Chris Hodson of Interpave answers the questions on why a responsible approach to rainwater drainage – in both new and existing homes – is key
WHAT’S WRONG WITH OUR DRAINAGE?
As our towns and cities continue to grow, more rainwater is captured on streets, car parks, drives and roofs – which then flow into sewers, rivers and streams. Existing drainage systems simply cannot cope, causing downstream flooding. In addition, roads, car parks and driveways collect pollution – from car exhausts, tyre-wear, oil leaks and other sources – and when it rains this gets washed into sewers, then harms our rivers and streams.
WHY DO WE NEED TO ACT NOW?
Climate change means more frequent and heavy rainstorms, and flooding. But ‘Sealing up’ our towns and cities also stops rainfall soaking into the ground. The water table levels drop causing more water shortages, streams and ponds to dry up, and trees to be deprived of water. It could also affect the foundations of your home, particularly on clay soils, as they dry out and shrink.
WHAT DIFFERENCE COULD MY GARDEN & DRIVE MAKE?
These problems are made worse when gardens are sealed-up and water runs straight to roads and sewers. Of course, one small area of paving in itself is not going to cause major flooding – but the cumulative impact can. For example, the area of paving in front gardens for car parking in London is already equivalent to around 5,200 football pitches. Simply allowing rainwater to run straight off roofs and paving into drains is no longer an option.
SO, WHAT ARE SUDS?
For years, it has been recognised that ‘Sustainable Drainage Systems’ (or SuDS) are essential in curbing flooding and pollution from rainwater runoff. SuDS manage rainwater by slowing it down and cleaning it up – copying, as closely as possible, the natural drainage from a site before any building work or paving takes place.
SuDS manage rainwater near the surface and as close to its source as possible, slowing the flow and providing additional benefits including clean water for wildlife or recycling. Typical SuDS include living (green) roofs and permeable paving, as well as planted features like rain gardens.
ARE SUDS NOW A LEGAL REQUIREMENT?
For new developments, SuDS will generally be required as part of the planning process, although this will depend on which part of the country. In some cases, a single house may be exempt but, increasingly, local policies affecting planning applications will demand SuDS. For example, the ‘Draft London Plan’ says that: “Development proposals for impermeable paving should be refused where appropriate, including on small surfaces such as front gardens and driveways.” In any case, using SuDS is a low-cost solution, and the responsible thing to do.
WHAT ABOUT EXISTING HOMES?
Installing drives or other paving anywhere in your garden used to be considered ‘permitted development’ – in other words an automatic planning permission. But changes a few years ago took away these rights from new or replacement paving, unless it’s ‘permeable paving’ or drains onto a permeable garden area or soak away within the property.
Otherwise, you will need to apply for planning permission, involving an application, drawings and a fee. Anyway, with policies in favour of SuDS, the application should be rejected. If you go ahead anyway, this could result in local authority enforcement action or legal problems when the house is sold.
OK, HOW DO I MAKE SURE PAVING IS PERMITTED?
Interpave’s ‘Paving for Rain’ guide offers two straightforward methods:
Concrete Block Permeable Paving – which allows rainwater to filter through grit-filled joints between the blocks into a sub-base below with enough space to store the water for a short time.
Rain Gardens – where conventional paving simply drains to a landscaped depression, maybe with plants, in the garden to temporarily hold water.
In both systems, water is held back on your property during heavy storms to avoid flooding, and filtered to remove or trap pollution. What happens next depends on the ability of the ground to absorb water (known as ‘permeability’) and whether you want to recycle the rainwater.
HOW DOES PERMEABLE PAVING WORK?
There is a wide choice of products available from Interpave manufacturers, specifically for permeable paving – with more shapes, styles, finishes and colours than ever. They are said to have the same performance as conventional concrete blocks and slabs: slip resistant, durable, strong and sustainable.
The difference with permeable paving is the presence of larger joints, which are filled with stone grit (but never sand or mortar). This ensures that water will continue to pass through the joints over many years. Concrete block permeable paving can be laid level and still avoids puddles without the need for drainage gulleys and pipes. It also provides a safe surface for all homeowners, unlike gravel and other loose material.
Chris Hodson is an architect and consultant to the trade association Interpave