Board-on-Board Fence Calculator
This calculator works out the boards, posts, and rails you need to build a board-on-board fence, also called a shadow board or good-neighbour fence, where vertical boards alternate on each side of the rails so both sides of the fence look the same and each side gets privacy. You enter your total fence length, fence height, post spacing, and number of horizontal rails, then your board width, board overlap, board thickness, and a wastage allowance. From these the calculator works out the board spacing (the centre-to-centre gap between boards on the same side, based on twice the board width minus overlap), then divides your fence length by that spacing to get boards per side, doubles it for both faces, and adds your wastage percentage. It returns the total number of boards including wastage, the number of posts including end posts, the total length of horizontal rails, and the board spacing in millimetres, alongside a full breakdown of the boards-per-side calculation and the post and rail counts. Use it to price timber before you order, to compare how overlap and post spacing change material quantities, or to check a quote from a fencing contractor. Post length needs extra for the in-ground portion, and gate posts, hardware, and concrete footings are not included. This is an indicative material estimate only; always confirm final quantities with your timber supplier or builder before ordering.
1. Fence Dimensions
2. Board Dimensions
Board Calculation
Posts and Rails
How a Board-on-Board Fence Works
A board-on-board fence (also called a shadow board fence or good-neighbour fence) uses vertical boards that alternate on opposite sides of the horizontal rails. Each board on one side overlaps the gap between the two boards on the other side. This alternating pattern provides:
- Privacy from both sides of the fence (neither side is the "ugly" back)
- Airflow through the fence (unlike a solid paling fence)
- A symmetrical appearance from both the street and the property
- Good wind resistance due to the gap allowing air to pass through
The Board-on-Board Formula
The key measurement is the board spacing, the centre-to-centre distance between two boards on the same side of the fence. Because a board on the opposite side sits in the gap and overlaps each neighbour by the overlap amount, the same-side spacing is:
Board spacing = 2 x (board width minus overlap)
For example, with 150 mm boards and a 25 mm overlap: spacing = 2 x (150 - 25) = 250 mm. Note that boards still alternate on the two faces every 125 mm, so 250 mm is the gap between successive boards on the same side.
The number of boards required per side is:
Boards per side = ceiling(fence length in mm / board spacing in mm)
Because boards appear on both sides of the rails, the total board count is:
Total boards = boards per side x 2
Then add your wastage allowance (typically 5%) to cover off-cuts, splits, and measurement errors.
Worked Example
| Input | Value |
|---|---|
| Fence length | 10 m |
| Board width | 150 mm |
| Overlap | 25 mm |
| Board spacing | 2 x (150 - 25) = 250 mm |
| Boards per side | ceil(10,000 / 250) = 40 |
| Total boards (both sides) | 40 x 2 = 80 |
| With 5% wastage | ceil(80 x 1.05) = 84 |
| Posts (at 2.4 m spacing) | ceil(10 / 2.4) + 1 = 6 |
| Spans | 6 - 1 = 5 |
| Rails (3 rails x 5 spans) | 15 |
Overlap and Privacy
The amount of overlap directly affects how much privacy the fence provides. A larger overlap means fewer visible gaps when viewed from an angle:
- 20 mm overlap: Slight gaps visible at an angle. Maximum airflow.
- 25 mm overlap: Standard. Good airflow, good privacy for most applications.
- 35 to 40 mm overlap: Near-solid privacy. Reduced airflow but very private.
A 25 mm overlap with 150 mm boards uses more timber than a solid paling fence of the same width, but significantly less than a double-boarded fence that provides the same coverage on both sides.
Post Spacing and Rails
Posts for board-on-board fences are typically spaced at 1.8 m to 2.4 m centres. Wider spacing requires heavier rails to avoid sagging. The number of rails depends on fence height:
- Up to 1.5 m: 2 rails (top and bottom)
- 1.5 m to 2.1 m: 3 rails (top, middle, bottom)
- Over 2.1 m: 4 rails
In New Zealand, common timber sizes for rails are 100 x 50 mm or 75 x 50 mm treated H3 or H4 pine. Posts are typically 100 x 100 mm or 125 x 125 mm treated H4 or H5 timber for in-ground use.
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Method: Board-on-board fence quantity formula based on standard construction practice: total boards = 2 x ceil(fence length / (2 x (board width - overlap))), with posts = ceil(fence length / post spacing) + 1 and rails = posts - 1 x number of rails per span. Wastage applied as a percentage uplift to the board count.
This calculator gives indicative material quantities only. Actual requirements depend on your specific site, fence design, timber sizes, and construction method. Always verify quantities with your supplier or builder before ordering materials. Prices, timber sizes, and spans vary by supplier and local conditions.