Water and external systems · Decks & Paving
Permeable Paving System
This entry explains permeable paving as two systems sharing one thickness, covering the openings that admit water, the sub-base that stores it, the liner or infiltration decision, any flow control, and the clogging behaviour that governs the pavement in service.
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 permeable paving is
A permeable pavement admits water instead of shedding it. Rain travels through open joints, through a porous surface course or through voids formed in the units, into a laying course that has to stay open, and then into a coarse sub-base built with enough void space to hold water while still spreading traffic loading. What happens after that is the decision defining the rest of the system.
Three outcomes are possible and they are not interchangeable. Water may soak into the ground beneath, in which case the pavement is disposing of it. It may be held and released slowly through a flow control, in which case the pavement is throttling it. Or it may be held above a liner and taken away entirely, in which case the ground beneath is being deliberately kept out of contact with it. A pavement built for one of these and expected to deliver another disappoints in a very predictable way.
The nearest sibling is the flexible paving system, and the test can be carried out from the pavement itself: look for a gully. Conventional unbound paving is shaped to move water across its face to an inlet and will have one. Permeable paving has no inlet within the field, because the field is the inlet, and its shaping exists to spread water rather than to gather it. Every layer beneath has to remain open for that claim to stay true.
The argument about resin bound and resin bonded surfacing belongs to the materials library rather than here, but its consequence belongs here. One lets water through and can sit above a storage layer; the other is a dressing over a closed surface and cannot. Treating a pavement as permeable because of what a product is called is precisely how a storage layer ends up sealed from above.
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.
- porous paving
- permeable pavement build-up
- source control paving
- infiltration paving
- SuDS paving
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.
- Take rainfall out of the surface water network at the point where it lands rather than collecting and conveying it elsewhere.
- Store water temporarily within the void space of a coarse sub-base while that same layer continues to spread traffic loading.
- Let water soak into the ground beneath where the ground, the setting and the design all permit it.
- Release water at a controlled rate where discharge to a drainage network is what the design requires.
- Reduce the volume of surface water that has to be gathered, piped and disposed of somewhere else on the site.
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 meaningful in this system. The roles below sit in sequence, and moving one relative to another changes what the assembly does. The sequence is conceptual only: it states no thickness, no dimension, no fixing and no order of work on site.
- Layer 1 of 8. Order is meaningful.Finish surfacePermeable surface course or open jointing
The trafficked face and the openings through it, whether enlarged joints between units, voids formed within them or a porous continuous surfacing. These openings do two jobs at once, admitting water and transferring load between units, and they are sized as a compromise between the two.
- Layer 2 of 8. Order is meaningful.SubstrateLaying course that stays open
The coarse bedding layer beneath the units. It has to seat the units and pass water on without becoming the place where fine material accumulates, which is why it is a different animal from the fine bedding used under conventional unbound paving.
- Layer 3 of 8. Order is meaningful.Primary supportStorage sub-base
The coarse layer that spreads traffic loading and holds water in the spaces between its particles at the same time. Its two duties pull against each other, since void space is what stores water and particle contact is what carries load.
- Layer 4 of 8. Order is meaningful.Control layerSeparation and filter layers
The sheets that keep soil out of the storage layer and keep the storage layer out of the soil. Where infiltration is intended they also have to pass water, so they are doing filtration and separation simultaneously rather than acting as barriers.
- Layer 5 of 8. Order is meaningful.Control layerLiner or infiltration interface
The base of the store, either open to the ground so water can leave downward, or lined so it cannot. This single decision determines whether the pavement disposes of water or merely holds it, and nothing visible at the surface reveals which was chosen.
- Layer 6 of 8. Order is meaningful.Drainage planeFlow control and exceedance route
The outlet arrangement where discharge is restricted, together with the route water takes when the store is full and rain continues. The control device is what makes storage into attenuation; without it the store is simply a void with a pipe attached.
- Layer 7 of 8. Order is meaningful.Edge and terminationEdge restraint and containment
The perimeter that confines the units and also contains water within the storage layer. It has an additional duty compared with conventional paving, because water leaving sideways into adjacent construction is a failure that leaves no trace at the surface.
- Layer 8 of 8. Order is meaningful.ProtectionCleaning regime against clogging
The scheduled removal of material from the surface openings. It is treated as part of the assembly because the pavement's capacity to admit water declines steadily in use and is recovered only by cleaning, making upkeep a designed component rather than an optional extra.
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.
Every layer is asked to do two things that pull against one another. Joint filling has to be coarse enough to pass water and tight enough to transfer load between units; the sub-base has to hold enough void space to store water and enough particle contact to spread traffic. Optimise either layer for one duty alone and the pavement disappoints at the other, which is why permeable construction is far less tolerant of casual substitution than conventional paving.
Permeability is a chain, and the chain is only as open as its most closed link. A surface passing water into a laying course contaminated with fine material, or into a store separated by a layer that has silted, is a permeable surface over a closed pavement. Water then behaves as water does: it fills the layer above the blockage and either ponds at the top or emerges at the edges, and neither symptom points at the layer actually responsible.
Clogging is this system's characteristic behaviour rather than a defect in it. Everything landing on the surface, from grit and leaf litter to soil tracked off a border and material spread in winter, migrates into the openings and narrows them. The pavement therefore has a capacity that declines with time and recovers with cleaning. Surrounding surfaces that shed silt onto it shorten the interval between cleanings without anything on the pavement itself having changed.
The liner decision reaches upward as well as down. Where infiltration is intended, the ground is part of the system and its willingness to accept water sets how quickly the store empties; where a liner is present, the store cannot empty downward at all and the flow control becomes the only exit. A full store is normal in one arrangement and a warning in the other, and the pavement gives no external clue as to which it is.
