What is Permeable Paving?
Like Extensive Green Roofs permeable paving is a form of Sustainable Urban Drainage System. Basically it refers to (usually) a hard surface that water can drain through. Asphalt and concrete are impermeable, which means water runs straight off them onto the neighbouring land, causing flooding and potentially pollution problems.
As a chartered Landscape Architect I have seen a great increase in the use of permeable paving over recent years. Many of my clients are keen to use permeable paving techniques to increase their eco-credentials through BREEAM and LEED. In this Squidoo Lens I will take you through the basics of permeable paving, its applications, benefits and how you can use these techniques to create a more sustainable patio or driveway, and do your bit for the environment.
Benefits of permeable paving
- Reduced flood risk
- Reduced demand for irrigation
- Benefits for tree roots
- Reduced urban heat island effects
Types of permeable paving
Permeable paving falls into three main categories; monolithic, unit, and fluid construction paving. Over the years I've used them all, and can tell you there is no real one size fits all approach. Let me explain what these different types are. Basically monolithic paving refers to paving that is laid in one smooth continuous layer like asphalt. Unit paving is made up of individual units like concrete or clay pavers. Fluid construction is any product that is poured into place and remains fluid, or movable, like gravel.
Here I shall list the main types of permeable paving, before discussing them in more depth later.
- Permeable asphalt (monolithic paving)
- Resin bound gravel (monolithic paving)
- Specialist interlocking clay and concrete pavers (unit paving)
- Permeable clay or concrete pavers (unit paving)
- Specialist plastic units for use with gravel or grass (technically unit paving)
- Gravel (fluid)
Resources on permeable paving
This book gives a detailed insight into hoe porous pavements and permeable paving work.
Permeable or Porous Asphalt
Monolithic permeable paving
The traditional black asphalt that you see on roads, pavements, an side walks now comes in a new permeable system. Normal non-permeable asphalt is made up of a mixture of sand and gravels bound together with bituminous material. In permeable, or porous asphalt the sand and fines are left out of the mixture leaving the larger gravel particles to be stuck together with voids in between. Water can then drain through these voids to the base layer underneath.
Some of the schools I have worked on as a Landscape Architect have used porous asphalt. Whilst it was the engineers who specified the mixes, I was interested to see how the system works, and observe the construction during site visits. With porous asphalt it is important that the base gravel layer is also porous. Once the water has filtered through the asphalt it can then either drain away into the sub base and sub soil, or be collected in porous pipes and taken away for storage and reuse or controlled release into the local water courses.
One school project I worked on proposed that the water collected from the children's playground be recycled (together with water from the roof), and stored in giant tanks under the school building.
Resin Bound Gravel
Monolithic permeable paving
Resin bound gravel is simply gravel that is bound together with a resin. The resin can be either clear to show off the natural colour of the gravel, or it can be coloured to give a more uniform appearance. Like permeable asphalt, resin bound gravel must also be laid on a permeable surface.
There is an important (if pedantic) distinction to be made between resin bound gravel and resin bonded gravel. I used to get these two mixed up all the time, much to the amusement of fellow colleagues and contractors. Resin bound gravel, like permeable or porous asphalt is made up of (relatively) large pieces of gravel held together with resin, with void in between the gravel stones. Resin bonded gravel is simply gravel that has been stuck to a surface using resin, and is not necessarily permeable.
Interlocking permeable pavers
Unit permeable paving
Interlocking permeable pavers are pavers that tessellate together in such a way that permanent voids are left in specific places between the blocks. Some are quite intricate tessellating shapes, whilst others are simply rectangular or square, but have small spacers on their sides to keep the blocks apart (to allow for drainage). This little spacers are often called nipples - This always caused much hilarity on the building sites I used to work on!!
Interlocking permeable pavers are laid on a specialist base and sub-base material of open single sized gravel. This allows the water to drain away. Personally I find it a bit tricky to get this type of bed to be 'level' or fall to the correct gradients. I must admit I prefer the old fashioned non-permeable method of laying on a bed of consolidated sharp sand. However, this might just be because I've laid more driveways with the old method, and I just need to adapt to survive.
