diff --git a/src/wall-thickness.ts b/src/wall-thickness.ts
index 1111111..2222222 100644
--- a/src/wall-thickness.ts
+++ b/src/wall-thickness.ts
@@ -2508,7 +2508,7 @@
-export function openingInnerFaceOffsetFromIndex(
+export function openingInnerFaceOffsetFromIndex(
   index: OpeningWallIndex,
-  const piece = centrePiece(selected);
+  const piece = tunnelFacePath(selected);