Calculate the total number of boards, posts, and rails needed for a board-on-board fence (also known as a shadow board or good-neighbour fence). Boards alternate on each side of the rails, overlapping to provide privacy from both sides.
Enter your fence length, board dimensions, post spacing, and number of rails to get a complete material list.
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Reviewed June 2026
Standard board-on-board fence construction method.
1. Fence Dimensions
m
m
m
2. Board Dimensions
mm
mm
Material Quantities
Total Boards
-
Including wastage
Posts
-
Including end posts
Total Rails
-
Horizontal rails
Board Spacing
-
Centre-to-centre (mm)
Board Calculation
Fence length-
Board width-
Overlap per board-
Board spacing (centre-to-centre)-
Boards per side-
Total boards (both sides, no wastage)-
Wastage allowance-
Total boards (incl. wastage)-
Posts and Rails
Post spacing (max)-
Number of spans-
Number of posts-
Rails per span-
Total rail lengths-
Fence height-
Board length needed-
Note: Post length should be fence height plus at least 600 mm for in-ground depth (typically one-third of total post length below ground). Rail lengths are based on post spacing. Allow for gate posts, gate hardware, fixings, and concrete for post footings separately. Actual board length should match fence height; boards are typically cut to length on site.
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.
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.
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