Enter the deck dimensions
Enter the length and width in metres. A 5 × 4 m deck has an area of 20 m².
Your deck
Check the highlighted fields.
Your deck layout
Decking boards Joists Supports Butt joint
Materials list from the plan
How to plan your deck
Choose boards and direction
Choose timber or composite, board width and stock length. Compare both laying directions in the drawing.
Adjust the subframe
Set the gaps, joist centres, support spacing and spare boards. Check the manufacturer’s installation instructions.
Review quantities and cost
Switch between decking and subframe views. Add optional prices and print both plans with your materials list.
Board direction, joints and subframe
Laying direction
Rows = (deck width + gap) ÷ (board width + gap), rounded up. With 140 mm boards and 5 mm gaps, a 4 m width needs 28 rows.
Butt joint
Long runs are divided into equal pieces. Each butt joint has two joists. Reused pieces allow a 3 mm saw cut. The last row may need ripping.
Subframe
The model includes edge joists and end supports. Timber uses two screws per crossing; composite uses intermediate and edge clips. Check product-specific spacing and fixings.
Single-level subframe with supports beneath each joist. Check structural sizes and spans with your supplier. Foundations, steps and railings are separate.
What affects decking installation cost?
The total includes only the prices you enter: whole boards including reserve, joists per metre, supports per item and installation per m². Foundations, fixings, delivery, steps and railings are excluded. Check what your installer’s quote covers.
Frequently asked questions
How many boards do I need for a 20 m² deck?
A 5 × 4 m deck with 140 mm boards, 5 mm gaps and 5 m stock lengths needs 28 boards along the 5 m side, or 31 with 10% extra. Direction and stock length change the result.
How do I compare decking cost per m²?
Enter your supplier’s board, joist and support prices and an optional installation rate. The entered-item total is divided by the deck area. Keep VAT treatment and quote scope consistent when comparing options.
What gap should I leave between boards?
The default is 5 mm. The model applies this gap both between rows and at butt joints. Actual side and end gaps may differ: follow the chosen product’s instructions and allow for moisture and thermal movement.
How far apart should the joists be?
Defaults are 40 cm centres for timber and 30 cm for WPC. These are starting settings, not a structural specification. The calculator distributes joists evenly without exceeding your chosen spacing and adds edge joists.
How are board joints and reusable offcuts counted?
Long runs are divided into equal pieces. Each butt joint has two joists. Reused pieces allow a 3 mm saw cut. The last row may need ripping.
How do timber and composite fixings differ?
The model includes edge joists and end supports. Timber uses two screws per crossing; composite uses intermediate and edge clips. Check product-specific spacing and fixings.
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