diff --git a/domain/sap10_ml/rdsap_uvalues.py b/domain/sap10_ml/rdsap_uvalues.py
index 845a72e2..1f37b222 100644
--- a/domain/sap10_ml/rdsap_uvalues.py
+++ b/domain/sap10_ml/rdsap_uvalues.py
@@ -857,9 +857,30 @@ def u_roof(
         return u
     if description is not None:
         desc = description.lower()
-        if any(marker in desc for marker in _ROOF_NO_INSULATION_MARKERS):
+        no_insulation = any(marker in desc for marker in _ROOF_NO_INSULATION_MARKERS)
+        limited_insulation = any(
+            marker in desc for marker in _ROOF_LIMITED_INSULATION_MARKERS
+        )
+        if (no_insulation or limited_insulation) and is_flat_roof and age_band is not None:
+            # FLAT roof reached here only when insulation_thickness_mm is None
+            # — the lodged thickness is undetermined ('ND'/'AB'/absent). Per
+            # RdSAP 10 §5.11.4 (PDF p.44) "U-values in Table 18 are used when
+            # thickness of insulation cannot be determined", so the column (3)
+            # flat-roof age-band default applies — NOT the uninsulated 2.30.
+            # The "no/limited insulation" text is RdSAP's as-built rendering:
+            # at old bands (A-D) the column (3) default IS 2.30 (so those certs
+            # are unchanged), but a newer-band flat roof carries the age-band
+            # insulation as built (band H = 0.35, F = 0.68, not 2.30). The
+            # roof's deterministic energy_efficiency_rating confirms it (e.g.
+            # cert 0390-2753 band H lodges rating 3 = moderate U, not the
+            # rating-1 that 2.30 implies). PITCHED roofs are deliberately NOT
+            # routed here — their "no insulation" text is load-bearing (the
+            # broad 'ND'→Table-18 reroute was empirically net-negative for
+            # pitched lofts).
+            return _FLAT_ROOF_BY_AGE.get(age_band.upper(), 0.4)
+        if no_insulation:
             return _ROOF_BY_THICKNESS[0][1]  # 2.30 W/m^2K
-        if any(marker in desc for marker in _ROOF_LIMITED_INSULATION_MARKERS):
+        if limited_insulation:
             return _ROOF_BY_THICKNESS[1][1]  # 1.50 W/m^2K (12mm row)
     if age_band is None:
         return 0.4
diff --git a/domain/sap10_ml/tests/test_rdsap_uvalues.py b/domain/sap10_ml/tests/test_rdsap_uvalues.py
index 2d563dd1..5783af45 100644
--- a/domain/sap10_ml/tests/test_rdsap_uvalues.py
+++ b/domain/sap10_ml/tests/test_rdsap_uvalues.py
@@ -878,6 +878,46 @@ def test_u_roof_unknown_flat_insulation_uses_table18_flat_column() -> None:
     assert abs(result - 0.35) <= 0.01
 
 
+def test_u_roof_flat_no_insulation_undetermined_thickness_uses_table18_by_age() -> None:
+    # Arrange — a flat roof lodged "Flat, no insulation" / "Flat, limited
+    # insulation" with an UNDETERMINED thickness (parsed to None from
+    # 'ND'/'AB') must take the Table 18 column (3) flat-roof age-band
+    # default per RdSAP 10 §5.11.4 (PDF p.44), NOT the uninsulated 2.30.
+    # The "no/limited insulation" text is RdSAP's as-built rendering — at
+    # old bands the column (3) default IS 2.30 (so they're unchanged), but
+    # a newer-band flat roof carries the age-band insulation as built.
+    # Cert 0390-2753 (top-floor flat, band H, "Flat, no insulation",
+    # thickness 'ND', roof rating 3 = moderate) drove a -31.78 SAP error at
+    # the 2.30 value; band H column (3) = 0.35.
+
+    # Act — band H "no insulation" → 0.35; band F "limited insulation" → 0.68;
+    # band C "no insulation" → unchanged 2.30 (column (3) default at C).
+    band_h = u_roof(
+        country=Country.ENG, age_band="H", insulation_thickness_mm=None,
+        description="Flat, no insulation", is_flat_roof=True,
+    )
+    band_f = u_roof(
+        country=Country.ENG, age_band="F", insulation_thickness_mm=None,
+        description="Flat, limited insulation", is_flat_roof=True,
+    )
+    band_c = u_roof(
+        country=Country.ENG, age_band="C", insulation_thickness_mm=None,
+        description="Flat, no insulation", is_flat_roof=True,
+    )
+    # A PITCHED roof "no insulation" with undetermined thickness is NOT
+    # rerouted — its text is load-bearing (2.30 stays).
+    pitched = u_roof(
+        country=Country.ENG, age_band="H", insulation_thickness_mm=None,
+        description="Pitched, no insulation", is_flat_roof=False,
+    )
+
+    # Assert
+    assert abs(band_h - 0.35) <= 0.01
+    assert abs(band_f - 0.68) <= 0.01
+    assert abs(band_c - 2.30) <= 0.01
+    assert abs(pitched - 2.30) <= 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.