Permeable paving design, detailing and maintenance

Permeable paving works as a storage tank with a surface you can park on. Rain passes through the joints between the blocks, down through a laying course, and into a sub-base of clean crushed stone whose voids hold the water until it infiltrates into the ground or is released through a flow control. Most of the failures have nothing to do with the blocks. They come from the sub-base being the wrong material, the edges being sealed with concrete, or nobody sweeping it for ten years.

For a self-builder there is also a planning reason to get it right. A hard surface in a front garden larger than five square metres needs planning permission unless it is made of porous materials or run-off is directed to a permeable area within the curtilage.

How permeable paving stores and treats water

Three things happen in the construction, and only the first is obvious.

Water enters through the surface. In permeable block paving it passes through joints or nibs filled with a small clean grit, typically a 2 to 6.3 millimetre grading with no fines in it. Porous asphalt and porous concrete let water through the material itself.

Water is stored in the sub-base. A clean, open graded crushed rock has a void ratio of roughly 30 per cent, so a 350 millimetre layer gives around 105 millimetres of storage depth across its area. That is the attenuation volume, and it is calculated in the same way as any other storage component.

Water is treated as it passes through. Sediment is intercepted at the surface and in the laying course, and hydrocarbons are broken down by biofilm on the aggregate surfaces. That treatment capacity is one of the reasons the SuDS Manual rates permeable paving well for water quality on car parks and driveways, and it is also why the surface silts up.

Sub-base depth, void ratio and loading

The sub-base has two jobs at once, structural and hydraulic, and the design has to satisfy both.

Hydraulically, the depth comes from the storage volume needed, which depends on the contributing area, the design rainfall event, the allowance for climate change and the permitted outflow. Those inputs come from the wider drainage strategy, not from a rule of thumb, and the discharge rate you are allowed is usually the controlling factor rather than the paving itself. Getting the permitted rate agreed early matters, which is why discharge rates and betterment on greenfield and brownfield sites is worth settling before the construction is sized.

Structurally, the depth comes from the subgrade strength and the traffic. A domestic driveway carrying cars occasionally is a light duty pavement. The same construction under a refuse vehicle access or a delivery yard is not, and the structural design follows the British Standard series for pavements constructed with pavers, with the sub-base thickness increased accordingly.

Two mistakes recur. Using a type 1 granular sub-base, which contains fines by design and will not store or transmit water. And specifying sand as a laying course, which clogs immediately. The materials have to be clean and open graded throughout, and that needs to be in the specification and checked on delivery.

Full, partial and no infiltration

Permeable paving comes in three hydraulic arrangements, and the choice is made by the ground, not by preference.

Full infiltration needs an infiltration test on the subgrade, carried out and interpreted properly rather than assumed from a soil map. Keep a clear separation between the base of the construction and the highest expected groundwater level, allow distance from building foundations, and be careful on shrinkable clay where introducing water near a structure creates its own problems.

Tanked construction is the default on contaminated sites, because infiltration can mobilise contaminants downwards towards groundwater. That question overlaps with the whole approach to sustainable drainage on a site with contaminated ground, and it needs settling before the layout is fixed.

Edges, falls and the details that fail

The surface should be close to level, with just enough fall to avoid ponding, because the water is meant to go down rather than across. Steep falls defeat the point: water runs to the low edge and leaves before it can soak in.

Edge restraints are structural. Blocks under traffic push sideways, and without a restraint the joints open and the pattern fails. The restraint has to be detailed so it holds the blocks without sealing the construction, which means no continuous concrete haunching across the full depth of the sub-base, and a geotextile at the vertical faces where fines could migrate in.

Three further details are worth drawing rather than leaving to the contractor.

1. Thresholds and door openings, where the finished level has to sit below the damp proof course with a drainage channel if necessary.
2. The junction with soft landscaping, where soil or mulch standing above the paving level will wash across it and silt the joints. Set beds lower, or use a hard edge.
3. Run-on from adjoining impermeable surfaces, which should be limited; the SuDS Manual advises keeping the ratio of contributing impermeable area to permeable area modest, and a roof discharging onto a small permeable drive will overwhelm it.

On a slope, subsurface baffles or check dams across the sub-base stop the stored water draining to the bottom corner and overtopping there.

Silt, sealing and loss of performance

Surface infiltration rate falls over time. That is expected and designed for, since a new permeable surface accepts water far faster than any rainfall event requires, so there is a large margin before performance becomes a problem.

What turns slow decline into failure is specific behaviour. Pressure washing drives fines down into the joints and seals them. Spreading sand for winter grip does the same thing deliberately. Stacking soil, aggregate or building materials on the paving during later works contaminates it. Overhanging trees shedding fine debris accelerate clogging in one area while the rest stays clear.

The other failure mode is sealing by goodwill. Owners who dislike the look of open joints fill them with kiln dried sand or, worse, grout them, and the surface becomes impermeable overnight. Tell the client in writing which jointing material is correct and that nothing else should go in.

The maintenance schedule and who carries it out

Write the schedule down and hand it over. For a domestic driveway it is short.

• Brush the surface and remove debris and leaf litter two or three times a year, more under trees.
• Have the surface vacuum swept periodically, with the frequency depending on the sediment load and the exposure, and do not substitute pressure washing.
• Inspect the joints annually for loss of jointing material and top up with the correct grit.
• Keep adjoining soil, mulch and gravel levels below the paving.
• Where infiltration has clearly declined, remove and replace the upper layer of jointing material rather than resurfacing.
• Keep any flow control chamber, perforated pipe and outfall accessible and clear.

Responsibility depends on ownership. A driveway is the householder's. Shared parking courts and private estate roads fall to a management company, and the arrangement should be in the transfer documents along with a copy of the maintenance schedule. Highway authorities rarely adopt permeable construction, and where they do it is usually with a commuted sum.

Schedule 3 to the Flood and Water Management Act 2010 contains a framework for approving and adopting sustainable drainage, with an approving body and national standards. As at October 2026 the commencement position in England needs checking, so confirm whether it applies before assuming an adoption route exists.

Where permeable paving is the wrong answer

It is not a universal solution, and offering it where it does not suit undermines the rest of the drainage case.

Avoid full infiltration on steep ground, where water migrates downslope within the construction instead of into the subgrade. Avoid infiltration where groundwater is high, where the ground is contaminated, in a groundwater source protection zone without checking the restrictions, and close to foundations on shrinkable soils. Avoid permeable surfacing where the use will deposit heavy sediment, such as an unsurfaced farm access feeding onto it, or where fuels and oils are handled without separate containment.

Heavy goods loading is a design problem rather than a prohibition: it can be done, with a thicker structural build-up and proper pavement design, but a standard domestic specification will rut.

An illustrative example. A single plot with a clay subgrade, a driveway of 60 square metres and a permitted discharge rate agreed with the lead local flood authority. Full infiltration is ruled out by the clay. The design becomes a tanked construction with a 400 millimetre open graded sub-base giving around 120 millimetres of storage across the area, a geomembrane, a perforated collector pipe and a flow control chamber discharging at the agreed rate, with the roof drained separately to avoid overloading the surface. The storage and rate calculations sit in the drainage strategy and the paving detail sits on the construction drawing, and the two have to agree.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.