Water and external systems · Drainage & Rainwater
Gravity Roof Rainwater Drainage System
A reference for how rainwater goods behave as a connected route rather than as a catalogue of profiles, and for the junctions at which roof, wall and buried drainage hand water on to one another.
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 roof rainwater drainage is
Rainwater leaves a pitched or flat roof at a defined edge or low point, and everything downstream of that edge belongs to this system: the collector that catches the sheet of water, the outlet that lets it leave the collector, the vertical pipe that takes it down, the secondary route assumed to be needed when the primary one blocks, and the point at which it is handed to the ground or to a buried network.
The assembly is a network rather than a layered build-up. Each downpipe serves a length of collector and works on its own under gravity, with the pipe running part full and air moving freely alongside the water. That independence is the defining behaviour: a blocked outlet floods its own length of gutter and no other, and a downpipe can be re-routed without reworking the rest.
The testable difference from a siphonic roof drainage system is whether a single outlet can be blanked off without changing the behaviour of the others. Here it can, because each outlet drains its own catchment under gravity. A siphonic installation runs full-bore as one primed circuit, which is why it collects horizontally at roof level and reaches the ground through very few stacks.
Roof form feeds the same system in two different ways. An eaves gutter sits outside the line of the wall and spills clear of it when it overtops; a parapet, valley or box gutter sits inside the building line, so overtopping discharges into the construction rather than away from it, and the secondary path stops being an optional refinement.
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 goods
- guttering and downpipe system
- eaves gutter and downpipe system
- gutters and downspouts
- gravity roof drainage
- roof drainage and gutter 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.
- Intercept water leaving the roof surface before it can run down the face of the wall or fall onto surfaces below.
- Carry that water under gravity to chosen discharge points rather than letting it find its own route off the building.
- Release it clear of the base of the wall, where saturated ground turns a roof problem into a below-ground one.
- Provide a secondary route that limits damage when the primary one is obstructed, treating obstruction as a normal condition.
- Allow the whole route to be reached, cleared and inspected without disturbing the roof covering.
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.
- Drainage planeEaves, valley or parapet gutter
The open collector that receives the sheet of water shed by the roof and holds it while it travels towards an outlet. Its position relative to the wall line, outside it at an eaves and inside it at a parapet or valley, decides what happens when it overtops.
- Edge and terminationOutlet, nozzle and offset bend
The transition from open channel to closed pipe. It is the throttle on the whole upstream length: the collector can only shed what the outlet accepts, so the outlet rather than the gutter profile usually governs how the collector behaves in heavy rain.
- Service zoneDownpipe
The vertical run that takes water to ground or to a connection point. Under gravity it carries a mixture of water and air rather than running solid, so a partial obstruction narrows the flow instead of sealing the run. Its standoff from the wall answers a different question: a weeping joint or an overflowing head then discharges clear of the face, and the back of the pipe stays reachable.
- ProtectionOverflow or secondary outlet
A designed release point that fixes where water goes once the primary outlet is obstructed. On a collector inside the building line it is the difference between visible spillage in a known place and water entering the construction unseen.
- ProtectionLeaf guard, outlet grating and balloon
Debris interception that keeps leaf litter, moss and roof grit out of the closed pipework by holding it where somebody can reach it. It relocates the blockage rather than removing the need to deal with one.
- AttachmentBrackets, clips and standoffs
The supports that hold the collector to its line and the pipe to the wall. They carry the weight of water and of the debris load that collects with it, and they are what holds the pipe at its distance from the wall face rather than letting it bear against the masonry.
- Jointing and sealingUnions, expansion joints and gaskets
The joints between lengths, which have to stay closed while the run lengthens and shortens with temperature. Long metal collectors and pipes move appreciably, so joints are where movement is either accommodated or converted into a leak.
- Edge and terminationShoe, back-inlet gully or sealed connection
The discharge detail where the system hands water on. Whether it is an open shoe onto a surface, a trapped gully or a sealed connection into a buried network decides how far water travels before it meets the ground beside the building.
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 outlet governs the collector, not the other way round. Water enters along the whole length of a gutter but leaves at a point, so the collector fills towards the outlet and the depth it reaches is set by how freely that outlet accepts flow. Enlarging the collector while leaving the outlet as it was changes little, which is why replacing a gutter on its own rarely alters how that length behaves in heavy rain.
The overflow means nothing except in relation to the outlet's assumed failure. It has to sit where water will actually reach it once the outlet is obstructed, and it has to discharge somewhere that reveals the problem rather than hiding it. An overflow set above the level a blocked gutter can reach, or one draining into the same obstructed pipe, is decoration.
Supports, joints and movement act on one another. Brackets fix the line the water follows; joints let the run change length as it heats and cools. Anchor a long run at many points and the movement is driven into the joints; leave the joints free with sparse support and the line sags, water stands, and standing water finds the joint.
The discharge point closes the loop back onto the building. Water taken cleanly off a roof and released at the foot of the wall re-enters the ground the foundation bears on, so this system is judged at its bottom end as much as at its top. Where it connects into a buried network, that network's behaviour at capacity is imported back up the downpipe.
Debris interception moves the failure rather than preventing it. A guard keeps litter out of the closed pipe and concentrates it at the collector or the grating, converting an unreachable obstruction into a reachable one. It only helps if somebody can reach it, which makes access a decision taken at the same time as the layout.
