What Counts as a Permeable Driveway Surface? Is Tarmac Permeable

Many people are aware that the regulations in England for paving over front gardens for driveways have changed to persuade people to put in permeable driveway surfaces. The other countries in the UK have separate arrangements but are likely to follow suit. Permeable surfaces are defined as being permeable to rainwater. Some people assume that any surface will do, but this is not so.

Increased Risk of Flooding

The regulations are largely to prevent water run-off overpowering drainage systems which exacerbates flooding, although there are other reasons. The details of the new regulations and the reasons behind them are covered in more detail in our article New Regulations for Paving Front Gardens.

But what options are there for house owners who want to convert their garden into a driveway? In urban areas, off-street parking is at a premium and a driveway will add value to a house where parking is scarce.

What Isn’t a Permeable Surface?

It might be easier to start with what isn’t considered a permeable driveway surface, although it must be pointed out that the regulations don’t actually define permeable and non-permeable surfaces. Those surfaces that we can be reasonably certain are not permeable are ConcreteTarmac and almost every sort of block or flag paving.

Having said that, larger stone or Concrete Blocks can be used for a permeable driveway surface, but only if they are laid with an absorbent substrate and there are large gaps between the flags or blocks. Those gaps need to be filled with gravel or sand to allow water to flow down into the soil below. However, it is difficult and expensive to make a driveway that is stable, so it’s best avoided in a domestic situation, particularly if you are laying the driveway yourself.

Is Tarmac Permeable?

No, it’s not. Tarmac is made up of small stones or tiny pieces of concrete that are bonded together with a tar-like surface. The small gaps between these small stones in tarmac combine to form a type of seal that would prevent water from seeping into the soil below.

Tarmac is used for driveways because it doesn’t hold water after rain, so there’s no puddling and it drains quickly. But that seal isn’t permeable.

For the same reason, tarmac can’t be installed over a permeable surface if you want to retain the permeability of that surface.

Is Block Paving Permeable?

Yes, it is. Permeable materials are those that allow water to seep through them and into the ground below. Flagstones, bricks and paving blocks all can be considered permeable materials because of their small gaps between the individual pieces of material.

Gravel – Perhaps the Easiest Option

It’s better to go with a driveway surface that is permeable from the outset. Gravel is a good choice, although there is a bit of a conflict when it comes to the laying of the foundations. The advice when making a foundation for a gravel drive is usually to have the foundation higher in the centre and sloping gently away to the sides to speed up drainage.

However, when laying a permeable driveway, we are looking to slow down drainage and allow rain to slowly soak through to the substrate below. For this reason it’s best to combine gravel with a reinforcing cellular structure. This is pegged down then the gaps are filled with gravel. A secondary advantage of this system is that it holds the gravel in place so it doesn’t escape into the surrounding areas.

These cellular systems can be used with materials other than gravel; anything can be used as long as it will allow the water to drain through it. Perhaps the best is soil, as grass and other small plants can be grown, giving back some of the wildlife environment that’s been lost by the installation of the driveway.

Permeable Products from Mainstream Suppliers

It’s no surprise that the paving industry has responded to the need for good-looking and permeable driveway surfacing products. Many manufacturers now have permeable versions of their block and flag products, which usually involve tabs on the blocks to increase the gaps between them to six to eight millimetres.

These gaps can then be filled with loose aggregate and water will drain away into the layers below. It is not enough to lay these products on a traditional foundation, though. They need a base of loose aggregate to be put down over a solid substrate or capping layer, and then the whole drive effectively becomes a soakaway. Each manufacturer should provide details of this.

Sustainable Urban Drainage Systems

Although the new regulations call for permeable paving, they do allow for non-permeable driveways to be installed as long as there is adequate provision for drainage within the property. This means that rainwater should be dispersed somehow, perhaps into the ground via a soakaway or into tanks for reuse within the property.

You will almost certainly need to go though a planning permission process for this but look into Sustainable Urban Drainage Systems (SUDS) for more information on the technology and techniques involved.

Conclusion

A permeable driveway surface is an important part of sustainable building. If you’re planning your own patio or driveway, it pays to know the difference between permeable and non-permeable surfaces so that you can make an informed decision about which one is best for your project.

There are many different types of permeable surfaces on the market, including gravel, porous paving blocks with gaps between them and even soil over tarmac or concrete pavers – depending on what looks good in your design scheme! But before you go ahead with any type of permeable paver product, it pays to consider whether your property will have adequate drainage when it rains. This is especially important if you live in an area prone to flooding or on hilly terrain.

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