diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 98feac78..d09a46e6 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -3993,6 +3993,26 @@ def _api_rir_detailed_surfaces(
                         ),
                     )
                 )
+    # Common walls — billed as external wall at the storey-below main-wall U
+    # (cascade `kind="common_wall"`), so no insulation thickness is read.
+    # Detailed BPs use the raw L × H area (RdSAP 10 §3.9.2; the cascade's
+    # common_wall branch applies the L × (0.25 + H) form only to Simplified
+    # BPs). The cascade deducts this area from the §3.10.1 residual roof.
+    common_wall_specs = (
+        (details.common_wall_length_1, details.common_wall_height_1),
+        (details.common_wall_length_2, details.common_wall_height_2),
+    )
+    for length, height in common_wall_specs:
+        if (
+            length is not None and height is not None
+            and length > 0 and height > 0
+        ):
+            surfaces.append(
+                SapRoomInRoofSurface(
+                    kind="common_wall",
+                    area_m2=_round_half_up_2dp(float(length), float(height)),
+                )
+            )
     if (
         details.flat_ceiling_length_1 is not None
         and details.flat_ceiling_height_1 is not None
diff --git a/datatypes/epc/domain/tests/test_from_rdsap_schema.py b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
index 5d29893a..054e9864 100644
--- a/datatypes/epc/domain/tests/test_from_rdsap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
@@ -2202,22 +2202,29 @@ class TestRoomInRoofDetailedSlopeAndStudWall:
             "stud_wall_length_2": 7.0, "stud_wall_height_2": 1.03,
             "stud_wall_insulation_thickness_1": "75mm",
             "stud_wall_insulation_thickness_2": "75mm",
+            "common_wall_length_1": 8.6, "common_wall_height_1": 1.2,
+            "common_wall_length_2": 8.6, "common_wall_height_2": 1.2,
         }
 
         # Act
         result = EpcPropertyDataMapper.from_api_response(cert)
 
         # Assert — both slopes + both stud walls reach the cascade, with the
-        # lodged thickness parsed and the L × H area to 2 d.p.
+        # lodged thickness parsed and the L × H area to 2 d.p. Common walls
+        # route to the `common_wall` kind (raw L × H, billed at main-wall U).
         rir_part = result.sap_building_parts[0].sap_room_in_roof
         assert rir_part is not None
         surfaces = rir_part.detailed_surfaces
         assert surfaces is not None
         slopes = [s for s in surfaces if s.kind == "slope"]
         studs = [s for s in surfaces if s.kind == "stud_wall"]
+        commons = [s for s in surfaces if s.kind == "common_wall"]
         assert len(slopes) == 2
         assert len(studs) == 2
+        assert len(commons) == 2
         assert abs(slopes[0].area_m2 - 9.8) <= 1e-9
         assert slopes[0].insulation_thickness_mm == 100
         assert abs(studs[0].area_m2 - 7.21) <= 1e-9
         assert studs[0].insulation_thickness_mm == 75
+        assert abs(commons[0].area_m2 - 10.32) <= 1e-9
+        assert commons[0].insulation_thickness_mm is None
diff --git a/datatypes/epc/schema/rdsap_schema_21_0_0.py b/datatypes/epc/schema/rdsap_schema_21_0_0.py
index fdb0d17c..4f7e7e40 100644
--- a/datatypes/epc/schema/rdsap_schema_21_0_0.py
+++ b/datatypes/epc/schema/rdsap_schema_21_0_0.py
@@ -228,6 +228,12 @@ class RoomInRoofDetails:
     stud_wall_insulation_type_2: Optional[int] = None
     stud_wall_insulation_thickness_1: Optional[str] = None
     stud_wall_insulation_thickness_2: Optional[str] = None
+    # §3.9.2 common walls of a Detailed RR — see
+    # `rdsap_schema_21_0_1.RoomInRoofDetails`. Previously dropped.
+    common_wall_length_1: Optional[float] = None
+    common_wall_length_2: Optional[float] = None
+    common_wall_height_1: Optional[float] = None
+    common_wall_height_2: Optional[float] = None
 
 
 @dataclass
diff --git a/datatypes/epc/schema/rdsap_schema_21_0_1.py b/datatypes/epc/schema/rdsap_schema_21_0_1.py
index db6d4c1a..37714034 100644
--- a/datatypes/epc/schema/rdsap_schema_21_0_1.py
+++ b/datatypes/epc/schema/rdsap_schema_21_0_1.py
@@ -267,6 +267,15 @@ class RoomInRoofDetails:
     stud_wall_insulation_type_2: Optional[int] = None
     stud_wall_insulation_thickness_1: Optional[str] = None
     stud_wall_insulation_thickness_2: Optional[str] = None
+    # The §3.9.2 common walls of a Detailed RR (the wall separating the RR
+    # from the rest of the cold roof void). Billed as external wall at the
+    # storey-below main-wall U (cascade `kind="common_wall"`). Detailed BPs
+    # use the raw L × H area (Simplified Type-2 BPs use L × (0.25 + H)).
+    # Previously undeclared → dropped → the RR undercounted wall loss.
+    common_wall_length_1: Optional[float] = None
+    common_wall_length_2: Optional[float] = None
+    common_wall_height_1: Optional[float] = None
+    common_wall_height_2: Optional[float] = None
 
 
 @dataclass