Building envelope · Roof Assemblies
Roof Stormwater Attenuation (Blue Roof) System
A reference account of a roof that is deliberately allowed to stand in water: how storage rewrites the duty of the layers beneath, why the flow restriction is the single element the concept depends on, and what has to exist for the moment storage runs out.
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 attenuation is
An attenuation roof holds rainwater on purpose. Instead of removing water as fast as it arrives, the outlet is deliberately restricted so that water backs up within the build-up or on its surface and drains away over a longer period. The roof is being used as temporary storage in the site's drainage strategy, and that use is the reason every layer beneath it is considered again.
The boundary against its nearest sibling, buried stormwater attenuation, is where the water is being kept. Look at the building: if the storage volume sits in a void within the roof and its level is defined by upstands and thresholds, this is a roof attenuation system. If the water is held in a tank or a granular void in the ground, the roof plays no part beyond delivering rainfall to it and the buried system owns the storage.
Holding water changes duties rather than adding a component. The waterproofing is asked to stand in water rather than shed it; laps and upstands that would normally sit above the run-off line now sit below the stored level; the structure carries a load that only exists during and after rainfall; and the thresholds and parapets that would be trim on any other roof become the walls of a reservoir.
Storage volumes, restricted discharge rates, rainfall intensity and structural capacity are engineering determinations and are deliberately outside the scope of this reference. What belongs here is the organisation: what the parts are, how they depend on one another, and what an owner should understand about a roof whose behaviour is designed around water staying put.
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.
- blue roof
- attenuation roof
- controlled-discharge roof
- stormwater detention roof
- restricted-outlet roof
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.
- Hold rainwater temporarily within or on the roof rather than passing it straight into the drainage network.
- Release the held water slowly through a deliberately restricted outlet so that discharge is spread over time.
- Define a storage level using the upstands, thresholds and perimeter of the roof rather than a separate vessel.
- Provide a route for water to escape safely once the intended storage has been used up.
- Keep the restriction reachable, so the element the whole concept depends on can be inspected and cleared.
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.
- Control layerWaterproofing under standing water
The layer resisting water in a roof where water is expected to stand rather than run off. Its laps, upstands and terminations are all working below a designed water level for part of their life, which is not the case in an ordinary flat roof.
- Cavity or voidAttenuation void or storage layer
The space in which held water sits, whether an open void beneath a surfacing, a cellular layer or simply the depth between the waterproofing and the storage level. It is the volume the whole concept is arranged around.
- Drainage planeFlow-restricting outlet
The device that meters the release. It is the element that makes a roof an attenuation roof rather than a roof with a drainage problem, and everything else in the assembly is arranged around the fact that it discharges slowly by design.
- Edge and terminationUpstands and thresholds defining the storage level
The perimeter that decides how much water can be held. The lowest point at which water could escape sets the limit, so a door threshold, a parapet opening or an edge trim is part of the storage design rather than a detail around it.
- ProtectionExceedance and overflow route
The intended path for water once the storage is full or the restriction is obstructed. Its purpose is to decide in advance where water will go, rather than leaving the roof to find its own way out at whichever point is weakest.
- Service zoneAccess to the restriction
The provision made for reaching, inspecting and clearing the flow-restricting outlet. Because the restriction is the single point of dependence, access to it is part of the design rather than a maintenance convenience.
- Primary supportStructure carrying the stored water
The deck and frame beneath, which now carry a load that appears during rainfall and disappears afterwards. What the structure can hold is an engineering determination, and the storage concept follows from it rather than the other way around.
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 storage volume is not chosen by the void; it is set by the lowest point at which water could leave. A door threshold, a weep in a parapet, an unsealed edge trim or a low point in an upstand each caps the level regardless of how much space exists above the waterproofing. Every perimeter detail is therefore a storage decision, and altering a threshold later alters what the roof does in a storm.
The restriction concentrates the whole idea into one element. Obstruct it and the roof continues to fill while releasing nothing, so the design condition is exceeded by an assembly that is behaving exactly as built; enlarge it and storage never forms at all, so the roof reverts to an ordinary drained roof and the site loses the attenuation it was relying on. Nothing else in the assembly can be relied on to compensate.
Standing water reaches parts of the build-up that running water never touches. Laps, upstands, penetration collars and outlet connections are held under water for extended periods rather than wetted and drained, so the terminations are working under conditions that an ordinary roof detail is not arranged for. That is why the waterproofing and the storage concept are specified together.
The exceedance route exists because the restriction can fail and the rainfall can exceed what was assumed. If no route is provided, the roof still finds one, and it will be the lowest point of the perimeter, which is usually a door, a parapet junction or the head of a wall. Deciding that route in advance is the difference between an overflow and water entering the building.
Falls and storage argue with each other. An ordinary flat roof is arranged so water leaves quickly, while this roof is arranged so it does not, which changes how the surface, the void and the outlet relate. The result is that a roof designed for attenuation cannot be judged by whether water is standing on it, because standing water is the intended behaviour rather than the symptom.
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.
Control layer
These waterproofing families are commonly encountered on roofs where water is retained temporarily. Whether a product may be used where standing water is designed into the roof is stated in the manufacturer's documentation and confirmed by the designer.
