diff --git a/packages/core/src/hooks/spatial-grid/support-host-patch.ts b/packages/core/src/hooks/spatial-grid/support-host-patch.ts
index f2e4556a8..3d5deb3ca 100644
--- a/packages/core/src/hooks/spatial-grid/support-host-patch.ts
+++ b/packages/core/src/hooks/spatial-grid/support-host-patch.ts
@@ -443,6 +443,9 @@ export function resolveWallConstruction(
   let resolvedNodes = nodes
   let sourceSupportUpdate: WallConstructionResolution['sourceSupportUpdate'] = null
   const constructionSourceNodeId = options?.constructionSourceNodeId
+  const constructionSourceIsSlab = constructionSourceNodeId
+    ? nodes[constructionSourceNodeId]?.type === 'slab'
+    : false
   if (constructionSourceNodeId) {
     const sourceNode = nodes[constructionSourceNodeId]
     const currentSupport =
@@ -475,13 +478,14 @@ export function resolveWallConstruction(
       !options.flatConstructionBase
         ? undefined
         : options
-    const preferredSupportSlabId =
-      wallOptions?.flatConstructionBase === true
-        ? GROUND_SUPPORT_ID
-        : (wallOptions?.preferredSupportSlabId ??
-          (wallOptions?.constructionElevation != null && terrainBase != null
-            ? GROUND_SUPPORT_ID
-            : null))
+    const flatConstructionBase =
+      wallOptions?.flatConstructionBase === true && !constructionSourceIsSlab
+    const preferredSupportSlabId = flatConstructionBase
+      ? GROUND_SUPPORT_ID
+      : (wallOptions?.preferredSupportSlabId ??
+        (wallOptions?.constructionElevation != null && terrainBase != null
+          ? GROUND_SUPPORT_ID
+          : null))
     const supportPatch = resolveWallSupportSlabPatch(wallWithParent, resolvedNodes, {
       maxElevation: wallOptions?.supportCap ?? null,
       preferredSlabId: preferredSupportSlabId,
@@ -496,8 +500,7 @@ export function resolveWallConstruction(
       wallOptions?.supportCap ?? null,
     )
     const groundDraft =
-      preferredSupportSlabId === GROUND_SUPPORT_ID &&
-      (terrainBase != null || wallOptions?.flatConstructionBase === true)
+      preferredSupportSlabId === GROUND_SUPPORT_ID && (terrainBase != null || flatConstructionBase)
     const supportOffset =
       groundDraft && wallOptions?.constructionElevation != null
         ? wallOptions.constructionElevation - sourceSupport.elevation
diff --git a/packages/editor/src/components/tools/wall/wall-drafting.test.ts b/packages/editor/src/components/tools/wall/wall-drafting.test.ts
index eaf7b662d..d2befe0b5 100644
--- a/packages/editor/src/components/tools/wall/wall-drafting.test.ts
+++ b/packages/editor/src/components/tools/wall/wall-drafting.test.ts
@@ -423,6 +423,136 @@ describe('createWallOnCurrentLevel', () => {
     expect(created?.supportSlabId).not.toBe(GROUND_SUPPORT_ID)
   })
 
+  test('a fresh-scene wall started on a slab node-top elects that slab plane-bound', () => {
+    const slab = SlabNode.parse({
+      id: 'slab_fresh_floor',
+      parentId: LEVEL_ID,
+      polygon: [
+        [-1, -1],
+        [5, -1],
+        [5, 5],
+        [-1, 5],
+      ],
+      elevation: 0.05,
+      thickness: 0.05,
+    })
+    seedLevel([], [slab])
+    spatialGridManager.clear()
+    spatialGridManager.handleNodeCreated(slab as AnyNode, LEVEL_ID)
+
+    const created = createWallOnCurrentLevel([2, 2], [3, 2], {
+      supportCap: 0.05,
+      preferredSupportSlabId: null,
+      constructionElevation: 0.05,
