How the decking calculator works
Deck boards run along the length of the deck, supported by joists running across it. The calculator works out the number of board rows across the width, the total length of boards and joists, and converts these into lengths to buy, with 10% for offcuts.
Typical deck frame
- Joists: 47 × 100mm or 47 × 150mm treated C16/C24 timber at 400mm centres.
- Posts / footings: about every 1.8m in each direction, set in concrete or on adjustable pedestals.
- Clearance: at least 150mm under the frame for airflow, with a weed membrane and gravel underneath.
- Fall: a slight slope (about 1:100) along the board direction so rain runs off.
Worked example: a 4m × 3m deck
With the default settings (144mm boards, 5mm gaps, 3.6m board lengths, joists at 400mm centres):
- Board rows: 3,000mm ÷ 149mm = 20.1, so 21 rows.
- Boards: 21 rows × 4m = 84m, plus 10% for offcuts = 92.4m. In 3.6m lengths that's 26 boards.
- Joists: 4m ÷ 0.4m + 1 = 11 joists across the 3m width, plus the two outer rim joists. With 10% extra that's 45.1m, or 10 lengths of 4.8m.
- Screws: 21 rows × 11 joists × 2 = 462 screws, so one box of 500.
- Posts: on a 1.8m grid, 4 × 3 = 12 posts, with two 20kg bags of Postcrete each.
Because 3.6m boards are shorter than the 4m deck, every row has a joint. Plan for it, as described below, or choose 4.8m boards and cut them down.
Board joints and layout
Every end joint must land on a joist. The neat way is to fit a double joist (two joists side by side) wherever joints fall, so each board end has its own timber to screw into. Stagger the joints from row to row, or line them up deliberately in a "breaker board" pattern. Avoid a random scatter of short offcuts, which look untidy and are weaker. Pre-drill board ends to stop them splitting, and keep screws about 15–20mm in from the end.
Lay boards bark side up, or as the manufacturer recommends, with a consistent gap. Grooved boards are often laid groove-down for better grip and drainage. The grooves are mainly there to reduce cupping, not to stop slipping.
Common decking mistakes
- No fall and no airflow. A dead-flat deck sitting in its own puddles rots far faster. Keep the 1:100 fall and the clearance underneath.
- Joists too far apart. Boards bounce and crack at 600mm centres. Stick to 400mm for softwood and the maker's figure for composite, which is often 300–350mm.
- Fixing the frame to the house without care. A ledger board against the house must stay at least 150mm below the damp-proof course, with spacers so water can drain behind it.
- Bare soil underneath. Weeds will push through the gaps. Lay a membrane and some gravel under the frame: the "Next for this job" link opens the gravel calculator with your deck size.
- Untreated cut ends. Brush end-grain preserver on every cut you make in treated timber, as the factory treatment doesn't always reach the middle.
Footings for the posts
Support posts are usually 100 × 100mm treated timber set about 450–600mm into the ground, depending on the soil and deck height, in concrete or Postcrete. On a sound patio or concrete base you can use adjustable pedestals instead. For mixing your own concrete, the concrete calculator has a round post-hole option. To fence or screen the deck afterwards, see the fence calculator.