Why does my patio flood when it rains?

Water on a patio is only a nuisance until it is against a wall, at which point it becomes a damp problem inside the house. Anything ponding within a metre of the building deserves prompt attention.

What you'll need

• Long spirit level or straight edge
• Tape measure
• Drain rods (optional)
• Jointing compound or mortar

Step by step

1. Check the fall.Rest a long straight edge on the paving with a level on it, and measure the drop over its length. You are looking for roughly 12mm per metre, running away from the house.
2. Check where the low point is.Pour a bucket of water on and watch. Water that runs to a gully is fine; water that stops in the middle or runs towards the wall is not.
3. Check the level against the damp-proof course.Paving should finish at least 150mm below the DPC — two brick courses. Paving raised over the years by relaying on top is one of the commonest causes of damp inside.
4. Clear the gully or channel.Lift the grate, clear the silt trap, and rod the outlet. A blocked gully makes a perfectly built patio flood.
5. Check the joints.Open or eroded joints let water into the bed, where it sits; solid joints keep it on the surface where it can run off. Repoint or refill with a proprietary jointing compound.
6. Look at where the water goes next.A patio that drains onto a lawn only works if the lawn can absorb it. Compacted or clay ground at the edge simply returns the water.
7. Add a channel drain if the fall is right but there is nowhere to go.A linear channel across the low edge, connected to a soakaway or surface water drain, deals with a patio that sheds water into a dead end.
8. Consider a permeable surface for the worst cases.Replacing sections with gravel, permeable paving or a planted strip lets water soak in rather than run. It is also the compliant approach for new hard surfaces at the front of a house.
9. Relay if the fall is genuinely wrong.There is no fix for a patio that falls towards the house other than lifting and relaying it to the correct gradient, on a proper compacted sub-base.

Tips

• 12mm of fall per metre, away from the house. Measure it before diagnosing anything else.
• Paving within 150mm of the damp-proof course is a damp problem waiting to happen.
• A bucket of water tells you more in ten seconds than an hour of looking at it.

Common mistakes

• Relaying new paving on top of the old to save lifting it — It raises the surface, usually breaching the 150mm clearance below the damp-proof course, and creates a damp problem in the house that is far more expensive than the patio.
• Sealing a patio to stop water sitting on it — Sealants change how the surface looks and how slippery it is; they do nothing about a missing fall. The water still has nowhere to go.

If it doesn't work

Questions people ask