Edge restraint carries an extra duty here. Beyond confining the units it has to keep water inside the storage layer and out of whatever is next to it, so a permeable pavement built against a building, a boundary or a lower terrace is a containment problem before it is a paving problem.
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.
Finish surface
Unit families of this kind are commonly encountered forming the face of a permeable pavement, laid with enlarged or voided joints. Whether a given unit is made in a form intended for permeable construction is a question for the manufacturer's documentation and the designer.
Edge and termination
Kerb, edging and haunching families are commonly encountered forming a perimeter that both confines the units and contains stored water. What that perimeter has to hold back, and where the water is meant to go instead, are design determinations.
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 layer against the formation
The formation here is not simply something to build on; where infiltration is intended it is also the outlet. Sealing or smearing the base of the excavation compromises the drainage function while leaving the load-spreading function apparently intact, so the two duties can fail separately.
Read about Pavement Foundation →Pavement acting as an infiltration device
An unlined permeable pavement is an infiltration system with traffic on top, and it inherits the same dependencies: what the ground will accept, how long the store takes to empty, and what happens during a storm that arrives before it has. Pre-treatment against silt is the shared concern.
Read about Infiltration Drainage →Storage volume feeding a flow control
Where discharge is restricted, the pavement is the storage half of an attenuation arrangement and the control device is the other half. Filling the store is expected behaviour, and it is the emptying rate, not the water level, that says whether the arrangement is working.
Read about Attenuation Storage →Adjacent surfaces draining onto the pavement
Where conventional surfaces or roofs discharge onto permeable paving, they deliver their silt as well as their water. That extra load is a design input rather than an incidental, since it changes both the volume the store must hold and how quickly the openings clog.
Read about Surface Water Collection →Storage layer near a building or a boundary
Holding water in a granular layer close to a wall, a foundation or a neighbouring plot is a decision with consequences beyond the pavement. Separation, containment at the edge and the position of any liner are settled with reference to what is next to the pavement rather than to the pavement alone.
Permeable surfacing over a rooting volume
A permeable pavement is the surfacing most compatible with a tree, since it lets air and water reach the ground beneath. That same openness means the rooting volume and the storage layer are exchanging water, and whether that exchange is wanted has to be decided rather than assumed.
Read about Tree Pit System →
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
- This assembly is intended to be wet, which inverts the usual questions. What matters is not keeping water out but knowing where it is being held, how long it stays, and what is standing next to a layer that is full of water after heavy rain.
- Maintenance and access
- Capacity declines as the openings fill and returns when they are cleared, so the assembly has a performance curve rather than a fixed condition. Who cleans it, how often and by what method are questions belonging to the design stage rather than to the first blocked winter.
- Durability
- The units and the store age differently. Surfaces can remain sound for a long time while the store silts up beneath them, so the age of a permeable pavement says little and its drain-down behaviour after rain says a great deal.
- Movement
- A coarse store behaves differently from a dense sub-base when it is saturated and loaded, and the migration of fines into the void space is what changes both its stiffness and its capacity. The two decline together and for the same reason.
- Interfaces
- Every edge of the store is a decision about containment: against a building, against a road, against a lower terrace or against untreated ground. Water leaving sideways is invisible from above and shows up somewhere else entirely.
- Buildability
- The store is exposed while it is being built and will accept anything that lands in it. Material washed or tracked into a partially completed store cannot be removed afterwards, so protection during construction affects the assembly permanently.
- Documentation
- Whether a store is lined, whether it discharges through a control and where its exceedance route runs are invisible once the surface is laid. A record of those decisions is the only way a future owner can tell what the pavement is actually doing.
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.
- Is this pavement intended to infiltrate, to attenuate, or to hold water above a liner and pass it on, and does everyone involved agree?
- What has been established about the ground's willingness to accept water, and does that come from testing or from optimism?
- What else drains onto this surface, and has the silt that arrives with the water been taken into account?
- Where does water go when the store is full and rain has not stopped, and is that route a designed one?
- What is immediately next to the store, and is a layer holding water an acceptable neighbour for it?
- Who will clean the surface, how often, and has that been agreed with whoever will own the pavement?
- Is any part of this surface likely to be resurfaced later with a closed material, and would that be noticed before it happened?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A permeable surface is not a permeable pavement; the description holds only while every layer beneath it also stays open.
- Ponding on the surface usually reports a blockage in the openings or the laying course rather than an inability of the ground to take water.
- A full storage layer is normal for an attenuating pavement and a warning sign for an infiltrating one, so the same observation means opposite things.
- Sealing or overlaying a permeable surface with a closed material converts a drainage asset into an ordinary pavement and removes capacity nobody may notice missing.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Which of the three arrangements is this design, and what changes if the ground turns out to accept less water than assumed?
- How was the contributing area worked out, and what surfaces are draining onto this pavement?
- Is a liner included, and if so what is it protecting and from what?
- Where is the flow control, how is it reached, and what does a blocked one look like from outside?
- Where does exceedance water run, and what does it run across on its way there?
- What cleaning regime does this design assume, and what happens to its capacity if that regime is not followed?
- How close is the store to a foundation or a boundary, and who has considered the effect of holding water there?
What this page does not do
- No storage capacity, void ratio, infiltration rate, drain-down time or rainfall assumption is stated here; all of these are design outputs from a qualified professional.
- Whether infiltration is permitted at a given location depends on the ground, on contamination, on proximity to structures and on the relevant authority, and none of that is settled by this entry.
- A pavement of this kind can be disabled by ordinary site activity, such as stockpiling soil on it or sealing part of the surface, long before anything looks wrong.
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.
Decks, Paving and Hard Landscape Systems
External surfaces people stand on, plus the substructure, edge, support and guarding that make one usable, each decided by the layer beneath and the route its water takes.
Browse all decks & paving entries →