Patio drainage: where should the water go?

Why patios need drainage

Paving changes the way rainfall moves through a garden. Instead of soaking straight into soil, water may travel across the surface towards a wall, a step or the lowest corner. Left standing, it makes the space less useful and encourages algae and slippery patches. Water gathering against the house also deserves attention rather than being accepted as normal winter behaviour.

The drainage plan needs to consider the roof downpipes, neighbouring levels and the area of paving together. A small courtyard enclosed by walls behaves differently from an open terrace beside a lawn. Sending every drop onto a narrow planting bed is not necessarily enough, especially if that bed already stays wet after rain.

A gentle fall away from the house

A patio laid with a gentle fall away from the house lets gravity move rain towards a planned collection point or suitable permeable area. The slope should feel comfortable underfoot while still doing its job. Its exact gradient depends on the paving material, surface texture and layout, so there is no single measurement that suits every project.

Finished paving levels also need to respect damp protection, air bricks and door thresholds. A low doorway does not justify simply raising the whole terrace against the wall. Where accessible thresholds or awkward existing levels complicate the design, the fall and any drainage channel need to be considered together before excavation starts.

What permeable jointing can and cannot do

Some jointing products allow water to pass between slabs, but that does not automatically make the whole patio permeable. Water must also be able to pass through the bedding and sub-base into suitable ground. A permeable joint over an impermeable construction may leave moisture trapped underneath rather than solve the drainage problem.

Choose jointing that suits the paving and the complete laying system, following the manufacturer's requirements. Narrow joints, dirt and accumulated organic matter can affect performance. Even where the construction is designed to drain through, sensible surface falls help deal with heavy rainfall and reduce reliance on the joints alone.

Channel drains and soakaways

A channel drain collects surface water along a line, often near a threshold or at the lower edge of paving. It needs an appropriate outlet; an empty channel with nowhere to discharge will simply fill up. Gratings should remain accessible so leaves, silt and debris can be removed without taking the patio apart.

A soakaway stores water temporarily while it infiltrates the surrounding soil. Whether it is suitable depends on ground conditions, groundwater, space and its position relative to buildings and boundaries. Poorly draining ground may make it unsuitable. Do not assume a patio drain can connect to an existing sewer: check the drainage arrangements, permissions and relevant requirements first.

Fixing water pooling on an existing patio

Watch what happens during rain and mark the places where water remains afterwards. Clear blocked grates and outlets before deciding the paving needs replacing. If the drains work but a puddle sits in the middle, settled slabs, an uneven base or an incorrect fall may be responsible. Adding more jointing will not correct a low spot.

Local lifting and relaying may work where the rest of the construction is sound. More widespread movement or a fall towards the house can require a larger repair. The patio repair and relaying service is the relevant starting point for existing paving. If lawns and beds are saturated too, consider garden drainage rather than treating the terrace in isolation.

Discussing the work before you commit

Ask where the water will be collected, where it will discharge and how the system can be maintained. Cost depends on access, excavation, paving that can be reused, ground conditions and the outlet needed. Dan runs Daniel John Landscapes from Sholing, Southampton, covering Eastleigh, Romsey, Winchester, Portsmouth and the New Forest within a working radius of up to 30 miles. Call 07841 510110 to discuss your garden.