diff --git a/packages/core/src/hooks/spatial-grid/spatial-grid-manager.ts b/packages/core/src/hooks/spatial-grid/spatial-grid-manager.ts
index 54a0b81e..f1a365d0 100644
--- a/packages/core/src/hooks/spatial-grid/spatial-grid-manager.ts
+++ b/packages/core/src/hooks/spatial-grid/spatial-grid-manager.ts
@@ -2,11 +2,14 @@ import type { AnyNode, ItemNode, SlabNode, WallNode } from '../../schema'
 import { SpatialGrid } from './spatial-grid'
 import { WallSpatialGrid } from './wall-spatial-grid'
 
+// ============================================================================
+// GEOMETRY HELPERS
+// ============================================================================
+
 /**
  * Point-in-polygon test using ray casting algorithm.
- * Returns true if point (px, pz) is inside the polygon defined by vertices.
  */
-function pointInPolygon(px: number, pz: number, polygon: Array<[number, number]>): boolean {
+export function pointInPolygon(px: number, pz: number, polygon: Array<[number, number]>): boolean {
   let inside = false
   const n = polygon.length
   for (let i = 0, j = n - 1; i < n; j = i++) {
@@ -20,6 +23,138 @@ function pointInPolygon(px: number, pz: number, polygon: Array<[number, number]>
   return inside
 }
 
+/**
+ * Compute the 4 XZ footprint corners of an item given its position, dimensions, and Y rotation.
+ */
+function getItemFootprint(
+  position: [number, number, number],
+  dimensions: [number, number, number],
+  rotation: [number, number, number],
+  inset = 0,
+): Array<[number, number]> {
+  const [x, , z] = position
+  const [w, , d] = dimensions
+  const yRot = rotation[1]
+  const halfW = Math.max(0, w / 2 - inset)
+  const halfD = Math.max(0, d / 2 - inset)
+  const cos = Math.cos(yRot)
+  const sin = Math.sin(yRot)
+
+  return [
+    [x + (-halfW * cos + halfD * sin), z + (-halfW * sin - halfD * cos)],
+    [x + (halfW * cos + halfD * sin), z + (halfW * sin - halfD * cos)],
+    [x + (halfW * cos - halfD * sin), z + (halfW * sin + halfD * cos)],
+    [x + (-halfW * cos - halfD * sin), z + (-halfW * sin + halfD * cos)],
+  ]
+}
+
+/**
+ * Test if two line segments (a1->a2) and (b1->b2) intersect.
+ */
+function segmentsIntersect(
+  ax1: number, az1: number, ax2: number, az2: number,
+  bx1: number, bz1: number, bx2: number, bz2: number,
+): boolean {
+  const cross = (ox: number, oz: number, ax: number, az: number, bx: number, bz: number) =>
+    (ax - ox) * (bz - oz) - (az - oz) * (bx - ox)
+
+  const d1 = cross(bx1, bz1, bx2, bz2, ax1, az1)
+  const d2 = cross(bx1, bz1, bx2, bz2, ax2, az2)
+  const d3 = cross(ax1, az1, ax2, az2, bx1, bz1)
+  const d4 = cross(ax1, az1, ax2, az2, bx2, bz2)
+
+  if (((d1 > 0 && d2 < 0) || (d1 < 0 && d2 > 0)) &&
+      ((d3 > 0 && d4 < 0) || (d3 < 0 && d4 > 0))) {
+    return true
+  }
+
+  // Collinear touching cases
+  const onSeg = (px: number, pz: number, qx: number, qz: number, rx: number, rz: number) =>
+    Math.min(px, qx) <= rx && rx <= Math.max(px, qx) &&
+    Math.min(pz, qz) <= rz && rz <= Math.max(pz, qz)
+
+  if (d1 === 0 && onSeg(bx1, bz1, bx2, bz2, ax1, az1)) return true
+  if (d2 === 0 && onSeg(bx1, bz1, bx2, bz2, ax2, az2)) return true
+  if (d3 === 0 && onSeg(ax1, az1, ax2, az2, bx1, bz1)) return true
+  if (d4 === 0 && onSeg(ax1, az1, ax2, az2, bx2, bz2)) return true
+
+  return false
+}
+
+/**
+ * Test if a line segment intersects any edge of a polygon.
+ */
+function segmentIntersectsPolygon(
+  sx1: number, sz1: number, sx2: number, sz2: number,
+  polygon: Array<[number, number]>,
+): boolean {
+  const n = polygon.length
+  for (let i = 0; i < n; i++) {
+    const j = (i + 1) % n
+    if (segmentsIntersect(
+      sx1, sz1, sx2, sz2,
+      polygon[i]![0], polygon[i]![1], polygon[j]![0], polygon[j]![1],
+    )) {
+      return true
+    }
+  }
+  return false
+}
+
+/**
+ * Test if an item's footprint overlaps with a polygon.
+ * Checks: any item corner inside polygon, or any polygon vertex inside item AABB, or edges intersect.
+ */
+export function itemOverlapsPolygon(
+  position: [number, number, number],
+  dimensions: [number, number, number],
+  rotation: [number, number, number],
+  polygon: Array<[number, number]>,
+  inset = 0,
+): boolean {
+  const corners = getItemFootprint(position, dimensions, rotation, inset)
+
+  // Check if any item corner is inside the polygon
+  for (const [cx, cz] of corners) {
+    if (pointInPolygon(cx, cz, polygon)) return true
+  }
+
+  // Check if any polygon vertex is inside the item footprint
+  // (handles case where slab is fully inside a large item)
+  for (const [px, pz] of polygon) {
+    if (pointInPolygon(px, pz, corners)) return true
+  }
+
+  // Check if any item edge intersects any polygon edge
+  for (let i = 0; i < 4; i++) {
+    const j = (i + 1) % 4
+    if (segmentIntersectsPolygon(
+      corners[i]![0], corners[i]![1], corners[j]![0], corners[j]![1],
+      polygon,
+    )) return true
+  }
+
+  return false
+}
+
+/**
+ * Test if a wall segment overlaps with a polygon.
+ */
+export function wallOverlapsPolygon(
+  start: [number, number],
+  end: [number, number],
+  polygon: Array<[number, number]>,
+): boolean {
+  // Either endpoint inside the polygon
+  if (pointInPolygon(start[0], start[1], polygon)) return true
+  if (pointInPolygon(end[0], end[1], polygon)) return true
+
+  // Wall segment intersects any polygon edge
+  if (segmentIntersectsPolygon(start[0], start[1], end[0], end[1], polygon)) return true
+
+  return false
+}
+
 export class SpatialGridManager {
   private floorGrids = new Map<string, SpatialGrid>() // levelId -> grid
   private wallGrids = new Map<string, WallSpatialGrid>() // levelId -> wall grid
@@ -243,6 +378,32 @@ export class SpatialGridManager {
     return maxElevation
   }
 
