What is Surface Water Drainage? A Guide for Roads, Car Parks and Commercial Sites
Estimated reading time 9 minutes
For any business with a car park, an access road or a paved yard, surface water drainage is a system that often goes unnoticed until it stops working. A sudden deluge of rain can see standing water accumulate in loading areas, car park bays, or customer entrances. Whilst it may not be a problem at first, it can grind the business to a halt.
Many people may choose to blame the weather, and to a point, understandably. Heavy rain can bring flooding, but the problem often lies with the drainage that wasn’t built to cope with it in the first place. To make things more problematic, it is often assumed that the council or water company are responsible for fixing the issue. When a commercial property sits on private land, this isn’t the case. Instead, the business or property owner is responsible for looking after the land and its drainage.
Surface water drainage in summary
- A network of falls, gullies, channels, pipes and outfalls that collect rainwater from hard surfaces such as roofs, roads, car parks and yards.
- Water is carried away safely to a public surface water sewer, watercourse or soakaway.
- It stops water from pooling on the surface
- Reduces damage to the surface and enhances safety for users
Surface water drainage systems at a glance
Most commercial sites rely on a combination of the following components rather than just one. Which ones are used depend on the size of the site, ground conditions and where water can be discharged.
| System | What it does | Typically used for |
| Gullies and yard drains | Collect water at low points and feed it into underground pipework | Car parks, yards, roadsides |
| Channel drainage | Intercepts water across the full width of the surface via a continuous slot or grate | Car park entrances, ramps, loading bays |
| Soakaways | Allow collected water to infiltrate gradually into the ground | Sites with permeable subsoil and no nearby sewer |
| Attenuation and SuDS | Hold water back and release it slowly to prevent downstream flooding | Larger developments and new build sites |
| Oil separators (interceptors) | Remove oil, fuel and silt before water is discharged | Fuel forecourts, HGV yards, larger car parks |
| Permeable surfacing (porous asphalt) | Lets rainwater drain through the surface, reducing runoff at source | Car parks, access roads, overflow parking |
What is surface water drainage?
Surface water drainage is the system that removes rainwater from hard surfaces like roads, paths, car parks and forecourts. When rain falls on these surfaces, it cannot naturally soak into the ground like it would in a field, so instead, it becomes runoff. A managed system that collects this water can prevent flooding.
A working surface water drainage system will transfer the water to an approved watercourse, like a stream or river, to a soakaway where the water soaks back into the ground, or a public surface water sewer.
Surface water drainage should not be confused with foul water drainage.
Foul water drainage
Foul water drainage is the wastewater produced inside a building. This could be from sinks, toilets or any appliances used there. Before it can be released, it needs to be treated. Surface water, in most cases, does not. Building Regulations state the two systems must be separate. If surface water enters the foul water system, there is a risk that the treatment works will overload, which could lead to flooding. If foul water gets inside the surface water system, contaminated water can end up in rivers and other watercourses.
How does surface water drainage work on roads and car parks?
On roads and car parks, effective drainage starts with the surface, not the pipework. A surface laid by experienced surfacing specialists includes carefully engineered falls, subtle gradients and crossfalls that help water run towards drainage points rather than sit still on the surface.
From there, gullies or channel drainage collect the water and carry it into underground pipework. On larger or more commercial sites, the water may also pass through a catchpit or oil separator before it reaches the outfall. The whole system is sized around the area being drained and the intensity of rainfall, so it can cope with a downpour and not just a shower.
If the falls are too shallow, the gullies positioned incorrectly or the surface has settled unevenly, water will collect on the surface rather than disperse. This means that on a busy commercial site, standing water can accumulate quickly in areas that have high footfall or continuous traffic.
Why poor surface water drainage will damage roads and car parks
Poor surface water drainage doesn’t just leave water where you don’t want it. It actively affects the lifespan of the surface.
Where water cannot drain away properly, it will find its way into small cracks and joints. When temperatures drop, it can freeze, expand and make small cracks considerably wider. The freeze-thaw cycle means it becomes easy for a small crack to become a pothole.
