diff --git a/domain/epc/property_overrides/roof_sloping_ceiling_guard.py b/domain/epc/property_overrides/roof_sloping_ceiling_guard.py
new file mode 100644
index 000000000..22ade4d28
--- /dev/null
+++ b/domain/epc/property_overrides/roof_sloping_ceiling_guard.py
@@ -0,0 +1,9 @@
+from __future__ import annotations
+
+from typing import Optional
+
+from domain.epc.property_overrides.roof_type import RoofType
+
+
+def roof_sloping_ceiling_guard(description: str) -> Optional[RoofType]:
+    raise NotImplementedError
diff --git a/tests/domain/epc/test_roof_sloping_ceiling_guard.py b/tests/domain/epc/test_roof_sloping_ceiling_guard.py
new file mode 100644
index 000000000..022b36460
--- /dev/null
+++ b/tests/domain/epc/test_roof_sloping_ceiling_guard.py
@@ -0,0 +1,42 @@
+"""The pitched-sloping-ceiling roof guard (ADR-0066).
+
+`PitchedWithSlopingCeiling: <As Built | Unknown | N mm>` is a slope-following
+ceiling (gov-API roof_construction 8), NOT a horizontal loft — so it must
+resolve deterministically to the `Pitched, sloping ceiling, …` family and never
+reach the LLM to be routed onto the `Pitched, N mm loft insulation` ladder. Same
+defect class the party-ceiling guard was created for (#1376): the LLM resolves
+identical inputs inconsistently, and the near-duplicate loft members are exactly
+the ambiguity. Returns None for anything that is not a sloping-ceiling marker so
+the LLM (and the party-ceiling guard) still handle the rest.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from domain.epc.property_overrides.roof_sloping_ceiling_guard import (
+    roof_sloping_ceiling_guard,
+)
+from domain.epc.property_overrides.roof_type import RoofType
+
+
+@pytest.mark.parametrize(
+    "description",
+    [
+        "PitchedWithSlopingCeiling: As Built",
+        "PitchedWithSlopingCeiling: Unknown",
+        "PitchedWithSlopingCeiling",
+        "PitchedWithSlopingCeiling: ",
+    ],
+)
+def test_sloping_ceiling_without_a_depth_resolves_to_as_built(
+    description: str,
+) -> None:
+    # ADR-0066: a sloping ceiling with no measured depth ("As Built" / "Unknown"
+    # / nothing) folds to `as built`, which defers the U-value to the age band.
+
+    # Act
+    member = roof_sloping_ceiling_guard(description)
+
+    # Assert
+    assert member is RoofType.PITCHED_SLOPING_CEILING_AS_BUILT