+  /**
+   * Get the slab elevation for an item using its full footprint (bounding box).
+   * Checks if any part of the item's rotated footprint overlaps with any slab polygon.
+   * Returns the highest overlapping slab elevation, or 0 if none.
+   */
+  getSlabElevationForItem(
+    levelId: string,
+    position: [number, number, number],
+    dimensions: [number, number, number],
+    rotation: [number, number, number],
+  ): number {
+    const slabMap = this.slabsByLevel.get(levelId)
+    if (!slabMap) return 0
+
+    let maxElevation = 0
+    for (const slab of slabMap.values()) {
+      if (slab.polygon.length >= 3 && itemOverlapsPolygon(position, dimensions, rotation, slab.polygon, 0.01)) {
+        const elevation = slab.elevation ?? 0.05
+        if (elevation > maxElevation) {
+          maxElevation = elevation
+        }
+      }
+    }
+    return maxElevation
+  }
+
   clearLevel(levelId: string) {
     this.floorGrids.delete(levelId)
     this.wallGrids.delete(levelId)
diff --git a/packages/core/src/hooks/spatial-grid/spatial-grid-sync.ts b/packages/core/src/hooks/spatial-grid/spatial-grid-sync.ts
index c78462a0..d978403a 100644
--- a/packages/core/src/hooks/spatial-grid/spatial-grid-sync.ts
+++ b/packages/core/src/hooks/spatial-grid/spatial-grid-sync.ts
@@ -1,6 +1,6 @@
-import type { AnyNode } from '../../schema'
+import type { AnyNode, AnyNodeId, ItemNode, SlabNode, WallNode } from '../../schema'
 import useScene from '../../store/use-scene'
-import { spatialGridManager } from './spatial-grid-manager'
+import { itemOverlapsPolygon, spatialGridManager, wallOverlapsPolygon } from './spatial-grid-manager'
 
 export function resolveLevelId(node: AnyNode, nodes: Record<string, AnyNode>): string {
   // If the node itself is a level
@@ -34,6 +34,7 @@ export function initSpatialGridSync() {
   }
 
