diff --git a/packages/core/src/services/model-service.ts b/packages/core/src/services/model-service.ts
index 80e808e..aec0e60 100644
--- a/packages/core/src/services/model-service.ts
+++ b/packages/core/src/services/model-service.ts
@@ -295,15 +295,22 @@ export class ModelService {
       nodesById.set(nodeId, mapped);
       childrenById.set(nodeId, childIds);
 
-      if (mapped.kind === 'instance' && findParentAsset(srcNode) == null) {
-        rootIds.push(nodeId);
-      }
       if (mapped.kind === 'property') propertyToSource.set(nodeId, srcNode.sourceNodeId);
       if (mapped.kind === 'action' && srcNode.parentSourceNodeId != null) {
         actionToMethod.set(nodeId, [srcNode.parentSourceNodeId, srcNode.sourceNodeId]);
       }
     }
 
+    // A root is an instance nothing in the model points to as a child, so that its
+    // parentId (parentIdOf) is null. Having no Object ancestor is not enough: an
+    // Object under a top-level Variable (e.g. its HA Configuration) has that Variable
+    // as parent.
+    const allChildIds = new Set<string>();
+    for (const ids of childrenById.values()) for (const id of ids) allChildIds.add(id);
+    for (const [nodeId, node] of nodesById) {
+      if (node.kind === 'instance' && !allChildIds.has(nodeId)) rootIds.push(nodeId);
+    }
+
     // ── Pre-fetch and map engineering units automatically ──
     const engUnitReqs: { propertyId: string; engUnitSourceId: string }[] = [];
     for (const node of nodesById.values()) {
diff --git a/packages/core/tests/model-service.test.ts b/packages/core/tests/model-service.test.ts
index 84068ee..fe028ea 100644
--- a/packages/core/tests/model-service.test.ts
+++ b/packages/core/tests/model-service.test.ts
@@ -171,6 +171,44 @@ describe('ModelService – browse path construction', () => {
     expect(idA).not.toBe(idB);
   });
 
+  it('an Object under a top-level Variable is not a root (conformance EXP-15)', async () => {
+    // Objects/Bool (historized Variable in a folder) -> HA Configuration (Object)
+    const nodes = [
+      sourceNode({
+        sourceNodeId: 'i=85',
+        nsuQualifiedName: 'nsu=http://opcfoundation.org/UA/:Objects',
+        typeDefinition: 'ns=0;i=61',
+      }),
+      sourceNode({
+        sourceNodeId: 'ns=2;i=1',
+        nsuQualifiedName: 'nsu=http://test.org/:Line',
+      }),
+      sourceNode({
+        sourceNodeId: 'ns=2;i=2',
+        nsuQualifiedName: 'nsu=http://test.org/:Bool',
+        parentSourceNodeId: 'i=85',
+        nodeClass: 'Variable',
+      }),
+      sourceNode({
+        sourceNodeId: 'ns=2;i=3',
+        nsuQualifiedName: 'nsu=http://opcfoundation.org/UA/:HA Configuration',
+        parentSourceNodeId: 'ns=2;i=2',
+      }),
+    ];
+    const svc = new ModelService(mockDataSource(nodes), nullLogger);
+    const model = await svc.getOrBuildModel();
+
+    const lineId = stableI3xId('nsu=http://test.org/:Line', 'instance');
+    expect(model.rootIds).toEqual([lineId]);
+    for (const id of model.rootIds) expect(svc.parentIdOf(model, id)).toBeNull();
+
+    const haConfig = [...model.nodesById.values()].find(
+      (n) => n.name === 'HA Configuration',
+    );
+    expect(haConfig).toBeDefined();
+    expect(model.nodesById.get(svc.parentIdOf(model, haConfig!.id)!)?.name).toBe('Bool');
+  });
+
   it('multi-namespace tree encodes both URIs in child browse path', async () => {
     const rootNsu = 'nsu=http://di.org/:DeviceSet';
     const childNsu = 'nsu=http://coffee.com/:Machine';