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.
What this diagram shows
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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