Capture an Alcove's Exact Shape With a Tread Gauge Before Cutting Shelves

The Problem

Shelves in an alcove or niche have to fit between two walls that are never quite parallel and never quite plumb. Measure the width with a tape and cut square, and the shelf binds at one end and gapes at the other — because the number was right and the angles were wrong. Each shelf becomes a cycle of cut, test, shave, test.

The Technique

A tread gauge — the adjustable template tool made for measuring stair treads between skirt boards — captures the whole geometry at once:

1. Expand the gauge in the alcove at the shelf's height until its ends press against both walls. The pivoting end plates seat flat against each wall, recording not only the width but the angle each wall actually makes.
2. Lock the gauge.
3. Carry it to your plywood, lay it down, and trace the ends — the true width and both wall angles transfer in one go.
4. Cut to the traced lines. A track saw makes the long clean cut, and the shelf slides in snug.

One capture per shelf position — walls change as they rise, so gauge each shelf at its own height rather than reusing the first tracing.