Permeable paving in a SuDS train, and its maintenance
Permeable paving attenuates water in the open-graded sub-base beneath it. Here are the three configurations, how storage volume is calculated, why systems clog, and what a lead local flood authority expects to see named.
Permeable paving works as a SuDS component because the sub-base beneath it is a reservoir, not because the blocks themselves hold water. That is the sentence to hold on to when a lead local flood authority asks for storage volume calculations. The surface controls how water gets in and how much silt comes with it. The open-graded aggregate underneath is where the attenuation actually happens, and the void ratio of that aggregate is the number your volume depends on.
Most rejections of a permeable paving proposal come down to one of three things: no infiltration testing, no maintenance schedule with a named responsible party, or a design that assumes infiltration on ground where it cannot happen.
Three configurations, and why the distinction matters
CIRIA C753, the SuDS Manual, separates permeable pavement systems into three types, and the planning consequences of each are different.
Total infiltration passes everything into the ground through the subgrade. There is no outfall, so no discharge rate to agree with an undertaker or a riparian owner, and no third party consent. It needs ground that will actually take water and a comfortable separation from groundwater.
Partial infiltration lets some water soak away and pipes the rest to a controlled outfall. It is the common compromise on sites with moderate permeability.
No infiltration, usually called a tanked system, wraps the sub-base in an impermeable membrane and discharges through a flow control. It is what you use over contaminated ground, in a source protection zone where infiltration is unacceptable, where groundwater sits high, or where the subgrade is clay. A tanked permeable pavement is still a legitimate SuDS component. It just does its work by attenuation and treatment rather than by disposal.
| Configuration | Needs | Typical use |
|---|---|---|
| Total infiltration | Measured infiltration rate, unsaturated zone beneath, clean ground | Granular subsoils, car parks away from sensitive receptors |
| Partial infiltration | Infiltration testing plus an agreed outfall and flow control | Mixed ground conditions |
| Tanked, no infiltration | Impermeable membrane, outfall, flow control | Clay, contaminated land, source protection zones, shallow groundwater |
Getting the storage volume right
The storage is the void space in the sub-base and, to a lesser extent, in the laying course. For open-graded crushed rock the voids ratio is commonly taken as around 30 per cent, but it should be confirmed against the aggregate actually specified, because a contractor substituting a denser grading quietly removes a chunk of your attenuation.
So a 300 millimetre sub-base over 500 square metres at 30 per cent voids gives roughly 45 cubic metres of storage. Deepen it to 450 millimetres and you get about 67 cubic metres. That linear relationship is why permeable paving is attractive on constrained sites: you buy volume with depth rather than with land.
The catch is that depth is limited by groundwater, by the subgrade's bearing capacity and by the practicalities of excavation. On a slope you also have to build in baffles or check dams within the sub-base so that water does not simply run downhill inside the structure and discharge at the low corner, a problem explored further in the treatment of runoff on a steeply sloping site.
Infiltration rates come from field testing, normally following the BRE Digest 365 approach, carried out at the depth and location of the proposed system rather than in a convenient corner of the site. A single trial pit is not evidence for a 2,000 square metre car park. And the rainfall data behind the calculation has to be stated, because an authority cannot audit a volume without knowing which dataset and which climate change allowance produced it, as the comparison of rainfall datasets in drainage calculations sets out.
Silt, clogging and the thing everyone gets wrong
Permeable paving fails from the top down. Sediment accumulates in the jointing material and the upper few millimetres of the laying course, surface infiltration drops, and water begins to pond and run off as if the pavement were impermeable.
The jointing material has to be the specified clean grit. Building sand brushed into the joints by a well-meaning contractor, or kiln-dried sand used as though this were a conventional block pavement, ruins the system on day one. Write the jointing specification on the drawing and repeat it in the construction notes.
Sediment input is the other half. Permeable paving should not receive run-on from unsurfaced areas, soil storage, landscaped banks that erode, or construction traffic routes. Where run-on is unavoidable, put a filter strip or a gravel margin in front of it as pre-treatment and accept that maintenance frequency goes up.
Maintenance itself is unglamorous and inexpensive relative to replacement. Vacuum sweeping once or twice a year on a lightly loaded private drive, more often on a retail car park or anywhere trees shed leaf litter, is the baseline. Inspections should record surface infiltration performance rather than appearance, because a pavement can look immaculate and be clogged. Remedial work means lifting blocks, replacing the jointing and the top of the laying course, and relaying. Winter gritting with sand rather than salt is a slow route to the same problem.
