What this answers
How many boards to buy for a deck or a fence, and how many joists or posts and screws go with them. You give the size, the width of a board, the gap between boards and the spacing you plan. The page counts; it does not tell you how far apart things have to be.
Boards
Boards of a width, laid with a gap between them, fill a width when n boards and n − 1 gaps are enough:
Rows = (width + gap) / (board width + gap), rounded up
On a deck the boards run along the length. The metres to cover are the rows times the length, plus the waste you choose, divided by the board length you buy and rounded up. The page assumes that what is cut off one board starts the next row. For a fence the same formula gives the number of boards along the run.
Joists, posts and screws
- Joists under a deck stand at your spacing along the length, with one at each end: the length divided by the spacing, rounded up, plus one.
- Posts along a straight fence are the sections plus one.
- Screws are the places a board meets a joist or a rail, times the screws you use at each.
Worked example
A deck of 4 × 3 m, boards 120 mm wide, 5 mm gap, 3.6 m boards, joists at 600 mm, 10% waste:
- (3000 + 5) / 125 = 24.04, so 25 rows
- 25 · 4 m = 100 m, plus 10% is 110 m, and 110 / 3.6 = 30.6, so 31 boards
- 4000 / 600 = 6.7, so 7 spaces and 8 joists
- 25 rows · 8 joists · 2 screws = 400 screws
What is not included
- How far apart joists and posts have to be. It depends on the load, the height and the rules where you are. The numbers on the page are examples you replace.
- Corners, gates and bearers. The page counts a straight run and a simple deck.
- Extra screws at a joint between two boards on one joist.
- Cost beyond the prices you type.
Common questions
Why is the cost empty?
The prices are yours. Type the price of a board, and of a post for a fence, and the cost appears.