diff --git a/domain/epc/property_overrides/roof_type.py b/domain/epc/property_overrides/roof_type.py
index 56ef9e8ed..bc3073f86 100644
--- a/domain/epc/property_overrides/roof_type.py
+++ b/domain/epc/property_overrides/roof_type.py
@@ -22,6 +22,28 @@ class RoofType(Enum):
     FLAT_NO_INSULATION = "Flat, no insulation"
     FLAT_NO_INSULATION_ASSUMED = "Flat, no insulation (assumed)"
 
+    # A flat roof with a known insulation depth. RdSAP 10 Table 16 col (1)
+    # ("joists at ceiling level and flat roof") scores flat roofs by thickness, so
+    # these carry the depth into the overlay (mirroring the PITCHED_LOFT_* ladder;
+    # no "loft" — a flat roof has none). ADR-0041. The value column is an FE-owned
+    # pgEnum, so these members are added to the Drizzle enum by the FE owner.
+    FLAT_12MM = "Flat, 12 mm insulation"
+    FLAT_25MM = "Flat, 25 mm insulation"
+    FLAT_50MM = "Flat, 50 mm insulation"
+    FLAT_75MM = "Flat, 75 mm insulation"
+    FLAT_100MM = "Flat, 100 mm insulation"
+    FLAT_125MM = "Flat, 125 mm insulation"
+    FLAT_150MM = "Flat, 150 mm insulation"
+    FLAT_175MM = "Flat, 175 mm insulation"
+    FLAT_200MM = "Flat, 200 mm insulation"
+    FLAT_225MM = "Flat, 225 mm insulation"
+    FLAT_250MM = "Flat, 250 mm insulation"
+    FLAT_270MM = "Flat, 270 mm insulation"
+    FLAT_300MM = "Flat, 300 mm insulation"
+    FLAT_350MM = "Flat, 350 mm insulation"
+    FLAT_400MM = "Flat, 400 mm insulation"
+    FLAT_400_PLUS = "Flat, 400+ mm insulation"
+
     PITCHED_INSULATED = "Pitched, insulated"
     PITCHED_INSULATED_ASSUMED = "Pitched, insulated (assumed)"
     PITCHED_INSULATED_AT_RAFTERS = "Pitched, insulated at rafters"
diff --git a/tests/domain/epc/test_roof_type_overlay.py b/tests/domain/epc/test_roof_type_overlay.py
index 75aab9204..1c39ec9f7 100644
--- a/tests/domain/epc/test_roof_type_overlay.py
+++ b/tests/domain/epc/test_roof_type_overlay.py
@@ -11,6 +11,26 @@ import pytest
 
 from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
 from domain.epc.property_overlays.roof_type_overlay import roof_overlay_for
+from domain.epc.property_overrides.roof_type import RoofType
+
+_FLAT_THICKNESS_MEMBERS: list[tuple[RoofType, int]] = [
+    (RoofType.FLAT_12MM, 12),
+    (RoofType.FLAT_25MM, 25),
+    (RoofType.FLAT_50MM, 50),
+    (RoofType.FLAT_75MM, 75),
+    (RoofType.FLAT_100MM, 100),
+    (RoofType.FLAT_125MM, 125),
+    (RoofType.FLAT_150MM, 150),
+    (RoofType.FLAT_175MM, 175),
+    (RoofType.FLAT_200MM, 200),
+    (RoofType.FLAT_225MM, 225),
+    (RoofType.FLAT_250MM, 250),
+    (RoofType.FLAT_270MM, 270),
+    (RoofType.FLAT_300MM, 300),
+    (RoofType.FLAT_350MM, 350),
+    (RoofType.FLAT_400MM, 400),
+    (RoofType.FLAT_400_PLUS, 400),
+]
 
 
 def test_pitched_loft_depth_maps_to_roof_insulation_thickness() -> None:
@@ -56,6 +76,24 @@ def test_flat_roof_depth_maps_to_roof_insulation_thickness() -> None:
     assert overlay.roof_insulation_thickness == 150
 
 
+@pytest.mark.parametrize(("member", "expected_mm"), _FLAT_THICKNESS_MEMBERS)
+def test_every_flat_thickness_member_resolves_to_its_depth(
+    member: RoofType, expected_mm: int
+) -> None:
+    # Each FLAT_*MM taxonomy member must stay lock-step with the overlay: its value
+    # resolves to the depth it names (so a landlord-stored member scores via Table
+    # 16 col (1), not the age-band default). ADR-0041.
+
+    # Act
+    simulation = roof_overlay_for(member.value, 0)
+
+    # Assert
+    assert simulation is not None
+    overlay = simulation.building_parts[BuildingPartIdentifier.MAIN]
+    assert overlay.roof_construction_type == "Flat"
+    assert overlay.roof_insulation_thickness == expected_mm
+
+
 @pytest.mark.parametrize(
     "roof_type_value",
     [