+      constructionHeight: 2.5,
+      constructionSourceNodeId: slab.id,
+      flatConstructionBase: true,
+    })
+
+    expect(created).not.toBeNull()
+    expect(created?.supportSlabId).toBeUndefined()
+    expect(created?.height).toBeUndefined()
+    expect(created?.supportOffset).toBeUndefined()
+    const support = spatialGridManager.getSlabSupportForWall(
+      LEVEL_ID,
+      created!.start,
+      created!.end,
+      created!.curveOffset,
+      created!.thickness,
+      created!.supportSlabId,
+    )
+    expect(support.electedSlabId).toBe(slab.id)
+    expect(support.elevation).toBeCloseTo(0.05)
+  })
+
+  test('a direct slab source does not pin a cross-slab wall away from the higher majority support', () => {
+    const sourceSlab = SlabNode.parse({
+      id: 'slab_source_low',
+      parentId: LEVEL_ID,
+      polygon: [
+        [-0.1, -1],
+        [0.1, -1],
+        [0.1, 1],
+        [-0.1, 1],
+      ],
+      elevation: 0.1,
+      thickness: 0.05,
+    })
+    const majoritySlab = SlabNode.parse({
+      id: 'slab_majority_high',
+      parentId: LEVEL_ID,
+      polygon: [
+        [0.1, -1],
+        [4.1, -1],
+        [4.1, 1],
+        [0.1, 1],
+      ],
+      elevation: 0.6,
+      thickness: 0.1,
+    })
+    seedLevel([], [sourceSlab, majoritySlab])
+    spatialGridManager.clear()
+    spatialGridManager.handleNodeCreated(sourceSlab as AnyNode, LEVEL_ID)
+    spatialGridManager.handleNodeCreated(majoritySlab as AnyNode, LEVEL_ID)
+
+    const created = createWallOnCurrentLevel([0, 0], [4, 0], {
+      supportCap: 0.6,
+      preferredSupportSlabId: null,
+      constructionElevation: 0.1,
+      constructionHeight: 2.5,
+      constructionSourceNodeId: sourceSlab.id,
+      flatConstructionBase: true,
+    })
+
+    expect(created?.supportSlabId).toBe(majoritySlab.id)
+    expect(created?.height).toBeUndefined()
+    expect(created?.supportOffset).toBeUndefined()
+  })
+
+  test('a grazing direct slab source does not override the commit elevation cap', () => {
+    const sourceSlab = SlabNode.parse({
+      id: 'slab_source_high',
+      parentId: LEVEL_ID,
+      polygon: [
+        [-0.1, -1],
+        [0.1, -1],
+        [0.1, 1],
+        [-0.1, 1],
+      ],
+      elevation: 0.6,
+      thickness: 0.1,
+    })
+    const cappedSlab = SlabNode.parse({
+      id: 'slab_capped_low',
+      parentId: LEVEL_ID,
+      polygon: [
+        [-0.1, -1],
+        [4.1, -1],
+        [4.1, 1],
+        [-0.1, 1],
+      ],
+      elevation: 0.1,
+      thickness: 0.05,
+    })
+    seedLevel([], [sourceSlab, cappedSlab])
+    spatialGridManager.clear()
+    spatialGridManager.handleNodeCreated(sourceSlab as AnyNode, LEVEL_ID)
+    spatialGridManager.handleNodeCreated(cappedSlab as AnyNode, LEVEL_ID)
+
+    const created = createWallOnCurrentLevel([0, 0], [4, 0], {
+      supportCap: 0.1,
+      preferredSupportSlabId: null,
+      constructionElevation: 0.6,
+      constructionHeight: 2.5,
+      constructionSourceNodeId: sourceSlab.id,
+      flatConstructionBase: true,
+    })
+
+    expect(created?.supportSlabId).toBe(cappedSlab.id)
+    expect(created?.height).toBeUndefined()
+    expect(created?.supportOffset).toBeUndefined()
+  })
+
   test('a non-ground draft never freezes the ghost height, even at a raised plane', () => {
     const created = createWallOnCurrentLevel([2, 2], [3, 2], {
       supportCap: 1.2,