Board on Board Fence Calculator

What board-on-board changes about a wood fence

A board-on-board fence is a privacy fence built from two runs of vertical boards that sit on opposite faces of the same rails, with each board overlapping the seam behind it. From the street it looks like a solid wall of wood. From the back yard it looks the same, which is the point: both neighbors get a finished face, and no gap opens up when the boards shrink in dry weather.

That construction changes the shopping list in a way that surprises people. A stockade fence counts boards by dividing the fence length by the board width, because the boards butt against each other. A board-on-board fence divides by the board width minus the overlap, because every board covers less ground than its face suggests. On a 150 foot run with 1x6 boards and a 2 inch overlap, the difference is 328 boards versus 515, which is 187 extra boards and a few hundred dollars before you buy a single post.

The overlap is also why the style holds up. Wood moves with the seasons, and a butted fence opens hairline gaps that widen over a few years. Overlapping boards slide past each other instead, so the wall stays closed. You pay for that in material, not in labor, since the extra boards go up at the same rate as the first layer.

Coverage width is the number that drives the whole estimate

Coverage width is the face width of the board minus the overlap you plan to run. It is the only number you need before you can count boards, and it is where most DIY estimates go wrong, because the label on the board is not the width of the board.

A nominal 1x6 is 5.5 inches wide when dressed, a 1x4 is 3.5 inches, and a 1x8 is 7.25 inches. Those dressed sizes are set by the American Softwood Lumber Standard, PS 20-25, which NIST publishes. The same standard is why a 2x4 is 1.5 inches by 3.5 inches. If you measure a board in the yard and get 5.5 inches, the standard is working as intended.

Take that 5.5 inch face and subtract a 2 inch overlap and you get 3.5 inches of coverage per board. That single subtraction turns a 5.5 inch board into a 3.5 inch board for counting purposes, and it raises the board count by 57% compared with butting the same boards edge to edge.

Overlap is a judgment call, not a code requirement. Two inches is the common target for a 6 foot privacy fence because it leaves room for seasonal movement without letting the wall open. One and a half inches works in mild climates and saves material. Three inches or more looks heavy, doubles the shadow line, and wastes boards. Anything under an inch defeats the purpose, since the boards can pull apart in the first dry spell.

Worked example: 150 feet of 6 foot fence

Here is the full takeoff for a 150 foot run, 6 feet tall, built from 1x6 cedar boards with a 2 inch overlap, posts every 8 feet, and three rails per bay. Every number below comes from the calculator on this page with those inputs.

The rounding matters. A run that divides evenly, such as 140 feet at 3.5 inches of coverage, gives 480 boards exactly, and then waste pushes it to 528. A run that lands on a fraction always rounds up, because you cannot buy two thirds of a board. The 10% waste allowance covers the ripped board at each end, the short cuts around a gate, and the two or three boards per hundred that arrive with a split end or a bad cup.

If you plan to cap the fence, add the cap rail separately. A cap is usually one 2x6 laid flat across the top, so it runs the length of the fence in linear feet, not in boards, and it needs its own fasteners every 16 inches.

Posts, bays and rails: the horizontal math

Vertical boards get the attention, but the frame sets the cost and the wind rating. Posts carry the load, and the spacing you choose decides how many you buy and how stiff the fence feels when someone leans on it.

Divide the run by the post spacing, round up to get the number of bays, then add one for the closing post. A 150 foot run at 8 foot spacing gives 18.75, which rounds to 19 bays and 20 posts. Tighter spacing means more concrete and more digging, but a 10 foot bay in a 6 foot tall board-on-board fence will flex, because the boards themselves catch wind and the rails have a longer unsupported span.

Rails run horizontally and hold the boards. Two rails work for a 4 foot fence. Three rails is the standard for 6 feet, and four is common above 6 feet or in an area with high wind. Rail count does not change with post spacing, since each bay gets the same number of rails, but the total linear feet of rail stays at fence length times rail count: 150 x 3 = 450 linear feet, whether you use 15 bays or 25.

Buy rails in 16 foot lengths and cut them in half for an 8 foot bay. That gives you two rails per board with no waste, and it is cheaper per foot than 8 foot rails. For a 6 foot bay, a 16 foot rail yields two 6 foot pieces plus a 4 foot offcut, so plan the offcuts for a gate frame or a short return.

Board counts by board size and fence length

Bigger boards cover more ground, but the overlap eats a larger share of a narrow board. The table below uses a 2 inch overlap throughout, which is the common spec for a privacy fence in a mixed climate.

A 1x4 with a 2 inch overlap covers 1.5 inches, so it takes 800 boards for 100 feet. That is why 1x4 board-on-board is rare: the overlap consumes more than half the board. A 1x6 keeps 64% of its face, and a 1x8 keeps 72%, which is why 1x6 is the sweet spot for cost and appearance.

Board length is a separate decision. Standard boards come in 6, 8, 10 and 12 foot lengths. For a 6 foot fence, an 8 foot board leaves a 2 foot offcut that is too short to reuse on the fence but fine for a gate brace. Buying 6 foot boards costs more per foot in most yards but produces no waste, so compare the two prices before you commit.

Waste, knots and the 10% rule

Ten percent waste is the working number for a straight run with no gates. It covers end cuts, boards that split when nailed near an edge, and the occasional board that arrives bowed. Add five points if the run has more than two corners, a gate, or a step down a slope, since each of those forces extra cuts and short pieces.

Knots and grade matter more than most buyers expect. A number 2 grade board can have knots up to a couple of inches across, and a knot near the edge will split when you face nail it. A number 1 grade costs more but wastes less. For a fence you plan to stain, buy the better grade on the street side and save the knotty boards for the back face, where they will not be seen.

Cedar and pressure-treated pine behave differently. Cedar is stable and takes stain well, and western red cedar heartwood resists rot without chemical treatment. Pressure-treated pine is cheaper and heavier, and it shrinks more as it dries, which is the reason to leave a little extra overlap on a green board. The USDA Forest Service Wood Handbook has the shrinkage numbers by species if you want to plan for movement rather than guess.

Height, wind and the rules that are not in the calculator

Most residential zoning codes cap a back yard fence at 6 feet and a front yard fence at 4 feet, and many require a permit above 6 feet. Those limits are local, so check with your city or county before you order material. A fence that is 6 feet 6 inches tall can trigger a permit, an inspection, and in some neighborhoods a neighbor notification, none of which the material count tells you about.

Wind is the other hidden variable. A solid board-on-board fence is a sail. Posts set 8 feet apart in 2 feet of concrete hold up in most of the country, but in a high wind area the same fence wants 6 foot spacing or deeper footings. The calculator gives you the count for the spacing you choose, so run it twice, once at 8 feet and once at 6 feet, and compare the post and concrete cost against the risk.

One more rule of thumb: keep the bottom board 2 inches off the ground. Boards that touch soil wick moisture and rot from the bottom up, and the 2 inch gap also lets water drain instead of pooling against the wood. It costs you nothing in board count, since the boards are cut to height anyway.