Three fences that all look solid and are not
Shadowbox, board-on-board and horizontal slat fences share one property: none of them is a single plane of boards with gaps, and none of them can be counted the way a picket fence is counted. Each has its own arithmetic and its own failure mode, and the failure mode in every case is that the boards move after they are fixed.
A shadowbox fence puts boards on both faces of the rails, alternating, so that each back board sits behind the gap between two front boards and laps over each of them. Seen square on, the laps close the gaps and it reads as solid. Seen from an angle, the two planes separate visually and the fence opens up.
A board-on-board fence puts all the boards on one face, each one overlapping the edge of the last, like weatherboarding turned vertical. There is no second plane, so the two surfaces that matter are separated only by a board thickness.
A horizontal slat fence runs the boards along the bay rather than up it, so the count is set by the fence height rather than the bay width, and the arithmetic is the picket calculation rotated ninety degrees.
The shadowbox count, derived
Everything follows from one relationship. Let the board width be the face dimension and the overlap be how far a back board covers a front board on each side. For the back board to make that overlap on both sides, the gap between two front boards must be the board width minus twice the overlap. A 5.5 inch board with a 1 inch overlap per side leaves a 3.5 inch gap between front boards.
The front boards therefore repeat at a pitch of one board plus that gap, which is nine inches in this example. Into a 92.5 inch bay with a front board tight against each post, that gives ten front boards and nine back boards: nineteen boards where a butted fence would use seventeen. The overlap has cost about twelve percent more material, which is the honest price of the appearance.
The leftover is handled the same way it is in a picket fence: it goes into the gaps between front boards, evenly. That widens the gap, and because the gap and the overlap are two sides of the same equation, widening the gap narrows the overlap. This is the number to watch. You ask for a one inch overlap, the bay does not divide evenly, and what you actually build has two thirds. That is still fine. Ask for three eighths and the spreading can take it to nothing.
The angle at which a shadowbox stops being private
Stand square to the fence and the overlaps close every gap. Walk sideways and the front boards appear to shift relative to the back boards, because they are in different planes. The apparent shift is the plane separation times the tangent of your viewing angle, and you can see straight through as soon as that shift exceeds the overlap.
So the angle at which the fence opens is the arctangent of the overlap divided by the plane separation. With a one inch overlap and boards fixed either side of a 2x4 laid flat, the separation is 1.5 inches and the fence opens at about 34 degrees off square. Ten feet back from the fence, that is about seven feet to one side. This is not a defect, it is what shadowbox is; it is also why a shadowbox fence between two patios that face each other obliquely disappoints the people who paid for it.
Board-on-board is much better on this measure and it is worth knowing why. The two overlapping boards are separated by a single board thickness rather than a rail depth, so the same one inch overlap gives an opening angle of about 53 degrees. The trade is that board-on-board looks the same from one side only, and the back of the fence shows the rails.
| Arrangement | Plane separation | 1 in overlap opens at | Both faces alike? |
|---|---|---|---|
| Shadowbox, boards either side of a flat 2x4 | 1.5 in | about 34 degrees | Yes |
| Shadowbox, boards either side of a 2x4 on edge | 3.5 in | about 16 degrees | Yes |
| Board-on-board, one face | 0.75 in | about 53 degrees | No |
| Butted boards, one face | n/a | never, until they shrink | No |
Shrinkage is the overlap you are really designing for
Treated softwood fence boards are frequently delivered wet, and they lose width as they dry. A board that measures 5.5 inches on the day it is fixed can measure appreciably less by late summer. On a butted fence that shows up as gaps opening between every board, which is the single most common complaint about a solid fence and the reason overlapping arrangements exist at all.
The consequence for this page is that the overlap you should specify is not the overlap you want to see, it is the overlap you want to still have after the boards have shrunk. If you build with a half inch lap out of wet stock, you may end up with very little. Building with a generous lap out of dry stock is the arrangement that stays looking the way it did on the day it went up, and it is worth checking what the wood movement calculator predicts for the species and the moisture change you expect before settling on a number.
For boards in a single plane with intentional gaps, the privacy fence board calculator is the right page and uses a different formula. Posts, holes and concrete are in the fence post calculator, the opening is in the gate sag and brace calculator, and sloping ground has to be settled before anything is boarded using the fence on a slope calculator. When it is built, the fence stain calculator counts the area, and an overlapping fence has considerably more of it than its face dimensions suggest.
Questions people ask
How much more wood does a shadowbox fence use than a solid one?
Around ten to twenty percent for typical dimensions, and the calculator gives the exact figure for yours. The comparison in the results is against boards butted edge to edge in one plane, which is the cheapest way to make a solid face. A 5.5 inch board with a one inch overlap per side comes out about twelve percent up. Push the overlap to two inches per side and the front gap shrinks to 1.5 inches, the pitch tightens, and the premium climbs sharply. The material premium is the entire cost difference, since the posts, rails, holes and concrete are identical either way.
Is a shadowbox fence actually private?
Square on, yes. From an angle, no, and the angle is calculable rather than a matter of opinion: it is the arctangent of the overlap divided by the distance between the two planes of boards. For common dimensions that lands somewhere around 30 to 35 degrees, which is close enough to head on that a neighbouring window looking obliquely at your fence will see through it. If genuine privacy from all angles is the goal, a single solid plane does it and shadowbox does not; what shadowbox buys instead is an identical appearance from both sides, and airflow through the fence, which matters more than people expect in wind.
What is a good neighbour fence?
It is a marketing term rather than a defined construction, and it usually means either a shadowbox fence, because both faces look the same, or a fence built with the finished face toward the neighbour. Neither meaning carries any legal weight. Whether a fence must present its finished side outward, how tall it may be, and whether it may be built at all are decided locally and vary between adjacent municipalities. Nothing about the style of a fence affects ownership, maintenance responsibility or where the property line runs, and those are the questions that cause actual disputes.
Can I run boards horizontally instead?
Yes, and the count is the same arithmetic applied to the height instead of the width, which is what the horizontal slat option here does. Two things change materially. Each board spans the whole bay unsupported, so bays have to be shorter than for a vertical fence or the boards sag and cup, and the upper face of every board holds water rather than shedding it, which shortens the life of the fence. Horizontal fences look excellent and demand better material and closer posts than the same fence built vertically.
My overlap came out smaller than I asked for. Why?
Because the bay does not divide evenly and the leftover has to go somewhere. The calculator fits the largest whole number of boards that will hold your overlap or better, then spreads the remainder into the gaps between the front boards. Widening those gaps necessarily narrows the overlap, since the two are linked by the board width. If the reduced overlap is too small to be comfortable, the fixes are to add one more board and rip the outer ones narrower, to shift the post spacing slightly so the bay divides more kindly, or to start from a wider overlap knowing it will come down.