diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index c12f87d4e..9818c6fa7 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -6,6 +6,7 @@ from datetime import date
 from decimal import ROUND_HALF_UP, Decimal
 from typing import (
     Any,
+    Callable,
     Dict,
     Final,
     List,
@@ -6031,6 +6032,8 @@ def _pashub_room_in_roof(
         details: Optional[List[RoomInRoofSurfaceDetail]],
         kind: Optional[str],
         insulation_thickness_mm: Optional[int] = None,
+        area_of: Callable[[float, float], float] = lambda length, height: length
+        * height,
     ) -> None:
         for d in details or []:
             if d.length_m is None or d.height_m is None:
@@ -6048,7 +6051,7 @@ def _pashub_room_in_roof(
             surfaces.append(
                 SapRoomInRoofSurface(
                     kind=surface_kind,
-                    area_m2=d.length_m * d.height_m,
+                    area_m2=area_of(d.length_m, d.height_m),
                     u_value=d.u_value,
                     insulation_thickness_mm=thickness,
                 )
@@ -6056,12 +6059,20 @@ def _pashub_room_in_roof(
 
     _add(rir.gables, None)
     _add(rir.slopes, "slope", roof_thickness)
-    # "Common wall" lodges as a Detailed-mode stud wall (Table 17 col 3):
-    # kind="common_wall" is a §3.9.2 Simplified-Type-2-only concept (different
-    # area formula, billed at the main-wall U); every worksheet-validated
-    # Detailed §3.10 fixture (000516/000487/000477/000480) uses stud_wall for
-    # this low internal wall.
-    _add(rir.common_walls, "stud_wall", roof_thickness)
+    if rir.roof_room_type is not None:
+        # Simplified approach ("Roof room type" lodged, RdSAP §3.9.2): a
+        # "common wall" is billed as an external wall at the storey-below
+        # main-wall U (SAP kind `common_wall`), spec area L × (0.25 + H) —
+        # NOT a Detailed §3.10 `stud_wall` roof surface, which would drop to
+        # the Table 18 col(4) "all elements uninsulated" default on a no-
+        # loft-access dwelling (78/72 North Road, deals 499538003156/
+        # 499627236560).
+        _add(rir.common_walls, "common_wall", area_of=lambda l, h: l * (0.25 + h))
+    else:
+        # Detailed §3.10: "common wall" lodges as a stud wall (Table 17 col
+        # 3, raw L × H area); every worksheet-validated Detailed fixture
+        # (000516/000487/000477/000480) uses stud_wall for this low wall.
+        _add(rir.common_walls, "stud_wall", roof_thickness)
     _add(rir.flat_ceilings, "flat_ceiling", roof_thickness)
     if rir.floor_area_m2 is None:
         return None