Calculating the Fall on your Driveway

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 1:80. 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.