As towns and cities grow and we pave, concrete or tarmac (asphalt) over more and more ground so there is less and less ground available for rain water to soak into.
This water has to go somewhere and can overwhelm the drainage system causing flooding and pollution. The solution is to create permeable driveways, patios and hard standings which the water can pass through and soak into the ground rather than run directly into our drainage system.
The options for permeable driveways, patios or parking on a front garden have increased significantly now that the problem has been recognised – and the planning rules have change to encourage it.
The Options for Removing Water from Driveways and Patios
There are essentially only three options for removing the rainwater that falls on to a driveway, in order of preference:
- Allow it to pass through the surface of the driveway and into the soil
- Carry it off to a soakaway, rain garden or harvesting system from where it can absorbed by the soil or removed without entering the drainage system
- Allow it to feed into the drainage system
The preference is based on creating a solution that does not overwhelm the drainage system and allows as much water as possible to be absorbed by the ground nearest to where it has fallen.
The ground conditions will determine what is possible, for example, clay soils do not allow water to pass through easily which makes them less suitable for solutions that allow absorption directly. In this instance you might have to revert to the drainage system.
It is possible to use a range of solutions. Continuing the example above, where the ground can no longer absorb the rainwater during/after a heavy downpour, then the excess can be removed through the drainage system.
Now that we know the broad types of drainage options available we can examine the advantages and disadvantages of the specific types of driveway. If you are looking for ideas about the different types of driveway that you might be able to create, have a look at our suggestions for driveways project here.
Types of Permeable Driveway and their Advantages or Disadvantages
There are a number of types of material or approaches to creating a permeable driveway or parking space in your front garden. We will briefly explain how they are constructed and their relative pros and cons:
Reinforced Grass Driveway and Parking
These are either plastic or concrete grid systems that reinforce the ground stopping your wheels creating ruts but allowing the grass to grow between the mesh. Plastic systems can also be used to stabilise a gravel driveway (see below) and stop the gravel spreading away from the driveway.
They are easy to install and look very natural once the grass is established. You will need to use the recommended grass species that will grow in the mesh. While this is the most natural looking and eco-friendly solution, it does require maintenance such as mowing.
If gravel is used this will need to be swept back, especially on sloping driveways. Grass driveway are not suitable if you car is parked there all the time as the lack of light will kill the grass.
Gravel or Shingle Driveways
Gravel driveways are very popular largely because they are really easy to create, can be very attractive and are very effective as permeable driveways. Find out here how you can lay a gravel driveway yourself. In summary, the gravel is laid over a sub-base and weed mesh.
Along with it being cheap and easy to make and attractive to look at, a gravel driveway is very easy to maintain, especially if you have used a weed mesh to stop any unwanted plant growing through.
Gravel is not good for steeper driveways and can be difficult if you need to wheel things across it, such as children’s buggies or wheelchairs. Maintenance is easy, but you will have to sweep up the gravel which scatters from time to time, and keep on top of any weeds that appear.
Wheel Tracks for Driveways
This is where you only pave the area of your driveway where the wheels of the car will go. This way the rest of the driveway remains porous and permeable. The paved tracks will need to be laid as normal (see here how to lay paving slabs), and the water should then run off on to planted permeable ground. Obviously the planning between the tracks needs to be low growing!
This is another cheap and easy solution although it will require some maintenance to keep the planting between the tracks tidy. Planning permission will not be required so long as the total paved area is kept below 52m. Find out more about the planning restriction for driveways below.
Permeable or Porous Hard surfaces
There is a wide range of permeable surfaces that allow the water to pass through while still providing a sound hard surface for driving and parking on. Porous asphalt, concrete and block paving either allow the water through the material or around the edges of the blocks. They have to be built on a porous sub-base.
This is a very durable surface with will require a minimal amount of maintenance. You will still have the look and feel of a normal paved area. The choice of materials and styles is huge.
The issue is that the construction in more complex and therefore much more expensive. You will need a contractor that knows what they are doing – which you can find here.
Building a Soakaway or Rain Garden
In some instances it is not possible or practical to have a permeable driveway so a traditional impermeable solution will need to be used. Or the porosity of the drive surface is not enough to cope with heavy rains and will lead to excess runoff on occasions.
