Garden Drainage Explained: Water Paths, Pressure & Structure

Designing water routes before you design the garden

Most garden drainage problems don’t come from “too much rain”. They come from water being forced to behave badly after we harden the ground.

This page explains how water actually moves through gardens, soil and structures — and why good drainage design is about pressure relief, load paths and long-term stability, not just pipes and channels. It underpins how we design patios, driveways, retaining walls and level changes across Haslemere, Hindhead, Grayswood and the wider Surrey/Sussex borders.

Where water really comes from

When gardens flood, the water usually arrives from several sources at once:

• Direct rainfall onto lawns, patios and planting
• Run-off from roofs, extensions and higher neighbouring ground
• Water moving laterally through soil on sloping sites
• Groundwater rising beneath compacted or sealed surfaces

Modern gardens tend to collect and concentrate water instead of letting it disperse naturally. Large paved areas, raised terraces and retaining walls all change the way water wants to travel. Drainage design starts by identifying where water enters the system, not just where it exits.

How soil controls water (more than drains do)

Soil type determines whether water disappears quietly or hangs around causing trouble:

• Sandy and greensand soils allow rapid infiltration but can still transport water sideways on slopes.
• Loams hold water long enough to saturate sub-bases if outlets are restricted.
• Clay soils store water near the surface and release it slowly, increasing pressure behind walls and beneath paving.

A common failure point is when water sits on top of compacted subsoil or an old, poorly built sub-base, creating a perched water layer. No amount of surface drainage will help unless that trapped water has somewhere to go. Understanding soil behaviour is as important as choosing the drain itself.

For a closer look at how different soils behave under load and in wet weather, see our soil types in Haslemere guide.

Surface water vs sub-surface water

Water acts very differently depending on whether it’s moving across the surface or through the ground:

• Surface water responds to falls, channels and edge detailing.
• Sub-surface water builds pressure, softens support layers and seeks the weakest escape route.

Hard landscaping can unintentionally act like a lid. A solid patio or driveway may shed rain efficiently, but it can also trap moisture beneath or push it sideways into lawns, beds or walls. Good drainage design accepts that water needs both surface routes and pressure-relief routes below ground.

Front gardens, impermeable surfaces & drainage responsibilities

In the UK, front-garden hard landscaping is subject to stricter rules for a reason. Large impermeable surfaces can overload surface-water systems and increase flood risk downstream. Guidance generally expects that new front-garden paving either:

• uses permeable surfacing, or
• drains to a permeable area or soakaway within the property boundary.

Rear gardens are less tightly regulated, but the hydraulic behaviour is the same. Moving significant volumes of water off your land — especially toward the road or neighbouring plots — is rarely good practice, even when the law doesn’t explicitly forbid it.

Drainage, neighbours & “common sense law”

There’s no official “Neighbourly Relations Act”, but English common law has long taken a dim view of one landowner making water someone else’s problem.

If landscaping works deliberately or carelessly collect surface water, concentrate it and discharge it onto neighbouring land, that can stray into nuisance or negligence — particularly if it leads to damp, flooding or long-term ground instability next door.

For that reason, good drainage design aims to:

• keep water calm and contained within your own plot
• avoid falling large hard surfaces straight toward boundaries
• provide relief routes before water reaches fences, walls or neighbouring foundations.

The goal isn’t box-ticking compliance. It’s simple foresight and keeping relationships — and structures — on good terms.

Drainage and retaining walls: pressure is the enemy

Most wall failures we see aren’t caused by weight alone. They’re caused by water pressure.

Saturated soil can exert far greater lateral force than dry ground. Without pressure relief, walls bow, crack or move over time. Proper retaining-wall drainage typically includes:

• free-draining backfill zones
• perforated drainage at footing level
• separation fabrics to prevent silting of the drainage zone
• controlled discharge away from the structure.

When patios, steps or driveways sit tight to a wall, drainage becomes structural, not optional. We cover wall design and safety in more depth in our retaining wall rules & safety guide.

Sub-bases as water-management systems

Sub-bases do more than support load — they also store and transport water. Depending on material and compaction:

• dense sub-bases can block downward movement and trap water near the surface
• open-graded layers can carry water laterally to drains or soakaways
• incorrect layering can keep moisture sitting beneath slabs for years.

This is why sub-base selection, depth and grading are part of drainage design, not separate from it. We explore the material options in more detail in our Sub-bases Explained guide.

Where water is allowed to go

Every drainage strategy needs a destination. Good solutions usually follow a simple hierarchy:

1. Allow water to soak into ground within the plot where practical.
2. Store and release it gradually via soakaways or infiltration trenches.
3. Only connect to existing drainage systems where appropriate and permitted.

Dumping water swiftly off-site may solve today’s puddle — but often creates tomorrow’s subsidence or neighbour dispute.

Design principle: water first, everything else second

We design landscapes from the bottom up:

• Let water choose its safe routes.
• Relieve pressure below ground.
• Then shape hard landscaping and planting around that logic.

That approach produces gardens that stay drier, structures that move less, and surfaces that age more gracefully over time. It’s the same structural thinking behind our guides to paving build-ups and driveway construction standards.

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Related deep-dive guides

Drainage only works properly when soil behaviour, sub-bases and retaining structures are all pulling in the same direction. These guides explore those pieces in more detail and link directly into the same structural logic.

Structural Guide

Drainage Channels Explained

How linear drainage channels actually work, where they should and shouldn’t be used, and why incorrect placement often creates more problems than it solves around patios, driveways, thresholds and retaining walls.

View drainage channels guide →

Materials Guide

Membranes & Geotextiles Under Hard Surfaces

Where separation fabrics, reinforcement grids and weed-suppressing geotextiles sit in the build-up — and how they interact with drainage and sub-bases.

View membranes & geotextiles guide →

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