In the same way as for permeable or porous asphalt, permeable pavers can either be free draining to the subsoil (if its free draining enough), or the water can be captured in perforated pipes and taken away to a drain or stored for future use.
Interlocking permeable pavers are visually attractive, and sometimes can be matched by manufacturer to existing adjacent products on site.
Unit permeable paving
Porous paver blocks have holes within them to allow water to drain through. These holes can be filled with either gravel or specialist free draining growing medium and planted with grass.
They are often extremely strong and can take larger and heavier vehicles whilst maintaining permeability. I have worked on projects where we used these to create a fire path (a safety zone roadway that can safely take the weight of a fire truck!).
Having worked with these kinds of materials in various landscape architecture projects I have grown to generally dislike them. I have yet to see an application where grass has successfully established in these blocks. Often they have the appearance of looking poorly maintained, and rather too industrial for a domestic setting. A better choice for the domestic market would be the specialist plastic units discussed below.
A WORD OF WARNING ABOUT PERMEABLE PAVING
I have some personal concerns about porous asphalt, resin bound gravel, and permeable paver blocks
Having spent many years laying, and relaying, standard block paver drive ways, I have seen how the small spaces between blocks can become clogged up. My concern is that the small voids in between the stones in porous asphalt and resin bound gravel, and the spaces in between permeable paver blocks will become filled with the dust, soil, and general detritus from cars and foot traffic.
I have been monitoring some projects I have worked on with permeable surfaces to see if they become less permeable over time. I think there is a strong likelihood that they will, but not in the last 5 years or so I have been observing them.
If a permeable surface does become clogged up with sand, silt and fines it is relatively easy to remedy for unit paving (simply lift and relay). Monolithic paving (such as porous asphalt and resin bound gravel) would require lifting and throwing away, and almost starting again. This is something to bear in mind when planning your project!
Specialist Plastic Units
Permeable unit paving
There are a plethora of products on the market that are based on cellular plastic units that spread loads and allow you to fill them with either gravel or specialist free draining growing media to sow with grass seed.
I have specified a few of these types of products over the years on various commercial and domestic projects, and generally found them to be good. Like other types of permeable paving they need to be laid on a bed of free draining marital.
Please see below for my top recommendations.
Specialist Plastic Units
A selection of products that I have used that I would recommend.
Bodpave are an international company supplying their plastic permeable paving units across the globe. I've used them on a few Landscape Architecture jobs where we have needed vehicular access. As well as being a good product I have also found that t
Now part of the international Geosynthetics group, Golpla was one of the first plastic units I used. In fact I think it was one of the first on the market. I have used this product on a few commercial Landscape Architecture projects and found it to b
Cedagravel is a product I have used many times in the UK for specifying domestic driveways. It has the added benefit of coming with an integral geosynthetic backing sheet making it quite a strong unit.
Fluid permeable paving
Last, but not least, we have gravel. When using gravel as a permeable paving material I always specify at least one, if not two geosynthetic membrane separation layers or landscape fabrics. These are sheets of thin material laid underneath the base layer, and/or in between the gravel and base layer. They will help stop the two layers mixing, and will also help spread the load, so that when you regularly park your car in the same spot it doesn't develop ruts over time. They also have the added benefit of suppressing weed growth from underneath. NOTE: Weeds can still get a foot hold in the gravel itself, but they are a lot easier to pick off if they can't get into the sub soil underneath!
Gravel is probably the least expensive form of permeable paving, and shouldn't be over-looked. When specifying or building a gravel area I always use a good edging. This stops the 'fluid construction' paving from flowing onto adjacent flower beds or paths etc.
Geosynthetic membranes - You know, those separating fabrics or landscape fabrics between the construction layers of permeable paving
If you're choosing a geosynthetic membrane to use as a separating fabric in permeable paving its important not to use a 'woven' style. In my opinion these are too slippery, and allow for the two separated layers to move laterally. Buying online, rather than from the store can save you a considerable amount of money. Especially if you're planning a larger project, trust me!
Another good choice, this membrane offers good value for money.