For many installers the wetter autumnal weather can often make installing driveways a challenge. Marshalls’ water management expert, Chris Griffiths, provides us with some top tips on how contractors and installers can overcome this by using permeable paving.
When installing a driveway, it’s important to know that there is legislation in place to reduce the impact of flooding. In simple terms, any new driveway over 5m2 must provide a drainage system for the water to run to a permeable area. If it doesn’t, then the homeowner will need to get planning permission.
As the UK has its fair share of rain, there are many problems that could lead to the increase of flooding of street drains and driveways caused by rainwater. But luckily you will find a number of innovative and sustainable drainage solutions suitable for any household or budget. One of these is permeable paving.
What is permeable paving?
Permeable block paving is a Sustainable Urban Drainage System (SuDS) for driveways. It has all the benefits of an efficient drainage system that returns rainfall to the water table, without the need for membranes, tanks or soakaways, and therefore homeowners do not require planning permission for a permeable driveway. In simple terms, the blocks are specially designed to allow rainfall to drain through the joints between the blocks, where it is stored in the sub-base until it can soak safely into the ground.
Why can it be installed in rainy conditions whereas regular block paving can’t?
Permeable driveways require a different sub-base to impermeable ones. The materials used for the installation of a permeable driveway do not become saturated by rain so there is no problem installing the driveway in wet weather.
Why is a permeable solution better for the environment?
Mainly, it reduces flood risk by managing surface water at the source and stops it flowing in high volumes and at high speeds into the surrounding sewerage network. In addition, during the process of flowing through the sub-base, the surface water is cleansed of any impurities it has picked up – so it returns to the water table pollutant-free.
What are the upsides of using permeable paving?
Well, it looks fantastic – and just like ordinary block paving, it comes in a wide range of different colours, sizes and textures so your customer can choose the best look to complement the front of their house. It’s always free of standing water so you’ll never see puddles or ponding and you don’t have to fiddle around installing channel drains and soakaways, so installation is simpler. Plus, it’s good for the environment and because of the interlocking nibs, Marshalls Driveline Priora creates a surface that’s structurally stronger than an impermeable driveway. Perhaps the only thing to be aware of is that, because the joints are wider than you find on standard block paving, you’re more likely to get occasional weeds growing between the blocks… but they’re easy get rid of by jet washing or just plucking out.
How long does permeable paving last without maintenance?
There’s a widely held misconception that without maintenance, permeable pavements become clogged with dirt and debris and stop working. To dispel this myth, in 2020 the Marshalls Design Team conducted American Standard ASTM tests on some of their older permeable pavements to see how they would perform years after installation. The research proved that all of the sites tested were still more than permeable enough to cope with the heaviest one-hour UK rainfall event in history! One site was over 17 years old at the time of testing, and that was more than twice as permeable enough to cope with that event. Using the collected data to predict the rate at which the infiltration rate might slow down, the team estimated that it would be 32 years before a pavement becomes too clogged to cope with the heaviest rainfall events the UK has seen. Of course, it’s really easy to restore permeability by a simple process of jet washing or brushing.
Top tips for installing permeable paving
Checking you have the right conditions might include digging a 400mm x 400mm x 400mm test hole. Once your test hole has been excavated, fill it to the top with water and, after 24 hours, check whether the water has disappeared. If it has, you have the right conditions to get started.
This process will determine whether the ground is porous enough to accept water at a fast enough rate. Other things to check are the services on the land, including gas, electric, sewer pipes and shallow foundations – it’s fine for services to run underneath a permeable pavement, but it’s useful to know where they run for access and repairs. Once you’ve carried out the necessary checks and know it’s a suitable site, you’re good to go.
- Excavate the site to the required depth of 310mm, making sure you keep below your DPC by 150mm.
- Compact the sub-soil.
- Install edge restraints to prevent any lateral movement of the paving.
- Install a 200mm layer of 20mm clean crushed stone with well defined angled edges, compacting with a vibrating plate at 100mm depths
- Install the next layer – 50mm of 6mm clean crushed stone with well defined angled edges. Compact with a vibrating plate.
- Lay the Priora block paving one at a time. Marshalls Priora permeable paving has a patented interlocking nib system which allows the water to drain freely through the voids between the blocks.
- Once the blocks are laid, sweep in 6mm clean crushed stone into the joints.
- Test by pouring a bucket of water onto your drive.You’ll be surprised by the way it immediately disappears into the sub-base and not into the overworked drains.
For further information on Marshalls’ permeable paving solutions visit https://www.marshalls.co.uk/gardens-and-driveways/driveways/permeable-pavin
