diff --git a/packages/domain/src/domain/sap/worksheet/heat_transmission.py b/packages/domain/src/domain/sap/worksheet/heat_transmission.py
index 43d6bef1..06abcde3 100644
--- a/packages/domain/src/domain/sap/worksheet/heat_transmission.py
+++ b/packages/domain/src/domain/sap/worksheet/heat_transmission.py
@@ -420,29 +420,32 @@ def heat_transmission_from_cert(
             for surf in rir.detailed_surfaces:
                 kind = surf.kind
                 area = surf.area_m2
-                rr_detailed_area += area
+                # Only (26)-(30) elements contribute to the external area
+                # aggregate (LINE_31) — gable_wall sits on (32) alongside
+                # the regular party walls, so its area is bookkept under
+                # `party` and excluded from `rr_detailed_area`.
                 if kind == "slope":
+                    rr_detailed_area += area
                     roof += area * u_rr_slope(
                         country=country, age_band=rir.construction_age_band,
                         insulation_thickness_mm=surf.insulation_thickness_mm,
                         insulation_type=surf.insulation_type,
                     )
                 elif kind == "flat_ceiling":
+                    rr_detailed_area += area
                     roof += area * u_rr_flat_ceiling(
                         country=country, age_band=rir.construction_age_band,
                         insulation_thickness_mm=surf.insulation_thickness_mm,
                         insulation_type=surf.insulation_type,
                     )
                 elif kind == "stud_wall":
+                    rr_detailed_area += area
                     roof += area * u_rr_stud_wall(
                         country=country, age_band=rir.construction_age_band,
                         insulation_thickness_mm=surf.insulation_thickness_mm,
                         insulation_type=surf.insulation_type,
                     )
                 elif kind == "gable_wall":
-                    # Treated as party-style at U=0.25 per Table 4. Lines
-                    # 196-197 of the U985 worksheet for 000477 confirm
-                    # this — RR gable walls land under (32) with U=0.25.
                     party += 0.25 * area
         floor += uf * floor_area_total
         party += upw * party_area
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py
index fc3d1290..7666ebd3 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py
@@ -85,11 +85,11 @@ def build_epc() -> EpcPropertyData:
                 ),
                 SapRoomInRoofSurface(
                     kind="slope", area_m2=5.71,
-                    insulation_thickness_mm=0, insulation_type="mineral_wool",
+                    insulation_thickness_mm=0,
                 ),
                 SapRoomInRoofSurface(
                     kind="slope", area_m2=7.02,
-                    insulation_thickness_mm=0, insulation_type="mineral_wool",
+                    insulation_thickness_mm=0,
                 ),
                 SapRoomInRoofSurface(kind="gable_wall", area_m2=7.55),
                 SapRoomInRoofSurface(kind="gable_wall", area_m2=7.55),
@@ -106,7 +106,12 @@ def build_epc() -> EpcPropertyData:
         sap_building_parts=[main],
         habitable_rooms_count=4,
         heated_rooms_count=4,
-        door_count=1,  # U985 line 42: single "Doors uninsulated" entry @ 3.70
+        # 000477 line 42 lodges total door area 3.70 m² = 2 × _DEFAULT_DOOR_
+        # AREA_M2 (1.85). Worksheet vocabulary calls it one "Doors
+        # uninsulated 1" entry but the area resolves to 2 physical doors
+        # (front + back, typical mid-terrace). U=3.0 (Table 26 age B
+        # uninsulated) × 3.70 m² = 11.10 W/K matches LINE_26 exactly.
+        door_count=2,
         percent_draughtproofed=100,
         low_energy_fixed_lighting_bulbs_count=SECTION_5_BULB_COUNT_LEL,
         sap_windows=list(SECTION_6_VERTICAL_WINDOWS),
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 2aa26947..732e51d2 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
@@ -1530,3 +1530,78 @@ def test_room_in_roof_detailed_per_surface_lodgement_routes_each_to_correct_line
     assert result.roof_w_per_k == pytest.approx(expected_roof, abs=0.001)
     assert result.party_walls_w_per_k == pytest.approx(expected_party, abs=0.001)
     assert result.walls_w_per_k == pytest.approx(expected_walls, abs=0.001)
+
+
+def test_room_in_roof_detailed_gable_wall_excluded_from_line_31_external_area() -> None:
+    """LINE_31 = Σ(26)-(30) net areas — the external-elements aggregate
+    that drives (36) thermal-bridging. Per the U985 worksheet, RR gable
+    walls sit on line (32) alongside party walls (Table 4 "as common
+    wall"), so their area must NOT contribute to LINE_31. Pre-fix the
+    detailed-RR loop summed every surface (including gable_wall) into
+    the external-area accumulator, overcounting LINE_31 by the gable
+    area and inflating (36) bridging by `y × A_gable`.
+
+    Synthetic dwelling (mirrors the routing test above; LINE_31 is
+    now the assertion):
+      - 1 storey × 40 m², 24 m perim, 2.5 m height → gross_wall 60,
+        net_wall 60 (no openings).
+      - Roof = ground_floor − RR_floor = 40 − 10 = 30 m².
+      - Ground floor = 40 m².
+      - RR detailed surfaces: slope 10 + stud_wall 5 + flat_ceiling 8
+        (on (30)) + gable_wall 7 (on (32)).
+
+    Expected LINE_31 = net_wall 60 + roof 30 + floor 40 + windows 0 +
+                       doors 0 + rr_external(slope+stud+flat) 23
+                     = 153 m². Pre-fix this would have been 153 + 7
+                     = 160 m² (gable double-counted).
+    """
+    # Arrange
+    main = make_building_part(
+        identifier=BuildingPartIdentifier.MAIN,
+        construction_age_band="B",
+        wall_construction=4,
+        wall_insulation_type=4,
+        party_wall_construction=1,
+        roof_construction=4,
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=40.0, room_height_m=2.5,
+                heat_loss_perimeter_m=24.0, party_wall_length_m=0.0, floor=0,
+            ),
+        ],
+        sap_room_in_roof=SapRoomInRoof(
+            floor_area=10.0, construction_age_band="B",
+            detailed_surfaces=[
+                SapRoomInRoofSurface(
+                    kind="slope", area_m2=10.0,
+                    insulation_thickness_mm=100, insulation_type="mineral_wool",
+                ),
+                SapRoomInRoofSurface(
+                    kind="stud_wall", area_m2=5.0,
+                    insulation_thickness_mm=100, insulation_type="mineral_wool",
+                ),
+                SapRoomInRoofSurface(
+                    kind="flat_ceiling", area_m2=8.0,
+                    insulation_thickness_mm=200, insulation_type="mineral_wool",
+                ),
+                SapRoomInRoofSurface(kind="gable_wall", area_m2=7.0),
+            ],
+        ),
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=50.0,
+        habitable_rooms_count=3,
+        country_code="ENG",
+        sap_building_parts=[main],
+    )
+
+    # Act
+    result = heat_transmission_from_cert(
+        epc, window_total_area_m2=0.0, window_avg_u_value=None, door_count=0,
+    )
+
+    # Assert
+    expected_line_31 = 60.0 + 30.0 + 40.0 + 0.0 + 0.0 + (10.0 + 5.0 + 8.0)
+    assert result.total_external_element_area_m2 == pytest.approx(
+        expected_line_31, abs=0.001
+    )