diff --git a/tests/domain/epc/test_roof_type_overlay.py b/tests/domain/epc/test_roof_type_overlay.py
index 050d76fa2..75aab9204 100644
--- a/tests/domain/epc/test_roof_type_overlay.py
+++ b/tests/domain/epc/test_roof_type_overlay.py
@@ -42,6 +42,20 @@ def test_each_loft_depth_is_parsed(roof_type_value: str, expected_mm: int) -> No
     ].roof_insulation_thickness == expected_mm
 
 
+def test_flat_roof_depth_maps_to_roof_insulation_thickness() -> None:
+    # Act — a flat roof with a known depth. RdSAP 10 Table 16 col (1) scores flat
+    # roofs by insulation thickness ("joists at ceiling level and flat roof"), so
+    # the depth must reach the overlay, not be discarded for the age-band default
+    # (ADR-0041).
+    simulation = roof_overlay_for("Flat, 150 mm insulation", 0)
+
+    # Assert — the flat shape and the real depth both ride on the overlay.
+    assert simulation is not None
+    overlay = simulation.building_parts[BuildingPartIdentifier.MAIN]
+    assert overlay.roof_construction_type == "Flat"
+    assert overlay.roof_insulation_thickness == 150
+
+
 @pytest.mark.parametrize(
     "roof_type_value",
     [