Board-on-Board Fence Calculator

Choose the fence cross-section

Look at the fence from above before choosing a style name. In this calculator, board-on-board places both rows on the same side of the rails. One row covers the gaps in the other. Shadowbox puts the two rows on opposite sides of the rails, with each back board centered behind a front-row gap. The top-view diagrams show the rail and row positions.

Shop names for these styles vary. A quote that says “board-on-board,” “shadowbox” or “alternating boards” may describe a different arrangement from the one you expect. Ask for a cross-section showing the rail and both board rows. Match that drawing to the picker before comparing material counts. The counting mathematics here is the same for both settings because both use one centered back board per front gap.

Front and back identify the two calculated rows. Either row can face your yard. The front row starts and ends flush with the boundaries of each board run. The back row starts behind the first gap and ends behind the last. There is no extra back board at either run end. If your detail adds end covers, corner returns or a continuous second face, count those pieces separately.

Enter width, overlap and run layout

Measure the board face

Use actual face width, not the nominal lumber name. The default is 5.5 in: this nominal 1 × 6 fence picket lists that actual width. Other products can differ. Measure several boards from the batch you intend to install, especially when replacing boards or combining deliveries. Both rows in this model use the same width; a narrower covering row needs a different layout.

Minimum overlap means how far each centered back board extends past each edge of the front gap in a straight-on projection. It is a minimum, so the finished overlap can be larger after the count rounds to whole boards. The result reports that actual overlap and the actual front-row gap. Do not use the requested overlap as a spacer dimension without reading the calculated gap.

For shadowbox, a negative minimum is allowed. A negative actual overlap means the back board is narrower than the front opening in projection. The tool labels the uncovered width as a see-through reveal. Zero means the projected edges align, with no overlap reserve. Rail depth and the viewer's angle still affect what can be seen through opposite-side boards; this calculation does not simulate a viewing angle.

Continuous runs or separate bays

Choose “Boards pass in front of the posts” when the board pattern continues across post locations. Each side is one board run unless gates divide it. Posts do not subtract width from that continuous face. At corners, measure each side to the boundary where its board pattern actually stops. Allow separately for any corner overlap or finishing detail your drawing requires.

Choose “Boards fit between posts” when the board layout restarts in each clear opening. The calculator divides each run into equal bays no longer than the maximum post spacing you enter. It treats that spacing as a center-to-center dimension, then subtracts one actual post width to get each bay's clear width. The same flush-end layout is calculated independently inside every bay.

This equal-bay estimate assumes matching post widths and new, evenly spaced framing. Measure each existing opening before buying. A width difference that crosses a whole-board threshold changes both row counts. If the layout leaves room for fewer than two whole front boards, the tool asks you to change the dimensions. It does not invent a ripped end board or a special narrow infill detail.

A negative gap in a single-row calculator is not enough to describe this pattern. It would place neighboring boards over each other without identifying a separate row. Here the front row always fits without overlapping itself, and the back row is counted explicitly. Use the picket spacing formula for a single row with clear spaces between pickets.

Count each row of boards

Call the usable run length L, actual board width w and front-row count N. The first front board begins at the run boundary and the last ends at the other boundary. That leaves N − 1 equal gaps. Put one back board at the center of each gap, so the back-row count is N − 1 and the combined installed count is 2N − 1.

Front gap g = (L − Nw) / (N − 1)
Overlap per side o = (w − g) / 2
Total boards = N + (N − 1)

The tool chooses the smallest whole N whose overlap meets your minimum. Increasing N reduces the front gaps and increases the cover behind their edges. It also checks that N whole front boards fit. When even the largest fitting count falls short of the requested overlap, the result carries a warning and shows that tightest layout. Treat it as a failed overlap check, then revise the design before ordering.

A 96-inch run

With L = 96 in, w = 5.5 in and a minimum overlap of 1 in, the calculation selects 12 front boards. They occupy 66 in, leaving 30 in across 11 gaps. Each gap is 2.7273 in. A centered back board overlaps each edge by (5.5 − 2.7273) / 2 = 1.3864 in.

