Decks & patios

How to build a deck or patio over soil: base, drainage and installation

A deck or patio should not be installed directly over loose topsoil. Assess the ground, choose a compatible system, build a base that spreads loads and maintain a route for water. Layer types and depths must suit the soil, climate and project; local standards and product documents take priority over generic recipes.

Project summary

DifficultyIntermediate to advanced, depending on the ground

Typical time2 to 5 days for 20 m², excluding design and compaction delays

Materials

separation geotextile where specified
designed foundation aggregate
edge restraints
compatible pedestals, supports or laying bed
surface material and system fixings

Tools

stakes, string line and tape measure
level or laser and straightedge
excavation tools
suitable plate compactor
personal protective equipment

Create my deck or patio project

Step 1

Choose the system before excavating

Decide whether the finish will sit on pedestals, a supporting frame, a granular bed or a designed slab. Each option has different build-up height, base, drainage and use limits.

Record thresholds, finished level, footprint, buried services and total available depth. Raised work, recent fill or uncertain soil should be assessed by a competent professional.

Calculate the deck or patio areaTesCalculs

Step 2

Assess and set out the ground

Assess and set out the ground — Decks & patios

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

Locate topsoil, soft or wet areas, roots, inspection covers and existing water routes. Mark the footprint and levels without directing runoff towards the building or neighbouring land.

Step 3

Excavate to the designed formation

Excavate to the designed formation — Decks & patios

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

Remove vegetation, organic matter and layers judged unsuitable. Excavation depth follows from finished level, the complete proposed build-up and the actual bearing condition of the formation.

The calculator measures a depth you have already established; it does not certify the soil or design the foundation.

Calculate the volume of soil to removeTesCalculs

Step 4

Separate the soil and build the base

Separate the soil and build the base — Decks & patios

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

Lay geotextile only where the system calls for it, using the overlap in its data sheet. Place the specified aggregate in layers suitable for the compaction equipment and restrain the edges.

Calculate geotextile rolls and overlapsTesCalculs

Step 5

Compact and form the drainage route

Compact every layer before adding the next and check levels over the whole area. Keep a positive outlet for water and remove low spots. Aggregate grading, layer depth and compaction requirements are project-specific.

Calculate aggregate volume for the baseTesCalculs

Step 6

Install supports and the finish

Install supports and the finish — Decks & patios

1Joists
2Pedestals
3Centre-to-centre grid

Place pedestals, joists or slabs to the approved layout and relevant product instructions. Include the extra supports required at edges, cuts and point loads, then keep ventilation and drainage accessible.

Calculate the pedestal gridTesCalculs

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

Can pedestals be placed directly on soil?

Not on loose topsoil. They need the stable, compatible support specified by the pedestal manufacturer and the foundation design.

Is a concrete slab always necessary?

No. Some systems use a compacted granular base, while others need a slab or designed point foundations. Ground, loads, height and finish determine the solution.

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
French public service — planning permission for a terraceOfficial information — France
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 →Calculate deck pedestals →

Related guides

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

Excavate, install geotextile, compact aggregate and manage drainage before building a pedestal deck or patio.

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

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Understand slope percentages, calculate fall, plan outlets and avoid low spots around doors and walls.

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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 →

Decking pedestals and joists: layout, spans and spacing

Position pedestals, choose joist centres, plan board-end joints and calculate a dependable support grid.

Decking pedestals and joists: layout, spans and spacing →

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 →

TesCalculs

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