diff --git a/packages/core/src/schema/nodes/roof-segment-trim.test.ts b/packages/core/src/schema/nodes/roof-segment-trim.test.ts
index d35de190c..ad2923107 100644
--- a/packages/core/src/schema/nodes/roof-segment-trim.test.ts
+++ b/packages/core/src/schema/nodes/roof-segment-trim.test.ts
@@ -32,6 +32,15 @@ describe('roof segment trim', () => {
     })
   })
 
+  // Scene loading casts migrated roof-segments instead of zod-parsing them, so a
+  // segment saved before `trim` existed reaches renderers with the field absent.
+  test('tolerates a segment carrying no trim at all', () => {
+    const trim = normalizeRoofSegmentTrim({ width: 8, depth: 6 })
+
+    expect(trim.left).toBe(0)
+    expect(trim.backRightZ).toBe(0)
+  })
+
   test('normalizes impossible side totals without inverting the footprint', () => {
     const trim = normalizeRoofSegmentTrim({
       width: 4,
diff --git a/packages/nodes/src/ridge-vent/renderer.tsx b/packages/nodes/src/ridge-vent/renderer.tsx
index 635917034..8398ae830 100644
--- a/packages/nodes/src/ridge-vent/renderer.tsx
+++ b/packages/nodes/src/ridge-vent/renderer.tsx
@@ -4,6 +4,7 @@ import {
   type AnyNodeId,
   getEffectiveRoofSurfaceMaterial,
   getEffectiveSegmentSurfaceMaterial,
+  normalizeRoofSegmentTrim,
   type RidgeVentNode,
   type RoofNode,
   type RoofSegmentNode,
@@ -29,7 +30,7 @@ import { buildRidgeVentGeometry } from './geometry'
 
 function ridgeVentSegmentGeometryKey(segment: RoofSegmentNode | undefined): string {
   if (!segment) return 'none'
-  const trim = segment.trim
+  const trim = normalizeRoofSegmentTrim(segment)
   return [
     segment.roofType,
     segment.width,