement sending the earliest and dirtiest part of an event to drain instead of to storage. What gets past here does not leave again; it settles in the vessel and stays.
- Cavity or voidStorage vessel and its restraint
The tank, buried or above ground, holding what has been collected, with the bedding, surround and anchorage keeping it where it was put. Light, warmth and disturbance change what happens to water standing in it, and a buried vessel standing empty in wet ground is acted on by that ground in ways its shell alone does not resist.
- ProtectionSeparation from the potable supply
The arrangement making a connection between stored water and the mains physically impossible rather than merely closed. It is the one role here whose failure affects something outside the building, and it is where any mains top-up has to be resolved.
- Service zoneDistribution set and controls
The pump, pipework, level sensing and switching moving stored water to the points using it. The controls decide what happens when the vessel is empty, and that decision is where a harvesting system either falls back gracefully or damages a pump.
- Jointing and sealingVessel penetrations
Every inlet, outlet, overflow and cable entry is an opening in the vessel, and each is a route for roots, vermin and ground water as well as for what it was made for. Vessel behaviour is decided at these openings rather than in the shell.
- Edge and terminationOverflow and its connection to drainage
The route taken by everything the vessel cannot hold, which across a wet season is most of what arrives. It is a drainage connection in every respect and needs the same protection against backflow and vermin as any other.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The collection surface, the filter and the vessel form a chain in which each stage decides what the next has to deal with. Anything the filter passes settles in the base of the vessel and stays there, so the draw-off is taken from neither the very bottom nor the surface, and the inlet is arranged to enter calmly rather than to stir what has already settled. A calmed inlet and a raised or floating draw-off only make sense in relation to the sediment the filter did not catch.
The overflow is what keeps the arrangement honest as a drainage system. Through much of a wet season the vessel is full and everything arriving passes straight to the drain, which makes the overflow a normal operating route rather than an emergency one. It also means the drain it joins can be full at the same moment, so the overflow has to behave when flow tries to come the other way.
The separation from the potable supply is the interaction that is not about performance at all. Where a mains top-up keeps the vessel usable in dry weather, the networks come closest exactly where stored water is dirtiest and pressure differences are least predictable, so the separation has to work as a physical arrangement rather than as a valve assumed to be shut. Nothing else in the system compensates for it.
The controls tie the supply side to the weather. A pump asked to draw from an empty vessel is being asked to do what the weather has made impossible, so level sensing, a fallback and the behaviour of the outlets when there is nothing to deliver form a single arrangement. Users notice a harvesting system mainly through this interaction, because it decides whether a dry spell passes unnoticed or becomes obvious.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Substrate
Collection surfaces are commonly encountered in these families, and each sheds something of its own along with the water, whether fines, coatings or organic matter. Which surfaces a scheme draws from is a water quality question for the designer rather than a drainage one.
Primary support
Storage vessels and the surrounds supporting them are commonly encountered in these families, in buried and above-ground forms. How a vessel is bedded, restrained and surrounded follows the manufacturer's documentation and the ground conditions on the site.
Jointing and sealing
Penetrations, covers and lid seals are commonly encountered detailed with these families. What is being excluded at these points, meaning roots, vermin, light and ground water, matters as much as what is retained, and the detail belongs with the vessel manufacturer.
Edge and termination
Screens, grilles, strainers and overflow terminations are commonly encountered in these families. Metals in contact with collected water are considered alongside the intended use of that water, which is a matter for the designer rather than a general preference.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Branch from the roof drainage
Harvesting is a branch off roof drainage rather than a replacement for it, so gutters and downpipes still have to work when the vessel is full. A diverter unable to pass full flow turns a storage decision into an overflow problem at the wall face.
Read about Roof Rainwater Drainage →Overflow into the buried network
The overflow joins the buried network and carries most of a wet season's collected water. The connection has to tolerate the drain surcharging while the vessel is full, which is the moment both systems are under pressure at the same time.
Read about Buried Drainage Network →The covering as the first stage of water quality
What a covering sheds, and what grows on it in shade, arrives at the filter. Moss and fines from a covering change how often a filter needs attention far more than the area of roof being collected from does.
Read about Tile and Slate Covering →Supply pipe and cable entry into the building
Where a pipe or cable from a buried vessel enters the building it crosses the envelope below ground and becomes a sealing detail rather than a plumbing one. It is a familiar route for water to enter a building through a system meant to serve it.
Read about Penetration Sealing →Overflow taken to the ground
Where the overflow goes to the ground rather than to a sewer, the vessel becomes the first stage of an infiltration arrangement, and the sediment and organic load leaving it become that system's problem to intercept.
Read about Infiltration Drainage →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- A buried vessel sits in ground that is wet for part of the year, and an empty vessel in wet ground behaves quite unlike a full one. Restraint, backfill and the condition of every penetration are what keep ground water on the correct side.
- Maintenance and access
- Filters, first-flush arrangements and the vessel base all need reaching, and a buried vessel under a lawn is easy to lose. Whoever will do this work should be able to open the covers without special equipment.
- Durability
- Sediment accumulates in the base over the years and gradually reduces what can be drawn on, while an inlet that stirs it keeps it in suspension. Neither is visible from a tap, and both are consequences of decisions made at the filter.
- Interfaces
- The system depends on parts of the building it does not own. Re-covering a roof, adding a moss treatment or altering a downpipe route all change what reaches storage without anything in the harvesting arrangement being touched.
- Documentation
- The position of the vessel, the routes into and out of it and the arrangement protecting the mains supply are what an owner, an occupier and any later trade need. Undocumented, a harvesting system is a hazard to whoever alters the plumbing next.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- What uses is the stored water intended for, and does anything else in the building rely on the same pipework?
- How is a connection between stored water and the mains supply made physically impossible?
- What happens in a dry spell, and does anything switch over automatically or does the supply simply stop?
- Where does the overflow discharge, and what happens when the drain it joins is already full?
- What is collected from, and what does that surface contribute besides water?
- Who empties the filter and the first-flush arrangement, and how often does that fall due?
- Is the vessel buried, and if so what holds it in place when it is empty and the ground is wet?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Collected rainwater is not clean by default; it carries whatever was on the roof, and storage changes it further.
- The overflow is not an emergency device, since for much of a wet season it is the route almost everything takes.
- A harvesting vessel does not reduce the peak leaving a site the way an attenuation store does, because it is intended to be full.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What arrangement separates the stored water from the mains supply, and how is it verified?
- What is the water intended for, and what does that intended use require of the filtration?
- How is the vessel accessed for inspection and cleaning once the ground above is finished?
- Where does the overflow go, and has backflow from that connection been considered?
- What are the controls expected to do when the vessel runs empty?
- How will this system be labelled so that a future trade does not connect it to something it should not reach?
What this page does not do
- Cross-connection between harvested water and a drinking-water supply is a public health matter governed by the relevant authority and handled by qualified professionals.
- This entry states no vessel capacity, no yield and no water quality outcome, all of which depend on the site, the collection surface and the intended use.
- Stored rainwater is not drinking water, and what any given installation may be used for is determined by the designer and the relevant authority.
- Pipework carrying harvested water is conventionally identified so that later alterations cannot mistake it for a potable supply.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Site Drainage and Rainwater Management Systems
Stages in the single route water takes from the surface it lands on to the point it leaves the site, each one defined by where it hands the water on.
Browse all drainage & rainwater entries →