Engineering • Drainage Geometry

How Much Fall Does a Patio Need?

Patio falls are not guesswork. They are precise engineering slopes designed to move water off the surface before it can soak into joints, bedding layers, and sub-bases. This guide explains the exact fall ratios patios need, how they’re measured, and why “just eyeballing it” causes long-term drainage failure.

Quick Answer

  • Minimum fall: 1:80 (12.5 mm per metre).
  • Preferred fall: 1:60 (16.7 mm per metre).
  • Steeper than 1:40 feels uncomfortable to walk on.
  • Falls must run away from buildings.
  • Falls must be built into the sub-base, not just the slabs.
Jump to your problem:

Correct Patio Fall Ratios

Patio falls are expressed as ratios that describe how much height change occurs over a horizontal distance.

  • 1:80 = 12.5 mm drop per metre (absolute minimum).
  • 1:60 = 16.7 mm drop per metre (industry best practice).
  • 1:40 = 25 mm drop per metre (maximum before it feels steep).

For most domestic patios, 1:60 provides the best balance between drainage performance and walking comfort.

*(Context: What Are Surface Falls? • Patio Drainage Basics)*

Why the Amount of Fall Matters

Too little fall and water pools. Too much fall and the patio becomes uncomfortable and unsafe.

  • Flat areas trap water and dirt.
  • Steep slopes increase slip risk.
  • Excessive fall creates uneven bedding thickness.

Incorrect falls also distort the bedding layer, causing weak zones that settle over time.

*(Related: Why Patios Hold Water • Why Patios Become Slippery)*

Correct Fall Direction

The direction of fall is just as important as the amount.

  • Falls must always run away from buildings.
  • Water must flow toward open edges or drains.
  • Falls should follow the shortest drainage path.

A technically correct slope in the wrong direction can cause more damage than a completely flat patio.

*(Related: Patios Against House Walls • Patio Drainage Design)*

How Patio Falls Are Measured

Falls must be measured during construction — not guessed once the slabs are down.

  • Laser levels for sub-base profiling.
  • Straightedges and spirit levels.
  • String lines for long runs.

Falls should be built into:

  • The sub-base surface.
  • The bedding mortar layer.
  • The final slab surface.

If the fall only exists in the slabs, the structure underneath is still flat — which guarantees future drainage failure.

*(Deep dive: What Is a Drainage Layer? • Patio Build-Up Explained)*

Common Patio Fall Mistakes

These errors appear repeatedly in failed patio inspections:

  • No fall at all.
  • Falls built only into slabs.
  • Conflicting fall directions.
  • Flat spots near thresholds.
  • Falls that change mid-run.

Each of these creates water traps that slowly destroy the patio structure.

*(Diagnosis: Why Patios Fail • What Are Surface Falls?)*

When Drains Are Still Needed

Surface falls handle normal rainfall, but they cannot cope with every site condition.

  • Large patio areas.
  • Clay soils.
  • Low-permeability paving.
  • Patios against buildings.

In these cases, falls must work alongside linear drains or soakaway systems.

*(Related: Do Patios Need Drainage? • Linear Drain Systems)*

What This Means For You

  • If water pools → increase the fall.
  • If slabs feel steep → reduce the fall.
  • If rebuilding → build falls into the sub-base.
  • If water runs toward the house → reverse the fall direction.
  • If installing new paving → design falls before excavation starts.

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