Patio Drainage: Why Patios Flood and How to Fix It
Updated: 3 days ago
A patio drains properly when it falls away from the house, sits well below the damp proof course, and sends its water somewhere it can go: a lawn, a border, a soakaway or a drain. Most flooded patios fail on the first two. If yours holds water against the back wall, the cause is almost always the levels, and the fix is a channel drain or relaying to the right fall. The same rules apply whether the patio is natural stone, resin or pattern imprinted concrete, which we lay as patios as well as driveways.
Why is my patio flooding? Causes, most likely first
It slopes towards the house. A patio laid flat, or with the fall running back to the wall, sends every drop of rain to the one place it should not go.
It was laid too high. Paving built up close to the damp proof course (DPC) leaves no room for a fall and lets splashback soak the wall.
Nowhere for the water to go. The fall is right but it runs onto compacted clay or a hard boundary, so it pools at the low edge.
A blocked or missing drain. An existing gully or channel full of leaves and silt, or a downpipe emptying onto the patio with no gully at all.
Settlement. A poorly compacted base sinks unevenly, leaving dips that hold puddles even when the overall fall is fine.
What fall does a patio need?
A typical patio falls away from the house at around 1 in 60, meaning a 1mm drop for every 60mm of width. Over a 3m wide patio that is a drop of 50mm from the house wall to the outer edge. Rougher surfaces such as riven stone may need a steeper fall of around 1 in 40, while smooth, well-draining surfaces can manage with a little less.
You can check your own patio after rain, or with a long spirit level and a straight timber laid from the wall outwards. If the bubble shows the patio rising away from the house, or water sits along the wall, the fall is wrong.
How far below the DPC should a patio be?
The standard rule, taken from Building Regulations guidance, is that paving should sit at least 150mm below the DPC. The DPC is usually the visible line in the brickwork two or three courses up, sometimes marked by a different coloured mortar joint or a slate strip. The 150mm gap keeps rain splashing off the paving from soaking the wall above the DPC and causing damp inside.
On small domestic jobs, getting a full 150mm is not always possible, especially where a patio meets a back door. In that case the paving should still be at least 75mm below the DPC, and a drainage channel should run along the wall to catch the water. Paving built up close to the DPC is the kind of thing surveyors notice, and it can lead to a lower valuation or a request for remedial work.
Patio drainage solutions
Relay to the correct fall
If the slope is wrong across the whole patio, the real fix is to lift it and relay on a properly compacted base at the right fall and level. It is more work than adding a drain, but it solves the cause.
Fit a channel drain along the house
A linear channel drain (a slot drain with a grating, set into the paving) along the back wall catches water before it reaches the brickwork. It allows the paving to come closer to the DPC than plain paving would, and it is the standard answer where a patio meets a door threshold. The channel must be connected to something: for a drive that falls towards the house, the government's paving guidance says to connect the channel to the drains that take the roof water.
Lead water to a lawn, border or soakaway
Where the fall runs to a lawn or planted border, water can simply soak into the ground. On heavy clay that will not happen quickly, so a soakaway or rain garden at the low edge may be needed. Keep soakaways and rain gardens away from the house: directing water into the ground next to foundations swaps one problem for another. Our guide to driveway soakaway regulations covers how they are sized and where they can go.
Choose a permeable surface
A permeable finish lets rain soak through the patio itself instead of running off. Resin bound laid on a permeable base drains straight through the gaps between the stones, which makes it a good choice for a patio that has nowhere for surface water to run. It still needs a fall away from the house for heavy downpours. We lay resin bound patios as well as drives.
Concrete and resin patios: getting drainage right from the start
On a new patio, drainage is decided before any surface goes down. The dig-out sets the level below the DPC, the compacted sub-base is laid to the fall, and channels or gullies go in at the base stage, not as an afterthought.
Pattern imprinted concrete is not permeable, so on an imprinted patio the falls and any channel drain have to be designed in from the start. Because the slab is poured as one piece, there are no open joints for water to sit in (only narrow movement joints), and a correctly laid fall clears it quickly.
Do patios need planning permission for drainage?
Usually not in a back garden: in England, the porous-surface condition only covers land between the front wall of the house and a road, and our guide to the 5m² front garden rule explains how it works. A back garden patio still should not throw water onto a neighbour's land or against your own walls, and listed buildings and flats have their own rules.
Patio drainage questions, answered
Can I just drill holes in my patio to drain it?
On some porous paving it helps a little, but it does not fix a fall running towards the house, and it lets water into the base. Water collecting in the sub-base can soften it and lead to settlement.
Can I connect a patio drain to the sewer?
Do not connect surface water to a foul drain without checking. If you are unsure which drain is which, ask a drainage contractor or your water company before connecting anything.
Is a patio sloping towards the house always a problem?
A slight slope towards the house over a short distance can be managed with a channel drain along the wall. A patio that slopes noticeably towards the house, with no channel, will push water against the brickwork in every heavy shower and should be corrected.
Patio flooding usually ends up with the same three fixes: correct the fall, protect the DPC, give the water somewhere to go. If you are in Wolverhampton or the Black Country and planning a new patio or replacing one that floods, we can check the levels and drainage on a free site visit before you decide on a surface.
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