What is a porous asphalt?

Yes, its usually a bit more expensive at the start than standard asphalt, mainly because of the build-up beneath and the extra design work that comes with it. However, it can save money elsewhere on the job by cutting down on gullies, pipes and separate drainage systems, and it can help with planning where SuDS and surface-water controls are a big issue. Whether it works out more expensive overall really depends on the project. For some sites, the drainage and compliance benefits more than offset the extra spend on surfacing for others, simple traditional asphalt may still be the better value.

Manages surface water at source.
Rainfall soaks through the surface into a stone reservoir below, which slows down runoff, reduces the risk of localised flooding, and helps recharge the ground or downstream drainage. For car parks, yards and low-speed roads, that often means you can reduce or simplify conventional gulleys, pipes and attenuation tanks.

Improves water quality and supports SuDS.
As water passes through the asphalt and stone, sediments and some pollutants are filtered out, improving the quality of runoff entering sewers or the ground. This makes porous asphalt a good fit for sustainable drainage SuDS schemes and supports planning and regulatory compliance for new developments.

Reduces standing water and spray.
Water soaks through the surface instead of sitting on top, reducing standing water, splashing, and surface film. That means less splash, better grip and clearer sightlines for drivers on low-speed roads in heavy rain.

Reduces winter ice formation.
The way porous asphalt drains and stores water in the sub-@base can reduce the amount of water left on the surface to freeze. This lowers black-ice risk.

Installed with familiar asphalt techniques.
The mix is produced at an asphalt plant and laid with standard paving equipment by trained surfacing crews, so the installation process is familiar to most road and surfacing contractors. That means construction sequencing and programme look very similar to a normal asphalt job.

Prone to clogging if not maintained.
Because the surface is full of tiny voids, it will slowly collect silt, dust, leaves and bits of winter grit if its left alone. Over time, that build-up blocks the drainage, and the surface starts to behave more like normal tarmac, so it does need occasional vacuum-sweeping or pressure washing to keep it working properly.

Needs the right ground conditions
Porous asphalt performs best on sites where the ground has been designed to take and manage the water that passes through it. If the subsoil is clay-heavy, slow-draining or contaminated, the design may need extras such as under-drains, liners or overflow points, which adds a bit more complexity and cost to the job.

Not suitable for all traffic
Porous asphalt is intended for car parks, driveways, access roads, and other low-speed, light-traffic areas. On very busy roads or heavily loaded industrial sites, the pores can close up, and the surface can wear more quickly if it hasnt been specifically designed for that duty.

Higher maintenance needs
Porous Asphalt requires regular maintenance to remove leaves and debris and help keep the surface free-draining. You may need to use vacuum sweeping instead of brushing, and avoid sand for winter treatment, or the system will slowly revert to an impermeable asphalt surface.

Porous asphalt is designed and supplied to recognised UK and European standards, so you know exactly what youre getting. In practice, mixes are produced in line with BS EN 13108-7, which sets out how porous asphalt should be made so its both strong and free-draining.

On-site, that surface course usually sits on top of an open-graded stone sub-base, which is sized to carry the traffic and hold back stormwater as part of a SuDS design. The exact specification we recommend for your job – mix type, layer thickness and stone depth – will depend on what the area is used for driveway, car park or access road and any local authority or planning requirements tied to drainage and runoff rates.