This water may also reach the sub-base. This can soften the foundation, which means the surface above can rut or break under traffic.
Without proper drainage or sufficient maintenance, you find yourself in a cycle of recurring repairs, reappearing potholes, and faster surface deterioration. In most cases, addressing drainage is more cost-effective than constantly looking for quick fixes.
What are the different surface water drainage systems?
A commercial drainage system is usually made of several elements working together. Understanding what each one does makes it easier to plan a site that stays dry.
Gullies and yard drains
These are the collection points set into the surface at its lowest levels. They collect the runoff and feed it into the pipes below. They are a simple addition, but will only work if the surface is graded to send the water in their direction.
Channel (linear) drainage
This runs as a continuous line across a surface, collecting water along its whole length rather than at one point. These are often found at car park entrances, loading bays or ramps where water needs to be stopped before it reaches a building or highway.
Soakaways
A soakaway is an underground structure that lets water infiltrate slowly into the surrounding ground. They are best for sites with permeable subsoil where a sewer connection is not available.
Attenuation and SuDS (sustainable drainage systems)
These hold water back and release it at a controlled rate. With a slower runoff before it enters a sewer or watercourse, the risk of flooding is reduced. For most major developments in England, some form of SuDS is now expected.
Oil separators
Oil separators are also known as interceptors; they remove oil, fuel and silt from runoff before it is discharged. They are used in places such as garage forecourts, large car parks, vehicle maintenance yards or anywhere else where hydrocarbons are likely.
Separators are defined and sized under BS EN 858 and CIRIA SuDS Manual (C753). Getting the right specification for your site is essential as discharging contaminated water into a watercourse is an offence.
Who is responsible for surface water drainage?
The responsibility for surface water drainage depends on where the assets sit. On commercial property, most of the responsibility lies with the property owner, rather than the water company.
| Party | Usually responsible for |
| Property owners | Gullies, downpipes, channels and all private surface water pipework up to the boundary |
| Tenant/facilities team | Day-to-day upkeep such as keeping gullies clear and reporting issues |
| Local authority | Highway gullies and public road drainage. Not private car parks or estate roads |
| Water company | Public surface water sewers and combined sewers beyond the property boundary |
For businesses, it is essential to know that water companies do not clear blockages or manage drainage on private land, and councils are not responsible for privately owned car parks and access roads. Knowing this in advance means that you can always get the right help if a problem arises. Having drainage correctly designed and installed from the outset means fewer problems and less recurring expense.
Implementing surface water drainage into new surfacing projects
The most reliable way to prevent surface water problems is to design the drainage into the surfacing from the outset. When a carpark, access road or commercial yard is built or resurfaced, the gullies, channels and outfalls should all be part of the same job.
This is where the surfacing type becomes a key consideration. Porous asphalt, for example, is a permeable surface that allows rainwater to drain straight through into a prepared sub-base. Designed and installed correctly, it reduces surface water, eases the load on the rest of the drainage system, and supports SuDS requirements on new developments.
At Tidey & Webb, drainage and surfacing are handled together as part of our commercial surfacing and car park work. Our case study from the Matthew Arnold School in Staines-upon-Thames is an example of a surface and its drainage being treated together.
When your surface should be inspected for drainage issues
Quite often, you can spot early signs of drainage problems. Puddles that sit long after the rain has stopped, water pooling at entrances or slow drainage after heavy rain could all indicate the drainage system is not working as it should.
If any of the above occur, or you notice the surface is cracking, rutting or developing potholes, it may be worth booking a site assessment from a civil engineering and surfacing contractor. They can identify where the issues are and the best course of action for your surface.
Keep your site safe with Tidey & Webb
Surface water drainage is easy to overlook until you have a problem. From flooding to potholes and from cracks to accidents, early treatment or implementation at the design stage means you can avoid issues that may cost a lot to rectify. At Tidey & Webb, we have more than 40 years of experience in civil engineering and surfacing. We cover Sussex, Surrey, Hampshire and Kent, installing and repairing car parks, roads, footpaths and commercial surfaces. Speak to our team today to learn more or request a free no-obligation quote.