   // 2. Then subscribe to future changes
+  const markDirty = (id: AnyNodeId) => store.getState().markDirty(id)
 
   // Subscribe to all changes
   store.subscribe((state, prevState) => {
@@ -42,6 +43,11 @@ export function initSpatialGridSync() {
       if (!prevState.nodes[id as AnyNode['id']]) {
         const levelId = resolveLevelId(node, state.nodes)
         spatialGridManager.handleNodeCreated(node, levelId)
+
+        // When a slab is added, mark overlapping items/walls dirty
+        if (node.type === 'slab') {
+          markNodesOverlappingSlab(node as SlabNode, state.nodes, markDirty)
+        }
       }
     }
 
@@ -50,6 +56,11 @@ export function initSpatialGridSync() {
       if (!state.nodes[id as AnyNode['id']]) {
         const levelId = resolveLevelId(node, prevState.nodes)
         spatialGridManager.handleNodeDeleted(id, node.type, levelId)
+
+        // When a slab is removed, mark items/walls that were on it dirty (using current state)
+        if (node.type === 'slab') {
+          markNodesOverlappingSlab(node as SlabNode, state.nodes, markDirty)
+        }
       }
     }
 
@@ -72,6 +83,10 @@ export function initSpatialGridSync() {
         if (node.polygon !== prev.polygon || node.elevation !== prev.elevation) {
           const levelId = resolveLevelId(node, state.nodes)
           spatialGridManager.handleNodeUpdated(node, levelId)
+
+          // Mark nodes overlapping old polygon and new polygon as dirty
+          markNodesOverlappingSlab(prev as SlabNode, state.nodes, markDirty)
+          markNodesOverlappingSlab(node as SlabNode, state.nodes, markDirty)
         }
       }
     }
@@ -81,3 +96,33 @@ export function initSpatialGridSync() {
 function arraysEqual(a: number[], b: number[]): boolean {
   return a.length === b.length && a.every((v, i) => v === b[i])
 }
+
+/**
+ * Mark all floor items and walls that overlap a slab polygon as dirty.
+ */
+function markNodesOverlappingSlab(
+  slab: SlabNode,
+  nodes: Record<string, AnyNode>,
+  markDirty: (id: AnyNodeId) => void,
+) {
+  if (slab.polygon.length < 3) return
+  const slabLevelId = resolveLevelId(slab, nodes)
+
+  for (const node of Object.values(nodes)) {
+    if (node.type === 'item') {
+      const item = node as ItemNode
+      // Only floor items are affected by slabs
+      if (item.asset.attachTo) continue
+      if (resolveLevelId(node, nodes) !== slabLevelId) continue
+      if (itemOverlapsPolygon(item.position, item.asset.dimensions, item.rotation, slab.polygon, 0.01)) {
+        markDirty(node.id)
+      }
+    } else if (node.type === 'wall') {
+      const wall = node as WallNode
+      if (resolveLevelId(node, nodes) !== slabLevelId) continue
+      if (wallOverlapsPolygon(wall.start, wall.end, slab.polygon)) {
+        markDirty(node.id)
+      }
+    }
+  }
+}
diff --git a/packages/core/src/systems/item/item-system.tsx b/packages/core/src/systems/item/item-system.tsx
index a5d94bf4..4722da9b 100644
--- a/packages/core/src/systems/item/item-system.tsx
+++ b/packages/core/src/systems/item/item-system.tsx
@@ -32,9 +32,11 @@ export const ItemSystem = () => {
           mesh.position.z = wallThickness / 2
         }
       } else if (!item.asset.attachTo) {
-        // Floor item: elevate by slab height
+        // Floor item: elevate by slab height (using full footprint overlap)
         const levelId = resolveLevelId(item, nodes)
-        const slabElevation = spatialGridManager.getSlabElevationAt(levelId, item.position[0], item.position[2])
+        const slabElevation = spatialGridManager.getSlabElevationForItem(
+          levelId, item.position, item.asset.dimensions, item.rotation,
+        )
         mesh.position.y = slabElevation
       }