Decks & patios

How to build a timber deck: frame, boards and finishing

A durable timber deck combines wood suitable for exterior use, a ventilated frame and details that shed water. Check sections, spans, centres, fixings and movement gaps against the documentation for the timber and system actually purchased.

Project summary

DifficultyIntermediate

Typical time2 to 5 days for 20 m² over a prepared base

Materials

decking boards suitable for the intended exposure
compatible, durable joists
supports, pads or pedestals suited to the base
compatible screws and connectors
end treatment or joist tape where specified

Tools

tape measure, string line and square
level or laser level
wood saw suitable for the product
drill driver and bits
installation spacers

Create my deck or patio project

Step 1

Select the timber and record its installation rules

Confirm the intended exterior use, durability, installation moisture range, board-to-joist compatibility and recommended fixings. Sort the boards and identify their installation face before planning the layout.

Movement gaps, overhangs, centres and fixing positions vary with species, section and profile. Take them from the applicable product documents; they are not universal figures.

Step 2

Measure the deck and draw the board layout

Measure the deck and draw the board layout — Decks & patios

1Boards
2Gap between boards
3Length to cover

Record length, width, thresholds, posts and access panels. Choose the board direction, place end joints and plan edge cuts before ordering.

Worked example: 5 × 4 m gives a gross area of 20 m². The order quantity must then reflect the actual layout and cuts.

Calculate the area of my timber deckTesCalculs

Step 3

Prepare a stable, free-draining base

Prepare a stable, free-draining base — Decks & patios

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

Over ground, remove unstable material and construct the specified compacted base. Over concrete, inspect condition, levels and drainage. In either case, keep timber away from locations where water can stand.

Calculate aggregate for beneath the deckTesCalculs

Step 4

Design and install the joist frame

Design and install the joist frame — Decks & patios

1Joists
2Pedestals
3Centre-to-centre grid

Joists normally run perpendicular to the boards. Provide edge support and reinforcement at end joints, steps and cut-outs. Determine board-support centres separately from the joist span between supports.

Use the most restrictive limit arising from the board, joist, loads and support system. Diagonal board layouts may change the limits stated by the manufacturer.

Calculate the pedestal and joist gridTesCalculs

Step 5

Calculate and install the boards

Calculate and install the boards — Decks & patios

1Boards
2Gap between boards
3Length to cover

Calculate rows from the effective board width and the specified gap. Start from a straight datum, pre-drill where required and keep checking alignment without forcing boards tightly together.

Calculate boards, joists and timber screwsTesCalculs

Step 6

Finish edges and plan maintenance

Keep perimeter ventilation clear, treat cut ends where the product requires it and preserve access to drainage. Inspect fixings, splinters, damp areas and contact with vegetation at least as part of regular maintenance.

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.

In the final row, 7 supports per joist is a deliberately entered calculation assumption. It does not define an allowable span; that value must come from the joist design.

Item	Assumption	Result
Area	5 × 4	20 m²
Board rows	4 m ÷ (145 + 5 mm)	27 rows
Joist lines	5 m ÷ 0.40 m, rounded + edge	14 lines, or 56 linear m
Pedestal grid	14 lines × 7 supports per line	98 before extra supports

Important checks

Frequently asked questions

What gap should I leave between timber deck boards?

The gap depends on species, board moisture at installation, width and site conditions. Use the supplier’s stated range and allow for seasonal shrinkage or swelling.

Do decking joists need protective tape?

Tape can protect the top face in some systems, but its benefit and compatibility depend on the construction detail. It must not trap water or substitute for durable, ventilated timber.

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
AFNOR — French NF DTU 51.4 P1-1 for exterior timber decking (official record)Normative reference — check scope
French public service — French guardrail rulesOfficial information — France
Légifrance — French Building Code article R134-59 (fall protection)Official information — France

Useful calculators

Calculate area →Calculate excavation →Calculate geotextile →Calculate the aggregate layer →Calculate slope and fall →Calculate deck pedestals →Calculate boards and joists →

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 →

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 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 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 →

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