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.
Drainage plane
Collector and downpipe families are commonly encountered in these metals as well as in moulded plastics. How any of them behaves in a given exposure, and alongside which other metals, is a matter for the manufacturer's documentation and the designer.
Edge and termination
Valley and parapet gutter linings, outlet aprons and the dressing around a rainwater outlet are commonly encountered in these families. Compatibility with the roof covering on either side of the junction sits with the roof system documentation.
Jointing and sealing
Union gaskets, lap seals and bedding at joints are commonly encountered in these sealant families. Whether any of them holds against standing water, movement and the particular metal or plastic involved is answered by the product literature.
Protection
Coatings are commonly encountered on ferrous rainwater goods and on the ironwork carrying them. Nothing here states a protective effect; the exposure, the substrate condition and the coating system govern, and the designer specifies.
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.
Eaves, fascia and the roof edge
The eaves detail decides where water leaves the covering and whether it lands in the collector or behind it. An underlay or drip stopping short of the gutter line sends water behind the collector and into the fascia, and no downstream component can recover it.
Read about Eaves System →Parapet and box gutter upstands
Where the collector sits inside the building line, the gutter lining and the parapet upstand are one continuous problem. How far water can rise before it passes the upstand, and where it goes when it does, is settled at this junction rather than in the pipework.
Read about Parapet System →Connection into the buried network
A sealed connection makes the downpipe part of the buried system and imports its behaviour, so surcharge below ground appears as water backing up at the connection. An open shoe over a trapped gully breaks that link but leaves an open route for debris.
Read about Buried Drainage Network →Diversion into storage
Where a downpipe feeds a store, the diverter, any first-flush arrangement and the overflow from the store all sit within the rainwater route. The store's overflow becomes part of this system's discharge path and has to be treated as normal flow, not an exception.
Read about Rainwater Harvesting →Discharge onto external surfaces
Where a downpipe releases onto paving, the receiving surface has to take that concentrated flow away from the wall. A shoe discharging onto a surface that falls back towards the building turns a working roof drain into a source of ground saturation.
Read about Surface Water Collection →
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
- Persistent wetting of a wall below a weeping joint or an overflowing collector is the usual route by which a rainwater fault becomes a fabric fault. What the wall is made of behind decides how much that matters and whether it can dry.
- Durability
- Dissimilar metals in contact, and run-off from one metal passing over another, are the classic slow failures in rainwater goods. Which combinations are acceptable in a given exposure is settled by the manufacturers' documentation rather than by appearance.
- Movement
- Long runs of metal collector and pipe change length with temperature, and the fixing pattern decides whether that movement is absorbed at the joints or forced into them. Where a run is anchored and where it may slide is a design decision.
- Maintenance and access
- Everything in the route silts, and the parts hardest to reach silt fastest because nobody reaches them. Whether a collector can be safely reached, and by what means, belongs in the design rather than being discovered after handover.
- Interfaces
- The junctions to the roof edge, to the parapet and to the buried network are where most failures occur; the middle of a gutter run rarely fails on its own. Each junction hands a duty on, and each has an owner in the documentation.
- Documentation
- Layout drawings, the assumed catchment for each outlet and the position of every overflow are what let a later alteration be judged. Removing a downpipe during a facade change is only safe if what it served has been recorded.
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.
- Where does each outlet discharge, and does that point sit clear of the ground the foundations bear on?
- Is the collector inside or outside the building line, and what does the overflow arrangement assume about that?
- Which parts of the route can be reached for clearing without specialist access equipment?
- Is the connection into the buried system sealed or broken by an open air gap, and what does each choice import?
- How is thermal movement in long runs accommodated, and where is the run anchored?
- What happens to the assumed catchment if the roof is later extended, re-covered or partly re-roofed?
- Are dissimilar metals in contact anywhere in the route, or does run-off from one pass over another?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A larger gutter profile is often assumed to cure overflowing, but where the outlet is the restriction, changing the collector alone changes very little.
- An overflow is treated as an optional refinement, when on a collector inside the building line it is the component deciding where a blockage shows itself.
- Leaf guards are read as removing maintenance, though they relocate debris to somewhere reachable and still need somebody to reach it.
- Discharging neatly at the foot of a wall reads as a finished job, when that ground is the one place water was worth keeping away from.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What catchment has each outlet been assumed to serve, and where is that assumption recorded?
- Where does the overflow discharge, and would it be noticed by somebody using the building?
- Is the discharge point separated from the ground the foundations bear on, and by what means?
- How should this route be inspected and cleared, and what access does that require?
- Are the metals in the collector, the fixings and the covering above compatible in this exposure?
- If the buried system surcharges, what happens at the connection to the downpipe?
What this page does not do
- Nothing here states a drainage capacity, a fall or a pipe size; those are design outputs for a qualified professional and depend on the catchment and the climate.
- Watertightness at a gutter or an outlet is a property of the completed and correctly detailed junction, not of the profile or the sealant used within it.
- Working at roof edges and from ladders carries risks that belong with a competent contractor and a method agreed in advance.
- Where a rainwater route crosses a boundary or discharges to a public system, the arrangement is a matter for the relevant authority and the parties involved.
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.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
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 →