Lap siding exposure vs. board width vs. overlap — why an 8¼-inch board covers only 7 inches

A lap board hides part of itself: the overlap is covered by the course above, so an 8¼″ board shows only 7″. Exposure (width − overlap) is what drives how much siding you buy — not the board width.

Cross-section of three stacked lap-siding courses. Each board is 8¼ inches wide, but its top 1¼ inches is hidden under the butt of the course above (the overlap), so only 7 inches shows — that visible part is the exposure. The top edge of each board is blind-nailed to the wall and covered by the next course. The exposure, not the full board width, is what sets how many courses a wall needs and how much siding to buy: exposure = board width − overlap, so 8¼ inches minus 1¼ inches equals 7 inches. — Source: James Hardie HardiePlank Installation / ASTM D3679 (1¼″ min overlap)

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Cross-section of three stacked lap-siding courses. Each board is 8¼ inches wide, but its top 1¼ inches is hidden under the butt of the course above (the overlap), so only 7 inches shows — that visible part is the exposure. The top edge of each board is blind-nailed to the wall and covered by the next course. The exposure, not the full board width, is what sets how many courses a wall needs and how much siding to buy: exposure = board width − overlap, so 8¼ inches minus 1¼ inches equals 7 inches.

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    <img src="https://remodelcalculators.com/api/figure-embed/siding-exposure-overlap" alt="Cross-section of three stacked lap-siding courses. Each board is 8¼ inches wide, but its top 1¼ inches is hidden under the butt of the course above (the overlap), so only 7 inches shows — that visible part is the exposure. The top edge of each board is blind-nailed to the wall and covered by the next course. The exposure, not the full board width, is what sets how many courses a wall needs and how much siding to buy: exposure = board width − overlap, so 8¼ inches minus 1¼ inches equals 7 inches." width="800" height="460" style="width:100%;height:auto;border:0" loading="lazy">
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    Diagram: <a href="https://remodelcalculators.com/construction-diagrams/siding-exposure-overlap" target="_blank" rel="noopener">Lap siding exposure vs. board width vs. overlap — why an 8¼-inch board covers only 7 inches</a> — courtesy of <a href="https://remodelcalculators.com" target="_blank" rel="noopener">RemodelCalculators.com</a>
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