Thermal layer
Boards from these families are commonly encountered in the build-ups attenuation roofs are formed on. Whether any of them belongs in a roof carrying held water depends on where it sits relative to the storage, and is a design question.
Finish surface
Surfaces of these kinds are commonly encountered above an attenuation void, where the storage is concealed under a usable or planted layer. What weight that adds, and how it interacts with held water, are determinations for the engineer and the designer.
Edge and termination
Perimeter trims, copings and overflow formations are commonly made from these families. Because the perimeter defines the storage level here, how each is formed and sealed is more consequential than on a conventionally drained roof.
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.
Storage held in the ground instead
Buried attenuation answers the same need without putting water on the building. Where both are used, the roof restriction and the buried storage are working in series, and the discharge from one becomes the inflow to the other.
Read about Attenuation Storage →The drainage the restriction feeds
Downstream pipework receives a metered flow rather than the roof's full run-off, which is the point of the arrangement. Any later change to outlets, gutters or downpipes therefore changes what the site's drainage was designed around.
Read about Roof Rainwater Drainage →Doors opening onto a roof that stores water
A threshold is a potential overflow point and therefore a limit on the storage level. Where a door opens onto an attenuation roof, the threshold has to be resolved with the storage concept rather than detailed as an ordinary entrance.
Read about Door Threshold Interface →Perimeter defining the reservoir
Parapets and upstands are the walls of the storage. Their height, the position of any weeps or overflows and the continuity of the waterproofing turning up behind them all determine how much water the roof holds and where it escapes.
Read about Parapet System →Storage beneath a usable surface
Where the roof is also a terrace, the storage void and the surfacing void are the same space. Access to the restriction then has to be arranged through the paving, and debris washed through the joints ends up where it can obstruct the outlet.
Read about Trafficked Roof Deck →Water taken for reuse
Harvesting removes water for use while attenuation delays its release; the two can coexist but they draw on the same rainfall. How they are arranged relative to each other, and which takes priority, is a design decision made for the site.
Read about Rainwater Harvesting →
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
- Water is held against the assembly by design, so every termination is working under conditions an ordinary roof detail does not face. Whether a given build-up can accept that is assessed by the designer and the product documentation.
- Maintenance and access
- The flow restriction is the element the concept depends on and the element most easily blocked by leaves, silt and debris. How it is reached and inspected is part of the design, and a roof that hides it has designed out its own upkeep.
- Durability
- Prolonged wetting, biological growth and debris accumulation are ordinary conditions for this roof rather than exceptional ones. What that means for a particular waterproofing is a matter for the manufacturer and the designer.
- Interfaces
- Thresholds, parapets, penetrations and the outlet connection all sit within a level water is designed to reach. Each has to be considered as part of the reservoir rather than as a detail around a roof that sheds.
- Documentation
- An attenuation roof looks like a flat roof holding water, which is exactly what a defective roof also looks like. Recording that the behaviour is intended, and how the restriction works, is what gives anyone arriving later a reason not to clear it.
- Buildability
- The perimeter and the outlet have to be built to a level rather than to an appearance, because the storage depends on it. Ordinary tolerances that are invisible on a drained roof are consequential where they set a water level.
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 is the storage intended to achieve for this site, and who has determined that the roof is the place to provide it?
- Where is the lowest point at which water could leave the roof, and does that point set the storage level?
- How is the flow restriction reached for inspection and clearing, and how often is that expected to be needed?
- What is the exceedance route when storage is full or the restriction is blocked, and where does that water go?
- Has the structure been assessed for the weight of held water in addition to everything else on the roof?
- Is the roof also going to be used or planted, and does the surfacing above the storage allow access to the outlet?
- How will a future owner or contractor know that standing water on this roof is intended?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Water standing on this roof is the design working, not a drainage fault, and clearing the restriction to make it drain faster defeats the purpose.
- The roof detains water and releases it; it is not a retention pond and is not intended to hold water permanently.
- The restriction is not simply a smaller outlet fitting, because the whole assembly beneath has been arranged around what it does.
- Storage on a roof does not remove the need for a drainage strategy on the ground; it changes the rate arriving at it.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What storage does this roof provide, and what does the site drainage strategy assume about its discharge?
- Which element sets the maximum water level on this roof, and has every perimeter detail been checked against it?
- How does the flow restriction perform if it becomes partly obstructed, and what happens next?
- What exceedance route has been designed, and where does water go once storage is exhausted?
- Has the structural engineer assessed the roof for the stored water as well as for the build-up and its use?
- What maintenance regime does this roof need, and who will be responsible for it once the building is occupied?
- How will the arrangement be recorded so that later work does not remove the restriction as a perceived blockage?
What this page does not do
- This entry sets out how the roof is organised as an assembly. It gives no storage volumes, discharge rates, rainfall assumptions or structural capacities, all of which are engineering determinations.
- Whether a roof may be used for stormwater storage at all is a matter for the relevant authority, the drainage engineer and the structural engineer.
- Watertightness is a property of the completed, correctly detailed and correctly installed assembly, and holding water against it places demands that must be designed for.
- Removing or enlarging the flow restriction, in the belief that the roof is draining poorly, disables the system the site's drainage was designed around.
- Adding storage to an existing roof changes its loading and the duty on its waterproofing, and is not an addition that can be made without assessment.
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.
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