Decks & patios

How to prepare the ground or concrete base for a deck or patio

The base must spread loads without local settlement and allow water to drain. The correct preparation differs for topsoil, an existing load-bearing base and a concrete slab.

Project summary

DifficultyIntermediate

Typical time1 to 3 days for 20 m²

Materials

suitable geotextile
graded aggregate for the base
levelling material if required
edge restraints

Tools

spade or mini excavator
straightedge and level
plate compactor
string line and stakes

Create my deck or patio project

Step 1

Assess the site

Assess the site — Decks & patios

1Finished level
2Sloping base
3Drainage away from the building

Identify topsoil, recent fill, clay, wet areas, buried services and the natural direction of water flow. Seek professional advice for questionable ground or uncompacted fill.

Step 2

Excavate to stable ground

Remove organic matter and loose soil. There is no universal excavation depth: it follows from the planned layers, finished level and bearing capacity of the ground.

The often-quoted depth of “about 150 mm” is a recommendation for some pedestal systems, not a rule suitable for every site.

Calculate how much soil to excavateTesCalculs

Step 3

Lay the geotextile

Lay the geotextile — Decks & patios

1Boards
2Joists
3Pedestals
4Compacted aggregate
5Geotextile
6Ground

Unroll it over the prepared formation and use the overlaps stated in its data sheet. Turn it up neatly at the edges where the system requires it. Geotextile does not replace compaction or drainage.

Calculate geotextile area and rollsTesCalculs

Step 4

Build and compact the base

Spread the specified aggregate in layers suitable for the compactor. Level and compact each layer, and restrain the edges to prevent sideways movement.

Calculate aggregate volume and weightTesCalculs

Step 5

Check levels and drainage falls

Check levels and drainage falls — Decks & patios

1Finished level
2Sloping base
3Drainage away from the building

The base must carry water away from the building without low spots. Over concrete, clean the slab and check cracks, fall and available height before placing pedestals.

Falls of 1–2% appear in many installation guides. Use the value specified for the selected system and surface.

Calculate the fall for my slopeTesCalculs

Worked example: 5 × 4 m deck

Teaching example only: assumptions are not prescribed dimensions. Replace them with the design and values for the selected system.

Item	Assumption	Result
Area	5 × 4	20 m²
Excavated volume	entered depth: 15 cm	20 × 0.15 = 3 m³
Compacted layer	entered thickness: 12 cm	20 × 0.12 = 2.4 m³
Fall	design slope: 1% over 5 m	5 × 0.01 = 5 cm

Important checks

Frequently asked questions

Do I need geotextile under a deck or patio?

It is often recommended over natural ground to separate soil from aggregate and help limit vegetation. The grade and installation depend on the system; geotextile alone will not stabilise loose ground.

How deep should the aggregate base be?

Depth depends on bearing capacity, climate, drainage, loads and the selected system. Design the layer build-up before entering a depth in the calculator.

Sources and status

Check technical values against the applicable document edition and selected product literature. Manufacturer instructions are not presented as statutory requirements. FCBA, CERIB and French public-service material provides French context only; check the local rules and standards that apply in your country.

Source	Status	Use
Text or standard applying to the system	Normative reference	Check its scope, edition and amendments for the actual build.
Guidance from the relevant professional body	Professional guidance	Understand design and installation without extending the guidance beyond its scope.
Product technical sheet	Manufacturer specification	Compatibility, spans, gaps, bases and fasteners for the selected system.
Planning authority	Official information	Check permissions and local rules before work.

FCBA / French timber sector — timber deck design and installation guideProfessional guidance
Jouplast — ground preparation for pedestal systemsManufacturer specification
CERIB — installation guide for large precast concrete slabsProfessional guidance
French Construction Quality Agency — building a ground-level terraceProfessional guidance

Useful calculators

Calculate area →Calculate excavation →Calculate geotextile →Calculate the aggregate layer →Calculate slope and fall →

Related guides

How to build a deck or patio: from planning to finishing

Choose the right build-up, prepare the base and calculate materials for timber, composite, pedestal, paving or concrete projects.

How to build a deck or patio: from planning to finishing →

How to build a deck or patio on pedestals: base, grid and levelling

Select pedestals, prepare the base, calculate the support grid and install paving slabs or a joisted deck while preserving drainage.

How to build a deck or patio on pedestals: base, grid and levelling →

How to build a concrete patio: preparation, placing and curing

Plan a concrete patio, prepare the ground, estimate volume and organise formwork, drainage, joints, placing and curing.

How to build a concrete patio: preparation, placing and curing →

How to lay patio slabs: on pedestals, a laying bed or adhesive

Compare three exterior slab methods, prepare the base, set out the pattern, calculate quantities and complete joints and edges.

How to lay patio slabs: on pedestals, a laying bed or adhesive →

TesCalculs

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