Fit a Board to a Rough Stone Wall by Sizing the Scribe Spacer to the Deepest Hollow

The Problem

Scribing copies an irregular surface onto a workpiece: you hold the workpiece near the wall, run a spacer along the wall with a pencil riding at the other side of it, and the pencil traces the wall's shape onto the board. Cut to that line and the board sits tight against the wall.

It only works if the spacer never loses contact. A washer that spans 10mm can follow a wall that wanders 5mm in and out. Put that same washer against rough stone that hollows back 30mm between proud faces and it drops into the hollows, hits the far side, and the line it draws is no longer a copy of anything — it's a copy of the parts of the wall the washer could reach.

The failure is quiet. The line looks like a scribe line. You cut to it carefully, offer the board up, and it rocks on two high spots with daylight behind it.

The Technique

The spacer sets the limit, so size the spacer to the wall.

1. Look along the surface and find the deepest hollow between the points that stick out furthest. That distance is what the spacer has to clear.
2. Pick a spacer at least that wide. On rough stone, a cut-off angle grinder disc is about right — it's rigid, perfectly round, and around 115mm or 125mm across, so it bridges hollows a washer falls into.
3. Set the workpiece parallel to the wall, held off it by at least the spacer's width, and fix or brace it so it can't shift while you trace.
4. Run the spacer along the wall face with the pencil held against its outer edge. Keep the spacer's travel square to the workpiece the whole way — if it tilts, the offset changes and the line drifts.
5. Cut to the line. The board now sits back from the wall by exactly one spacer width along its whole length, so trim the whole board by that amount, or set it out that far from the start.