diff --git a/domain/sap10_calculator/rdsap/tests/test_golden_fixtures.py b/domain/sap10_calculator/rdsap/tests/test_golden_fixtures.py
index 9ef94470..e8cb2bc8 100644
--- a/domain/sap10_calculator/rdsap/tests/test_golden_fixtures.py
+++ b/domain/sap10_calculator/rdsap/tests/test_golden_fixtures.py
@@ -74,9 +74,9 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
     _GoldenExpectation(
         cert_number="0240-0200-5706-2365-8010",
         actual_sap=73,
-        expected_sap_resid=-14,
-        expected_pe_resid_kwh_per_m2=+12.4933,
-        expected_co2_resid_tonnes_per_yr=+0.6957,
+        expected_sap_resid=-10,
+        expected_pe_resid_kwh_per_m2=+0.0542,
+        expected_co2_resid_tonnes_per_yr=+0.0626,
         notes=(
             "Detached house, TFA 118, age J, oil boiler PCDB-listed + PV + "
             "RR on BP[0]. Mapper DOES extract sap_room_in_roof.room_in_roof_"
diff --git a/domain/sap10_calculator/worksheet/heat_transmission.py b/domain/sap10_calculator/worksheet/heat_transmission.py
index 71abd946..1c0a6f0c 100644
--- a/domain/sap10_calculator/worksheet/heat_transmission.py
+++ b/domain/sap10_calculator/worksheet/heat_transmission.py
@@ -636,7 +636,8 @@ def heat_transmission_from_cert(
             or _described_as_insulated(wall_description)
         )
         party_construction = _int_or_none(part.party_wall_construction)
-        roof_thickness = _parse_thickness_mm(getattr(part, "roof_insulation_thickness", None))
+        raw_roof_thickness = getattr(part, "roof_insulation_thickness", None)
+        roof_thickness = _parse_thickness_mm(raw_roof_thickness)
         floor_ins_thickness = _parse_thickness_mm(getattr(part, "floor_insulation_thickness", None))
 
         ground_fd = next(
@@ -676,15 +677,27 @@ def heat_transmission_from_cert(
             )
         # When the per-bp `roof_insulation_thickness` is explicitly lodged
         # as 0 (uninsulated — e.g. cert 001479 Ext2 PS sloping ceiling
-        # age C from Slice 91's `_api_resolve_sloping_ceiling_thickness`)
-        # the global `epc.roofs[].description` ("Pitched, insulated" from
-        # another bp) must NOT override the per-bp truth via u_roof's
-        # Table 18 footnote (2) assumed-insulation path. Drop the
-        # description in that case so the cascade returns the spec
-        # uninsulated U-value (Table 18 row 0). Cohort Summary mappers
-        # leave `epc.roofs` empty so description is None there anyway.
+        # age C from Slice 91's `_api_resolve_sloping_ceiling_thickness`,
+        # which returns the int 0 sentinel) the global
+        # `epc.roofs[].description` ("Pitched, insulated" from another bp)
+        # must NOT override the per-bp truth via u_roof's Table 18 footnote
+        # (2) assumed-insulation path. Drop the description in that case so
+        # the cascade returns the spec uninsulated U-value (Table 18 row 0).
+        # Cohort Summary mappers leave `epc.roofs` empty so description is
+        # None there anyway.
+        #
+        # The "NI" string sentinel is the OPPOSITE signal — it means
+        # "thickness not indicated; defer to description" per RdSAP 10
+        # §5.11.4 (PDF p.44): "If retrofit insulation present of unknown
+        # thickness use 50 mm". `_parse_thickness_mm` collapses BOTH int(0)
+        # and "NI" to 0, so we distinguish by inspecting the RAW lodgement
+        # value before the parse — explicit `int(0)` drops the description,
+        # `"NI"` keeps it so `u_roof`'s §5.11.4 branch can fire.
+        roof_thickness_explicitly_zero = (
+            isinstance(raw_roof_thickness, int) and raw_roof_thickness == 0
+        )
         effective_roof_description = (
-            None if roof_thickness == 0 else roof_description
+            None if roof_thickness_explicitly_zero else roof_description
         )
         # RdSAP 10 §5.11 Table 18 page 45: column (3) "Flat roof" applies
         # when the per-bp roof construction lodges as a flat roof and the