Patio drainage, falls and levels against the house
Part of our patios guides
Standing water is the most common complaint about a patio and it is almost always designed in rather than a defect that appeared later. Falls and levels are set before the first slab goes down.
How much fall a patio needs
Paving needs a consistent fall away from the building so water leaves the surface rather than sitting in the middle or running back to the wall. Around one in eighty is a common working figure for smooth paving — enough to drain, gentle enough that furniture still sits level and nobody notices the slope.
The fall has to be continuous. A patio that falls correctly for two metres then flattens out will pond at the flat spot, and that spot is where the algae and the frost damage start.
The damp proof course
Paving should finish at least 150mm below the damp proof course. Where garden levels make that impossible — a common problem on raised or terraced plots — the answer is a drainage detail against the wall, such as a channel drain or a gravel-filled aco strip, rather than simply butting paving up to the brickwork at a higher level.
Where the water goes
Falling a patio to a lawn or border is usually the simplest solution and keeps the water on your own plot. Where that is not possible, options include a linear channel drain to a soakaway, permeable jointing over a free-draining base, or a discreet gravel margin along the low edge.
- Fall to a lawn, border or gravel margin inside the boundary
- Linear channel drain connected to a properly sized soakaway
- Soakaway positioned clear of foundations and drains
- Permeable build-up where the ground below genuinely takes water
Full mortar bed, not five dabs
Spot-bedding leaves voids under the slab. Water collects in them, freezes, and the slab cracks or rocks; on porous stone the voids also show through as dark patches. Every slab should be bedded fully on mortar — it uses more material and takes longer, and it is the difference between a patio that lasts and one that is relaid in five years.
Frequently asked
Can a patio be completely level?
It can be built that way, but it should not be. Without a fall, water sits on the surface, promotes algae and works into the joints. A correctly set fall is not noticeable underfoot or under furniture.
My patio is above the damp proof course — what can be done?
Either the level comes down, which may mean re-grading the garden and adding a step, or a drainage channel is built along the house so water is intercepted before it reaches the wall. Which is appropriate depends on the levels you actually have; it is something to look at on site.