Who maintains it, and how the authority secures that
Schedule 3 to the Flood and Water Management Act 2010, which would have created a SuDS approving body with adoption duties, has not been brought into force in England. So the maintenance question is answered by planning condition and by private arrangement rather than by a statutory adopter.
Highway authorities remain cautious about permeable paving in the adopted public highway, and on many schemes the pavement sits in a private shared surface or a management company area instead. The authority will therefore want three things named in the submission: the maintenance operations, their frequency, and the body legally responsible for carrying them out for the life of the development. A management company, a residents' company with a service charge, or a single owner on a one-plot scheme are all acceptable. A blank is not.
Expect a condition requiring a verification report after construction, confirming that the system was built as designed, with photographs of the sub-base before it was covered. Take those photographs. Nobody can prove the aggregate grading afterwards.
Where permeable paving is the wrong answer
It is not suitable on steep gradients without substantial internal baffling, and the SuDS Manual steers designers towards shallow slopes. It is poor where heavy goods vehicles turn repeatedly, because block rotation and joint widening follow. It should not be used where fuel or chemical spillage is likely unless the treatment design and the membrane account for it, and it is generally unacceptable for infiltration within a source protection zone 1 or over contaminated ground, where the risk of mobilising contaminants outweighs the drainage benefit. Any of those situations pushes you towards a tanked configuration, a different component, or both.
Illustrative example, not a real scheme: a 14 space staff car park on a site with a measured infiltration rate too low for total infiltration, and made ground of uncertain history, is designed as a tanked permeable pavement with a 400 millimetre sub-base, a hydrobrake restricting discharge to the pre-development greenfield rate, and a filter strip along the landscaped edge. The attenuation sits entirely under the parking, the drainage plan shows no above-ground basin, and the maintenance schedule names the building owner with twice-yearly vacuum sweeping. The same car park on granular subsoil would have been a total infiltration system with no outfall at all.
What to show on the drainage plan
Put the configuration type in words on the plan, not just in the report. Show the extent of the pavement, the sub-base depth, the membrane where there is one, the location and type of flow control, the outfall and its invert, the baffle positions on sloping ground, and the pre-treatment. Add a layer schedule with the aggregate grading and the jointing material.
Then carry the same figures into the drainage strategy and, where the site is in Flood Zone 2 or 3 or over a hectare, into the flood risk assessment, so the storage volume quoted in one document is the storage volume in the other. Officers do check.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Infiltration testing, ground investigation and hydraulic modelling should be carried out by a suitably qualified engineer, and councils usually expect a suitably qualified author for drainage submissions.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
How much water does permeable paving store?
Storage comes from the void space in the open-graded sub-base, commonly taken as about 30 per cent of its volume, confirmed against the aggregate specified. A 300 millimetre sub-base over 500 square metres therefore gives roughly 45 cubic metres. Depth buys volume, so constrained sites often use permeable paving instead of a basin.
Can permeable paving be used on clay?
Yes, as a tanked system. The sub-base is wrapped in an impermeable membrane and discharges through a flow control to a sewer or watercourse at a restricted rate. The pavement then attenuates and treats rather than disposes. Tanked configurations are also used over contaminated ground, where groundwater is high, and where infiltration is unacceptable in a source protection zone.
Why does permeable paving stop working?
Sediment clogs the jointing material and the top few millimetres of the laying course, so surface infiltration falls and water runs off instead. The usual causes are the wrong jointing material, run-on from soil or landscaped banks, construction traffic, leaf litter and winter gritting with sand. Vacuum sweeping once or twice a year prevents most of it.
Who is responsible for maintaining it?
Whoever the planning submission names, because Schedule 3 to the Flood and Water Management Act 2010 has not been brought into force in England and there is no statutory SuDS adopting body. Highway authorities are often reluctant to adopt permeable paving, so a management company, residents' company or single owner usually holds the obligation, secured by condition or planning obligation.
Does a permeable driveway need planning permission?
Hard surfacing in the curtilage of a house can fall within Part 1 Class F of the General Permitted Development Order 2015 where the surface is porous or where runoff is directed to a permeable area within the curtilage. Non-porous front garden surfacing above five square metres needs permission. Check the designation position, since restrictions apply on designated land.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.