Building a wooden deck: the basics for a successful outdoor project
Building a wooden deck is a project within reach of a motivated DIYer, provided you prepare the ground well and respect a few layout rules. A well-built deck lasts 15 to 20 years with regular maintenance. This guide gives you the basics: soil preparation, laying the joists, fixing the decking boards and finishing, with the essential tools and measurements.
The materials and tools needed
- Decking boards in autoclave-treated pine class 4 or exotic wood (doussie, ipe), thickness 27 mm minimum
- Joists in treated pine class 4, section 45 x 70 mm
- Stainless steel A2 or A4 screws of 4 x 50 mm for the joists and 4 x 60 mm for the boards
- Fixing plates for clip-on boards (if laying on joists with clips)
- Circular saw or jigsaw, spirit level, tape measure, marking chalk line
- Drill/driver with wood bit, suitable pilot bit, mallet and eye/ear protection
Preparing the ground and the support
- Mark out the surface with a chalk line: allow a setback of 40 to 60 cm from the house
- Excavate the ground 15 to 20 cm deep and level it with a rake
- Spread a layer of compacted gravel (drainage) over 10 cm, then a 3 cm layer of sand
- Plan a slope of 1 to 2% for water drainage, always sloping away from the house
- Lay concrete slabs or adjustable plastic deck pedestals to support the joists
Laying the joists
The joists support the whole structure: they must be perfectly level, otherwise the deck will be wobbly. Lay them perpendicular to the direction of the boards, every 40 cm (maximum spacing for 27 mm boards). Check the level on each joist and shim the low points with plastic wedges. Leave a 5 cm gap between the ground and the joists for wood ventilation.
Laying the decking boards
- Cut the boards to length with a circular saw, allowing 10 mm of play on each side
- Fix the first board at the edge, checking the squareness with a builder's square
- Leave a gap of 5 to 8 mm between the boards for drainage and expansion
- Screw two screws per board/joist intersection, drilling a pilot hole to avoid splitting
- Stagger the board joints by 40 to 60 cm for more strength and aesthetics
Finishing and maintenance
- Cut the protruding boards and sand the corners with a belt sander
- Apply an oil or exterior wood stain before the first rain
- Renew the oiling every 1 to 2 years depending on exposure
- Brush off dead leaves regularly and check the screw fastenings in spring
- Install screw caps or clip-on boards for a result without visible screws
The most suitable wood species
The choice of species depends on your budget and the desired look. Autoclave-treated pine class 4 is the most common: economical, it is deeply treated against fungi and insects, and stains easily. Douglas fir, naturally durable and weather-resistant, offers a beautiful reddish tint that grays over time. Exotic woods like ipe or cumaru are very hard and almost rot-proof, but more expensive and harder to cut: plan for carbide drill bits and saw blades. Whatever the choice, favor a certified wood that guarantees sustainably managed sourcing.
The cost and duration of the project
- Autoclave-treated pine: 25 to 45 € per m² of boards, excluding structure
- Douglas fir: 40 to 60 € per m², a durability/price compromise
- Exotic wood: 60 to 90 € per m², for a very long lifespan
- Structure (joists, pedestals, screws): 15 to 25 € per m² extra
- Allow 2 to 3 days of work for 20 m², preferably with two people
The mistakes to avoid
- Laying on unstabilized ground: the deck deforms with settling
- Using class 3 wood in contact with the ground: it rots in a few years
- Forgetting the slope: water stagnates and the wood degrades quickly
- Spacing the joists more than 50 cm apart: the boards sag under weight
With careful ground preparation, level joists and well-spaced boards, your wooden deck will be stable, durable and pleasing to the eye. The biggest part of the work happens before laying the boards: take the time to check every level, and you will enjoy your deck for decades.