diff --git a/packages/domain/src/domain/ml/rdsap_uvalues.py b/packages/domain/src/domain/ml/rdsap_uvalues.py
index a38d05b7..27069827 100644
--- a/packages/domain/src/domain/ml/rdsap_uvalues.py
+++ b/packages/domain/src/domain/ml/rdsap_uvalues.py
@@ -417,6 +417,14 @@ def u_roof(
        as uninsulated.
     3. Table 18 age-band default.
     """
+    if insulation_thickness_mm == 0 and _described_as_insulated(description):
+        # Spec §5.11.4 (page 44 footnote): "If retrofit insulation
+        # present of unknown thickness use 50 mm". The cert encodes
+        # "thickness unknown but retrofit insulation present" as the
+        # "NI" sentinel which `_parse_thickness_mm` parses to 0. Without
+        # this override the Table 16 row-0 lookup below returns the
+        # uninsulated 2.30 W/m²K.
+        return 0.68  # Table 16 row 50, "Insulation at joists at ceiling level"
     if insulation_thickness_mm is not None:
         # nearest tabulated thickness <= supplied
         u = _ROOF_BY_THICKNESS[0][1]
diff --git a/packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py b/packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py
index 90c13cce..afc96334 100644
--- a/packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py
+++ b/packages/domain/src/domain/ml/tests/test_rdsap_uvalues.py
@@ -466,6 +466,49 @@ def test_u_wall_uses_rdsap_unknown_thickness_default_of_50mm_when_insulated_but_
 # ----- Roofs -----
 
 
+def test_u_roof_ni_thickness_with_insulated_description_applies_50mm_per_section_5_11_4() -> None:
+    # Arrange — 346 corpus certs lodge roof_insulation_thickness="NI"
+    # (Not Indicated, parsed to 0 by _parse_thickness_mm). When the
+    # description also signals retrofit insulation ("Pitched, insulated
+    # (assumed)" / "Flat, insulated" / "Roof room(s), insulated
+    # (assumed)"), RdSAP 10 §5.11.4 (page 44) footnote applies:
+    #   "If retrofit insulation present of unknown thickness use 50 mm".
+    # That maps to Table 16 row "50 mm at joists at ceiling level" = 0.68
+    # W/m²K — vs the current 2.30 we return when thickness=0 hits the
+    # Table 16 row-0 lookup.
+
+    # Act
+    result = u_roof(
+        country=Country.ENG,
+        age_band="C",
+        insulation_thickness_mm=0,  # parsed from "NI"
+        description="Pitched, insulated (assumed)",
+    )
+
+    # Assert
+    assert result == pytest.approx(0.68, abs=0.01)
+
+
+def test_u_roof_ni_thickness_with_no_insulation_description_stays_at_2_30() -> None:
+    # Arrange — 706 corpus certs lodge "Pitched, no insulation
+    # (assumed)" which can co-occur with thickness="NI". The
+    # description-based override for retrofit-insulated roofs must
+    # respect the "no insulation" negation: `_described_as_insulated`
+    # returns False on "no insulation" substring, so the Table 16
+    # row-0 lookup applies and U = 2.30 W/m²K stays.
+
+    # Act
+    result = u_roof(
+        country=Country.ENG,
+        age_band="C",
+        insulation_thickness_mm=0,
+        description="Pitched, no insulation (assumed)",
+    )
+
+    # Assert
+    assert result == pytest.approx(2.30, abs=0.01)
+
+
 def test_u_roof_age_band_j_pitched_returns_table18_value() -> None:
     # Arrange — Table 18, pitched insulation between joists, age J -> 0.16 W/m^2K.
 
diff --git a/packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py b/packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py
index cf5950f6..fdbed35a 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/test_heat_transmission.py
@@ -31,6 +31,51 @@ from domain.sap.worksheet.heat_transmission import (
 )
 
 
+def test_roof_insulated_assumed_with_ni_thickness_uses_50mm_per_section_5_11_4() -> None:
+    # Arrange — 346 corpus certs lodge roof_insulation_thickness="NI"
+    # with descriptions like "Pitched, insulated (assumed)". The
+    # retrofit-insulation signal in the description must flow from
+    # epc.roofs[0].description through heat_transmission_from_cert
+    # into u_roof so the 50 mm assumption fires per RdSAP 10 §5.11.4.
+    # Geometry: 100 m² ground floor → top floor area also 100 m² →
+    # roof_area = 100 m². roof_w_per_k expected = 0.68 × 100 = 68
+    # W/K (vs uninsulated 2.30 × 100 = 230 W/K).
+    main = make_building_part(
+        identifier="Main Dwelling",
+        construction_age_band="C",
+        wall_construction=3,
+        wall_insulation_type=4,
+        party_wall_construction=1,
+        roof_construction=4,
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=100.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=40.0, floor=0,
+            ),
+        ],
+    )
+    # roof_insulation_thickness "NI" mimics the cert field — parses to 0.
+    main.roof_insulation_thickness = "NI"
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=100.0,
+        country_code="ENG",
+        sap_building_parts=[main],
+    )
+    epc.roofs = [
+        EnergyElement(
+            description="Pitched, insulated (assumed)",
+            energy_efficiency_rating=4,
+            environmental_efficiency_rating=4,
+        ),
+    ]
+
+    # Act
+    result = heat_transmission_from_cert(epc)
+
+    # Assert
+    assert result.roof_w_per_k == pytest.approx(68.0, abs=2.0)
+
+
 def test_floor_insulated_assumed_with_ni_thickness_uses_50mm_per_table19_footnote() -> None:
     # Arrange — 2 413 corpus certs lodge floors with thickness="NI" and
     # description "Solid, insulated (assumed)". The retrofit-insulation