diff --git a/domain/sap10_ml/rdsap_uvalues.py b/domain/sap10_ml/rdsap_uvalues.py
index 44c7e79d..4dc8a40e 100644
--- a/domain/sap10_ml/rdsap_uvalues.py
+++ b/domain/sap10_ml/rdsap_uvalues.py
@@ -837,6 +837,24 @@ def u_roof(
         # ("Average thermal transmittance X W/m²K"); spec §5.11 opening
         # clause defers to the assessor's value when present.
         return measured
+    if (
+        age_band is not None
+        and description is not None
+        and "unknown" in description.lower()
+        and (insulation_thickness_mm is None or insulation_thickness_mm == 0)
+    ):
+        # RdSAP 10 §5.11.4 (page 44): "U-values in Table 18 are used when
+        # thickness of insulation cannot be determined." A roof lodged
+        # "Unknown loft insulation" carries thickness "NI" (Not Indicated,
+        # parsed to 0) or "ND" (None) — the thickness is UNDETERMINED, not
+        # zero — so it takes the Table 18 age-band default (column (1)
+        # pitched / column (3) flat), NOT the uninsulated 2.30 the Table 16
+        # row-0 lookup would give for a parsed-0 thickness. Distinct from a
+        # genuine "no insulation" lodgement, which keeps 2.30 (below). The
+        # discriminator is the deterministic "Unknown" text RdSAP renders
+        # for an undetermined-thickness observation.
+        table_18 = _FLAT_ROOF_BY_AGE if is_flat_roof else _ROOF_BY_AGE
+        return table_18.get(age_band.upper(), 0.4)
     if (
         is_sloping_ceiling
         and age_band is not None
diff --git a/domain/sap10_ml/tests/test_rdsap_uvalues.py b/domain/sap10_ml/tests/test_rdsap_uvalues.py
index 00cf7164..bc06a68c 100644
--- a/domain/sap10_ml/tests/test_rdsap_uvalues.py
+++ b/domain/sap10_ml/tests/test_rdsap_uvalues.py
@@ -821,6 +821,46 @@ def test_u_roof_ni_thickness_with_no_insulation_description_stays_at_2_30() -> N
     assert result == pytest.approx(2.30, abs=0.01)
 
 
+def test_u_roof_unknown_loft_insulation_uses_table18_default_per_section_5_11_4() -> None:
+    # Arrange — "Pitched, Unknown loft insulation" lodges
+    # roof_insulation_thickness 'NI' (Not Indicated, parsed to 0) — the
+    # thickness is UNDETERMINED, not zero. RdSAP 10 §5.11.4 (page 44):
+    # "U-values in Table 18 are used when thickness of insulation cannot
+    # be determined." So a pitched roof takes the Table 18 column (1)
+    # age-band default (age A = 0.40), NOT the uninsulated 2.30 the
+    # Table 16 row-0 lookup gives for a parsed-0 thickness. Cert
+    # 9836-5829-1500-0803-7206 (top-floor flat, age A).
+
+    # Act
+    result = u_roof(
+        country=Country.ENG,
+        age_band="A",
+        insulation_thickness_mm=0,  # parsed from "NI"
+        description="Pitched, Unknown loft insulation",
+    )
+
+    # Assert
+    assert abs(result - 0.40) <= 0.01
+
+
+def test_u_roof_unknown_flat_insulation_uses_table18_flat_column() -> None:
+    # Arrange — an "Unknown" flat-roof lodgement with no determinable
+    # thickness (None) takes Table 18 column (3) "Flat roof" age-band
+    # default (age H = 0.35), per §5.11.4 — not 2.30.
+
+    # Act
+    result = u_roof(
+        country=Country.ENG,
+        age_band="H",
+        insulation_thickness_mm=None,
+        description="Flat, Unknown insulation",
+        is_flat_roof=True,
+    )
+
+    # Assert
+    assert abs(result - 0.35) <= 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.