Patio Drainage: How to Get the Fall Right and Where the Water Should Go

Good patio drainage comes down to two things: a gentle fall across the surface so water runs off rather than sitting, and a planned place for that water to go. The fall should always run away from the house, and the finished patio should sit comfortably below the damp proof course, usually by at least 150mm. When the water has nowhere natural to go, a channel drain or a soakaway finishes the job.

What fall does a patio need?

Most patios are laid with a fall of between 1:60 and 1:80. A 1:60 fall means the surface drops 1cm for every 60cm of length, so a 4m-deep patio falls about 6.5cm from the house to the far edge. Many slab manufacturers recommend a fall in this range for their products.

Smooth porcelain sheds water well and can sit at the shallower end. Riven sandstone and textured slabs hold more water in their surface and are better nearer 1:60. Too little fall and water ponds in the dips; too much and the patio feels like a slope and furniture wobbles.

Which way should a patio fall?

Away from the house, always. Water running towards a wall soaks the brickwork, can bridge the damp proof course and ends up as damp inside. On a patio that runs along the side of the house as well as the back, the fall may need to run in two directions, which takes careful setting out.

Where falling away from the house isn't possible (for example, a garden that rises steeply away from the back door), the water has to be caught before it reaches the wall. That's the job a channel drain along the house does.

How far below the damp proof course should a patio be?

The usual guidance is that paving should finish at least 150mm below the damp proof course (DPC). That gap stops rain splashing up above the DPC and stops the patio itself acting as a bridge for moisture into the wall.

Patios laid tight to the DPC, or above it, are one of the most common causes of rising damp complaints we're asked to look at after someone else's work. If a level threshold is wanted at bifold doors, the answer is a slot or channel drain at the door line, with the detail designed so water is taken away before it reaches the frame.

Patio drainage solutions

There are four ways to deal with the water once it runs off the surface.

• Run-off to a lawn or border. The simplest option. The patio falls towards the garden and the water soaks into the soil. It works well on free-draining ground and for patios that aren't too large.

• Gravel margin. A strip of gravel along the low edge collects water and lets it soak away slowly. Useful where the patio meets a fence or a path.

• Channel (linear) drain. A slotted channel set into the paving, usually along the house wall or across a door threshold, connected to a gully, a surface water drain or a soakaway.

• Soakaway. A buried crate or rubble-filled pit that holds water and lets it disperse into the ground. It has to be sized for the area it serves and placed well away from the house.

Water from a patio should go into a surface water system or the ground, not into a foul drain. If you're not sure which drain is which, a building control officer or a CCTV survey will tell you.

Why patios flood on clay soil

Clay soils, common across much of Northamptonshire, drain slowly. Water that runs off a patio onto clay may sit on the surface of the lawn for hours or days, and a soakaway dug into clay may barely empty between storms. On clay, the drainage needs more thought: a larger soakaway, a connection to an existing surface water drain, or a gravel-filled trench running to a lower part of the garden.

This is one of the reasons we test the ground and look at where existing drains run before we quote for patios in Kettering.

Fixing a patio that already floods

If an existing patio ponds or runs towards the house, the options depend on the cause.

• Small low spots: individual slabs can be lifted and rebedded to the correct level.

• Wrong fall across the whole patio: the only proper fix is lifting and relaying, which is why getting it right first time matters.

• Right fall but nowhere for water to go: adding a channel drain, a gravel margin or a soakaway at the low edge often solves it without touching the slabs.

• Patio too high against the house: a channel drain along the wall can help, but lowering the paving near the house is usually the lasting answer.

Front gardens and driveways

Different rules apply at the front of the house. Paving over 5 square metres of a front garden needs to be permeable, or drain to a lawn or border within the property, or it needs planning permission. That's usually handled as part of a driveway job, and our driveways team designs the drainage in from the start.

Getting it right at the build stage

Falls and drainage are set out before a single slab is laid: the sub-base is shaped to the fall, string lines are set to the finished levels, and drains go in first. Our step-by-step guide on how to lay a patio shows where each of these fits in the build.

If you have a patio that floods, or you're planning a new one and want the drainage designed properly from the start, we're happy to come and take a look. Get in touch to arrange a visit.