This can happen if the driveway is on a slope; gravel might be impractical and a porous hard surface cannot absorb all the rainwater before it runs off, or it is simply out of your budget. If you happen to be on a clay soil, which is slow to absorb water, even if you driveways in highly permeable the water will not be able to pass through the soil quick enough so with back up and puddle on top of the drive in a heavy downpour.
The solution is to create a “trap” or “sink” of some sort where any excess water can collect and then drain away into the ground (or be used elsewhere) over a longer period of time. There are essentially three solutions that you can use:
- A Soakaway
- A Rain Garden
- Rainwater Harvesting
All these solutions can deal with either the excess water from a permeable driveway, or they can be used with a traditional drive construction. All the runoff water should be channelled to them to be absorbed or removed, meaning that it does not end up in the drainage system.
Here are the relative advantages and disadvantages of each solution:
1. Using a Soakaway to absorb excess water from your drive
A soakaway is essentially a large hole in the ground which is filled with large and course aggregate where rain water collects and then slowly seep into the soil.
We have a whole project on the construction of a soakaway, so we will not get into it here.
The advantage of a soakaway is that it is relatively easy to construct and can, in the right conditions, absorb significant amounts of excess water. While the construction is relatively easy, this is not to say in might not be a significant project which will require deep excavations and considerable space. If not constructed properly they are not as effective as they could be and in some instances will not be suited to particular soil types.
2. Absorbing Excess Runoff in a Rain Garden
A way to think about a rain garden is that it is an open soakaway with plants planted in the top to help absorb the excess water. Depend on the requirement and amounts of water to deal with these can be a depression where excess water is directed which has been planted sympathetically or laid with cobbles to slow the runoff and allow it to be absorbed.
For more information about rain gardens and how to build one, have a look at the Royal Horticultural Society website.
A rain garden can be a low ecological impact and highly attractive feature for your garden. If you have the space and with the right soil conditions they are very effective and absorb significant amounts of water. However they can be a fair amount of work involved to build one and it is important to have an understanding about the soil and plants that should be used.
3. Rainwater Harvesting
In its simplest form this could be a water butt collecting water for reuse later, say for watering the garden or washing the car. A more complex solution would involve underground tanks where the water would be channelled for storage. This can be used for all sorts of non-potable water needs in the house such as flush toilets or feeding washing machines. To drink it the water would need to be treated.
For detailed information about how they work and are constructed, please read our project all about rainwater harvesting, however here we will summarise the benefits and constraints for use as a solution to excess driveway runoff.
The big benefit, other than the obvious environment benefit of using the water that falls on to your property, is that it can save on mains water and it cost, particularly if you are on a metered supply.
The down side is that this is a specialist job to install which will make it expensive, prohibitively so if retrofitting on most occasions. They also require an overflow (into the mains drainage system) for when the system is full and cannot cope with more rain.
How are Permeable or Porous Driveways Built
The driveway will need to be dug out and then built back up to the desired surface level. The depth you need to dig out the driveway does depend on the local conditions; if the soil is loose and will not provide a firm base you will need to dig out more to provide a deeper sub-base to provide this stability. As a rule of thumb, if you can drive a 150mm (6 inches) peg into the ground then it is not strong enough to support your driveway and will need digging out.
After digging out, a weed mesh or geotextile should be laid to prevent the weeds growing up and through from the soil below, but also to bind the sub-base above which stops it sinking into the soil. It is porous so water will be able to pass through it easily.
Over the weed mesh 150mm (6 inches) or sub-base is laid. For a traditional impermeable driveway the sub-base would have been made from hardcore or scalpings that contained silt and sand.
This is often referred to as MOT Type 1 sub-base and can be purchased from almost any builders merchant. The problem is that when compacted the sand and silt will stop water passing through it easily even if the drive surface is permeable.
The solution is to use a sub-base that has been “open graded” so that there are none of the finer particles that stop the water flowing through. With only the larger aggregates it must be compacted in the same way but the holes between the particles will remain allow the water through. Ask for 4/20 or Type 3 sub-base.
