Guide · Patios
Where patio water actually goes
A new patio adds a large hard surface to a lot already managing its own rainfall. Deciding where that runoff goes is the part of the design that protects the house.
Destination first, route second
Patio drainage resolves into two questions, answered strictly in this order:
- Where will the water end up? It needs a real destination with real capacity: a lawn that drains, a planting bed, a swale or a drain with an outlet. Where none exists, creating one is part of the job.
- How will it get there? Only with the destination fixed can falls be set and the need for a channel or catch basin be judged.
Failed jobs usually answered the second question first. The patio sloped correctly away from the house, then delivered its water to a corner that could not absorb it.
Surface fall
The patio pitches in one deliberate direction, away from the building. The slope should be too slight to see or to tip a chair, yet unbroken across the whole field. It is formed in the base, before bedding goes down. Correcting fall in the bedding creates uneven thickness, and uneven bedding settles unevenly.
When one direction cannot work, as with an L-shaped patio or one enclosed on two sides, the surface is split into planes. Each plane falls to a defined point, and every point is drained.
The house junction
This is the detail to be strict about. Finished paving must sit low enough against the sill, threshold and any weep or damp-proofing that runoff along the wall cannot enter the building. Raising a patio flush with a door where grade does not allow it is among the costliest errors in residential landscaping.
Roof water
A downspout emptying onto a patio delivers a whole roof's runoff to one spot. The surface was designed only for the rain falling on it. The fall is overwhelmed, the base saturates there, and winter ice lifts the surrounding units.
Treat it as part of the patio scope. Extend the discharge beyond the paving or pipe it underground to the same destination as the surface water. It is a modest line item that removes a whole class of failure.
When a fall alone is not enough
- A linear channel drain along the low edge, where a large surface must shed to a single line. Typical spots are the foot of a slope or a house wall on a lot graded the wrong way.
- A catch basin at a defined low point, where the surface must fall inward because there is no other outlet.
- A French drain in the ground beyond, catching water upslope before it reaches the patio.
- A dry well or soakaway, where there is nowhere to daylight and the soil below can absorb the volume.
Each one needs an outlet. A drain feeding a pipe to nowhere simply stores the problem out of sight.
Permeable construction
Permeable paving passes water through its joints into an open-graded stone reservoir, which releases it slowly into the ground. On free-draining soil it removes surface runoff entirely. On tight clay the reservoir fills and stalls, so it suits specific sites rather than every drainage problem.
The build differs throughout: open-graded aggregate the full depth, no fines anywhere, wider joints filled with clean chip, and joints kept free of silt. Built as standard paving with different jointing, it fails.
How a good result looks
Pour a bucket of water anywhere on the finished patio. It should run off in one clear direction and leave nothing behind. An hour after heavy rain, expect no puddles at all: not in corners, not along the wall, not at the lawn edge. The rest of the build is covered on the patios page, and footprint planning in the patio design guide.
Warning signs
What standing water does while you wait
Weakens the base
Water on the surface is also in the bedding and base below. Saturated base material loses strength and shifts under load.
Freezes and lifts
Water in joints and bedding expands as it freezes, rocking and raising the units around a persistent puddle.
Washes out fines
Repeated pooling and draining carries fine particles out of the base, turning minor settlement into major settlement.
Grows weeds
Constant damp plus debris in an open joint makes a good seedbed. Weedy joints usually signal a drainage fault.
Moves toward the house
A low spot beside a wall does not stay put. The water migrates into the soil against the foundation.
Gets worse on its own
A puddle deepens the very hollow that holds it, which makes it the defect least worth waiting on.
Questions
Drainage questions, answered
How much fall does a patio need?
A gentle, consistent slope that is easy to measure and hard to see. It must shed water promptly without rocking furniture. Consistency matters as much as amount, since a correct average fall with one flat patch still leaves a puddle on that patch.
Which way should it drain?
Away from the house and toward somewhere that can absorb the volume. A lawn qualifies if it drains well. A neighbouring property does not, and many municipalities can require you to stop directing runoff onto an adjoining lot.
Are permeable pavers a good idea?
They work well where the sub-grade drains and the water has nowhere else to go. The construction differs from the base up: open-graded stone throughout, wider joints filled with clean chip, and no fines anywhere. On tight clay the benefit shrinks, because water passing through still has nowhere to drain.
Do I need a drain, or is grading enough?
Grading alone suffices when a downhill destination can take the water. Add a drain when the patio sits in a hollow, is enclosed by the house on two sides, would otherwise fall toward the building, or sheds more runoff than the ground beyond can absorb.