Lap Siding Calculator

Exposure is the number that matters

Lap siding works by overlapping: each course sits over the top of the one below so water runs down the face and off. The visible part of each board is its exposure, and the exposure is what covers the wall.

Board width minus the minimum overlap gives the maximum exposure. An eight inch board with an inch and a quarter overlap can show six and three quarters at most, and that figure — not eight — is what the wall divides by.

The material consequence is direct: coverage per linear foot is exposure divided by twelve, not board width divided by twelve. Ordering from board width understates the requirement by the overlap fraction, which is between fifteen and thirty percent on common profiles.

It is the same class of error as using nominal deck board width, running in the opposite direction and considerably larger.

Divide the wall, do not repeat up it

The instinct is to set the exposure at the maximum and start nailing from the bottom. That works until the top, where whatever is left over becomes a part-height course under the eave.

A narrow strip at the top is visible from the ground on every elevation, and it reads as a mistake because it is one. The alternative costs nothing: count how many courses the wall needs at maximum exposure, round up, and divide the wall height by that number. Every course is then identical and slightly under the maximum.

The layout then goes onto a story pole — a straight batten marked with every course line — which is transferred to each corner. That carries the layout around the building so courses meet correctly at the corners, and it prevents the accumulating error that comes from marking each course off the last board.

Small errors compound. A sixteenth of an inch per course over twenty courses is an inch and a quarter of drift, which is plainly visible where the siding meets a window on the other side of the wall.

Landing on the windows

The detail that separates a good siding job from an adequate one is that course lines meet window and door trim cleanly rather than being cut through mid-course.

Exposure has a usable range — anywhere between the maximum and something noticeably tighter — and a small adjustment within that range moves the course lines a long way over the height of a wall. Choosing an exposure that puts a course line at the window head, and often at the sill as well, removes the awkward cut entirely.

Where the head and sill cannot both be satisfied, the head matters more because it is at eye level and because water management there is more critical. Where neither works with a single exposure, using one exposure below the windows and a slightly different one above is invisible in practice — the change happens across the break in the wall where nothing lines up anyway.

One nail per stud per course, placed so it clears the top edge of the board below. Nailing through both boards pins them together across the grain, and the wood splits at the nail the first time it moves. It is the most common lap siding failure and it is entirely avoidable.