The back row therefore needs 11 boards and the installed total is 23. Reducing the front row to 11 boards would give only 0.9750 in of overlap per side, below the requested minimum. The single-face coverage benchmark is ceil(96 / 5.5) = 18 boards before end trimming. The two-row result is 23, compared with 36 for doubling that single-face count.

A 100 ft run: two different layouts

For a continuous 100 ft run with the same board width and overlap input, the front row needs 134 boards and the back needs 133, totaling 267. Its actual front gap is 3.4812 in. The single-face coverage benchmark is 219 boards before end trimming, with no intentional gap or waste allowance.

Between posts, 100 ft with an 8 ft maximum center spacing needs 13 equal bays. Their actual center spacing is 92.3077 in. Subtracting a 3.5 in actual post width leaves 88.8077 in per bay. Each bay needs 11 front and 10 back boards, or 21 boards. Multiplying by 13 gives 273 installed boards.

Restarting the pattern at each bay changes the total because each opening has its own whole-board rounding and two flush ends. The difference is not a universal percentage. Keep the chosen run layout on your printed estimate so a supplier or installer can reproduce the same count.

Keep gates and spares separate

Enter each side's full length, including its gate openings, then enter the gate count on that side. Gate width is shared by all gates in this estimate. The calculator subtracts count × width from that side and splits the remaining footage into count + 1 equal runs. Each run starts a fresh board pattern. This is an estimating convention; it does not record actual gate positions.

For example, a 20 ft side with one 4 ft opening leaves 16 ft of fence, split into 2 runs of 96 in. With the default width and overlap, those runs need 24 front boards and 22 back boards, totaling 46. The gate face is excluded. Count it from the gate leaf's own usable width and board arrangement, including any separate frame or trim.

If the gate is close to a corner, the two remaining runs will differ. Count the actual runs separately with zero gates to replace the equal-run assumption. For gates of different widths, subtract the measured openings yourself and enter the resulting board runs. Avoid subtracting an opening once manually and again through the gate field.

Waste is a purchase allowance applied after all installed boards have been added. At the default 10%, the continuous 100 ft example goes from 267 installed boards to ceil(267 × 1.10) = 294 boards, including 27 spares. The tool rounds the overall purchase total up once. It does not round an allowance separately for each row or bay.

Choose the allowance for your cutting plan and the boards you accept. Spares do not repair a failed overlap check, add missing gate faces or account for a different end detail. Print or save PDF keeps the current counts, run assumptions and inputs together. Copy link saves those inputs in the URL so another person can restore the same estimate.

Board-on-board vs shadowbox: privacy, wind and framing

Positive overlap covers the front gaps in a straight-on projection. Opposite-side shadowbox separates the rows by the framing depth, so oblique sight lines differ from a same-side arrangement. Before committing to a privacy detail, check a physical sample from the viewing directions that matter. A negative-overlap reveal is intentionally open in projection and should not be described as full privacy.

Overlap depends on the widths you measure today. Viance's treated-wood guidance describes shrinkage as damp treated boards dry. If board faces narrow, their edge coverage changes. The calculator does not predict that movement or choose a moisture allowance. Follow the selected product's installation instructions and set the minimum overlap accordingly.

Do not use the board count to infer a wind rating. Air paths between staggered boards depend on the full cross-section, and this tool calculates neither wind pressure nor structural capacity. Fence height, exposure, post foundations and connections still need their own design. A shadowbox label alone does not establish that a fence is suitable for a particular site.

The extra boards add weight and need attachment to the framing. Check the rail and fastener detail for both rows, including how the covering boards are supported. This count supplies no rail size, fastening schedule or connection capacity. Carry the selected cross-section and installed quantities into the wood fence cost guide discussion. Use the fence materials and cost calculator for posts, rails, concrete and cost planning, and the single-row picket fence calculator for a conventional single-row picket purchase estimate.