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Calculating the Fall on your Driveway

By: Chris Hogan MSc - Updated: 4 Jan 2018 | comments*Discuss
 
Driveway Drive Fall Gradient Concrete

You might think all driveways are flat, after all, they look flat. But even if you are lucky enough to be putting a drive down on level ground, you have to build in a gradient, or fall, or rainwater won't run off into the drains. We covered the basics in our article on Finding The Levels For Your Driveway, this article will tackle calculating the fall itself.

Distinguishing Between Flat and Level

The first thing to do is make sure that you know the difference between flat and level. A driveway can be flat but not level and vice versa, and in many cases, depending on the lie of the land, there may be more than one gradient to cope with.

Level means close to horizontal, and flat means that the surface doesn't have any undulations, so it's perfectly possible to have a driveway that's on a severe gradient but is still flat. Every drive plot is going to be different so we will try to give guidelines that you can use to work out the fall on any plot.

Calculating the Fall

The fall required for Good Drainage is between 1:60 and 1:80. The major factor that affects this is the material, with solid surfaces like Concrete and Tarmac needing the steeper 1:60 Blocks and paving can usually get away with the shallower angle.

What this means is that over the length of the driveway the end with the drainage gullies should be one unit lower for every 60 or 80 units of length. For example, the end of a 20ft (6m) long driveway, big enough for most family cars, would need to be 3in (around 8cm) lower than the start for a tarmac drive, and 2in (5cm) lower for block paving.

Central Camber

You should also make sure that the central part of the drive is higher than the edges. Again this is to make sure the rain falls off to the sides rather than pooling in the middle. Use the same ratios to calculate the fall either side. Remember also that if any part of the drive butts up against a house wall, the top surface of the drive needs to be at least 6in (around 15cm) below the damp-proof course.

This is a very simple example, of course. If your driveway is longer, curved and has junctions or turning bays, then calculating and marking out the different falls can be quite challenging. But as long as you decide on a fixed starting point then measure out from there, it should all fall into place.

What About Permeable Paving?

Finally, if you are converting a front garden into a driveway, then the latest government rules make it very difficult to install anything other than permeable paving. This is no bad thing, to be honest. But remember that gradients and falls are not so important with most permeable paving systems, as they tend to be laid flat to encourage water retention.

This increases the amount of water that returns to the water table rather than running off into the drains. Refer to the manufacturer's instructions to make sure you get the fall right for a drive of this type.

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[Add a Comment]
Hello, I hope this is the correct area to ask a question. I have recently purchased a new build home with a rather large driveway which I am mostly pleased with. However, the mouth or entrance where the driveway meets the pavement the driveway inclines rather steeply and both my wife's car and my car both bottom out causing damage to the bottom of the vehicles. I have raised this with the builder and been told that this is not their problem. I should mention that neither my wife's car nor mine are particularly low sports cars, so I can't see it being a fault of the cars. Is there any advice that anyone has (other than move as we love the house and the area)? Thanks,
Boz - 4-Jan-18 @ 9:56 AM
I have a dark red brick semi detached house with a front garden that is approx. 400 sq ft in area - a little wider by the house than it is at the entrance. I rent this out and now want to get the front garden area paved over. Bearing in mind that I want the cheapest material but one that will be durable have you any advice on what might be a suitable material to use. I am not keen on the idea of tarmac/bitmac because I understand it is adversely affected by fuel/engine oil drips which can destroy the top surface causing this to crumble and resulting in potholes and unsightly repairs. Any advice please about materials and costs
Bob - 30-Dec-15 @ 3:59 PM
@hucky. If you want to avoid having to construct a drainage channel, or soakaway etc then you could look at the more permeable options. You can buy interlocking blocks which allow to grasses to grow through but maintain a firm surface and provide a greener appearance. Some of these block solutions are quite sophisticated with a wicking design that pulls water between them and downwards. You could also consider a semi permeable self binding gravel surface - often used in parks but great for paths and drives...it appears to hold water when it first rains, but it's surprising how quickly it absorbs it (you would need to get as flat a surface as possible though)
DrivewayExpert - 8-Jan-15 @ 9:56 AM
Hi, I have an area of lawn (approximately 5m2) that is used as a pathway for wheelie bins / pedestrian access / motorbike that I would like to convert to a more suitable finish (slabs / gravel etc) The problem I have it is both off camber, and on a gradient (both approximately 1:5) do you have any suggestions ? Many thanks
Hucky - 5-Jan-15 @ 11:15 AM
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