With the sub-base in place and compacted the driveway surface can be laid. This will be in the region of 200-250mm (8-10 inches). This makes the total depth you have to dig out to be approximately 350-400mm (14-16 inches).
The type of driveway you are laying will dictate how it is done. We explain more in the projects about the different options:
Constructing a permeable driveway is a project that requires a greater understanding than an impermeable one, so it is wise to use a qualified contractor. See here for reliable contractors.
If you have a clay type soil which does not readily absorb water then extra work might be required to ensure that the water does not soak through the drive surface and simply sit in a “puddle” in the sub-base. It might be necessary to have a pipe in the sub-base to take excess water to your water drainage system for the rest of the house.
A similar overflow pipe will be required for a rain garden if it cannot drain and standing water is collecting in it for over a day (12 hours). Soakaways and rain gardens should not be built near the foundations of your home. Typically they need to be 3 or ideally 5 metres away, but this should be confirmed with your local building control who will know more about the local soil conditions.
Maintenance Consideration for a Permeable Driveway
The precise maintenance required for a driveway will depend of the type or driveway, but there are a few important rules of thumb that you can follow to ensure that it continues to work well.
The key thing to remember is that they work by allowing water to pass through them and soak away into the soil below. For this to happen there needs to be gaps and pores for the water to travel through, so these should not be allowed to become blocked up.
Therefore your maintenance of a permeable driveway should include:
- Clear any leaves or mud that collects on the driveway – this can break down and pass through the surface and block the pores
- Brush away any dirt or dust that collects on the surface or use a pressure washer
- Remove any weeds, rather than using a weed killer as the weed can then rot down and block the pores
- Don’t put any materials, like sand or cement, or oils on the surface of the driveway which could reduce the porosity
- Ensure that soil and dirt for the garden are not washed onto the driveway
If you understand how they work it is easy to understand what is needs to be done to ensure that you permeable driveway remains permeable and porous.
Planning Permission for a Permeable Driveway and other Factors to Consider
Until 2008 and 2013 in Wales (similar rules apply in Scotland too), creating a driveway in your front garden was considered a permitted development. However to reduce the amount of water entering the drainage systems, which were becoming overwhelmed leading to flooding, these rights have been taken away – except if a permeable driveway is being created.
If your driveway or paved area is impermeable and greater than 5 meters square, you will need to get planning permission for a new or replacement. If the drive way is permeable or the water is directed to a boarder or lawn to drain way it will be considered a permitted development.
For more information about a driveway in the front garden, see the Planning Portal.
Other Areas Around Your House
There are no restrictions for patios and paving, like a driveway, that is not part of your front garden, however you might need permission if there are significant works. For more information see the Planning Portal.
To get to your front garden might involve driving across the foot path or pavement from the street. You will need to apply to the council for permission to do this. This is required so that they can check that there are no utilities (water pipes, TV cables, etc) which could be damaged or disrupted by your plans.
There are a few other things that will affect what you can do and the suitability of a permeable driveway. Here are some of the most common, but for a very detailed look as the factors see the guidance from the Precast Concrete Paving and Kerb Association.
- Slope – the drive should slope away from the house, but if it doesn’t you will need a drainage channel to collect any excess before it gets to the house. Slopes in excess of 1:20 are unlikely to be suitable for permeable driveways, so you will have to use a soakaway or other solution. You should not direct water on to a neighbour’s property.
- Services – As we have mentioned you need to check with the council about dropping the curb, predominantly so that there is no issue with any utilities. The same will apply to your own drive. Check there are no pipes or cables running underneath where you want to put the drive. If there are, you will have to arrange to have them moved by the relevant utility company
- Contaminated sites – If the site has been contaminated by previous use, then there might be a restriction on allowing water to soak into the ground. If this is the case you should seek specialist advice.
- Access – We have already discussed the drop curb but you should also consider whether you are going to inconvenience neighbours or have suitable access yourself
The reason for building a permeable driveway, rather than a traditional impermeable one, is to reduce the amount of excess runoff that is overwhelming the drainage system. This is encouraged by the planning regulations. Hopefully you will see that there are good reasons to going this route and now understand all the options for the different types of permeable driveways and porous hard surfaces.