Terrace planner: plan your deck and build it step by step
A timber terrace goes up from the ground: footings, bearers, joists, boards. Here is each stage with the dimensions the Kavandy planner uses – and the planner does the same for your own terrace, as a 3D build you can watch and a guide you can download.
Building a deck in 7 steps
- Measure and set out. Mark the deck’s outline with string and pegs and check that the diagonals are equal – nothing later puts that right. In Kavandy, draw the shape and get the footing positions as a drawing.
- Put in the foundation. Choose it by the ground and the height: blocks on compacted gravel, screw-in post feet, concrete deck legs, or cast piers below frost depth. Bring them all to one level, no more than 1,500 mm apart under a bearer.
- Fit the bearers. Bearers (C24 50×150, say) sit on the footings or on posts, up to 1,800 mm apart. Put a bitumen strip between concrete and timber, and join bearers only over a support.
- Fit the joists. Joists (C24 50×100, say) lie across the bearers at right angles to the boards, usually at 500 mm, or 400 mm under diagonal and herringbone boards. Tape their tops with joist tape.
- Fix the deck boards. Leave about 5 mm between boards and fix each with two stainless screws at every joist. Join boards over a joist and stagger the joints from row to row. Seal every cut end at once.
- Add the fascia, steps and railing. The fascia covers the joist ends. A deck over 25 cm high wants steps; one over 60 cm, a railing.
- Finish and maintain. Oil treated timber after 2–4 weeks, once it has dried, and then once a year.
The frame: bearers and joists
The frame has two layers. Bearers (usually C24 50×150) sit on the footings, up to 1,800 mm apart. Across them, at right angles to the boards, lie the joists (usually C24 50×100). A low deck, up to about 35 cm, has no bearers: its joists sit straight on adjustable pedestals.
Joist spacing: 400, 500 or 600 mm?
- A timber board spans about 21 times its thickness: a 28 mm board about 590 mm, a 26 mm board about 545 mm.
- 500 mm is the usual choice. 600 mm is the limit of a 28 mm board and too far for 26 mm thermo pine.
- 400 mm or less for composite boards and for diagonal and herringbone layouts, where a board spans 1.41 times the joist spacing.
Choosing the foundation
- Adjustable pedestals on gravel – for a low deck, about 15–35 cm.
- Concrete blocks on compacted gravel – quick and simple, up to about 70 cm, on firm ground.
- Concrete deck legs – set about half a metre into the ground.
- Screw-in post feet – no digging; suit slopes and soft ground.
- Cast concrete piers – the strongest, taken below frost depth.
Boards: gaps, screws and joints
- About 5 mm between boards; 5 or 6 mm with hidden clips.
- Two screws at every joist: about 38 per m² with 120 mm boards on joists at 500 mm.
- Stainless screws only in thermo timber and larch.
- Join boards over a joist, stagger the joints, and seal every cut end.
How much material?
A 4 × 5 m deck takes 11 joists, 4 bearers, 20 footings and 32 rows of boards – about €1,700 of materials in treated pine. Exact quantities and prices: the deck configurator.
Frequently asked questions
Which deck foundation suits soft ground?
Screw-in post feet suit soft and sloping ground: they are screwed in without digging. Cast concrete piers below frost depth carry the most. Blocks on compacted gravel suit firm, well-drained ground and decks up to about 70 cm high. On peat, fill or very wet ground, ask a geotechnical engineer first.
What joist spacing should a deck have?
About 21 times the board’s thickness for timber: up to about 590 mm for a 28 mm board and 545 mm for 26 mm; up to 400 mm for composite. 500 mm is the usual choice, and 400 mm for diagonal or herringbone boards, which span 1.41 times further between the same joists.
What gap should be left between deck boards?
About 5 mm with screws (4–6 mm). With hidden clips the clip sets it, at 5 or 6 mm. Composite boards expand in the heat and want a clip that lets them move.
When does a deck need a railing?
Kavandy advises a railing once the deck is more than 60 cm above the ground, with openings under 100 mm where children use it, and steps for a deck over 25 cm high.
Does the calculation replace a structural engineer?
No. The load check is a first estimate for an ordinary house deck. A high deck, a roof, a special load such as a hot tub, or poor ground needs an engineer.
Is Kavandy free?
Yes. Designing, the 3D view, exact quantities, the price, the cutting plan and the PDF and Excel files are free.