diff --git a/domain/sap10_calculator/worksheet/heat_transmission.py b/domain/sap10_calculator/worksheet/heat_transmission.py
index dff61a08..a961d60f 100644
--- a/domain/sap10_calculator/worksheet/heat_transmission.py
+++ b/domain/sap10_calculator/worksheet/heat_transmission.py
@@ -980,21 +980,28 @@ def heat_transmission_from_cert(
         # U_main_wall per spec page 23 ("Common wall U-value is inferred
         # from the U-value of the main wall in the building part below";
         # gables fall under the same Table 4 rule).
-        rr_a_rr = geom["rr_simplified_a_rr_m2"]
-        rr_common = geom["rr_common_wall_area_m2"]
-        rr_gable = geom["rr_gable_area_m2"]
+        # `rr_roof_area` is the worksheet's simplified A_RR (the notional
+        # room-in-roof roof area, RdSAP 10 §3.10.1). Its perimeter common
+        # walls + gables are billed to walls; the leftover residual is the
+        # roof-going area that takes the roof U-value.
+        rr_roof_area = geom["rr_simplified_a_rr_m2"]
+        rr_common_wall_area = geom["rr_common_wall_area_m2"]
+        rr_gable_area = geom["rr_gable_area_m2"]
         rr_detailed_area = 0.0
-        if rr_a_rr > 0:
+        if rr_roof_area > 0:
             rir = part.sap_room_in_roof
-            assert rir is not None  # rr_a_rr > 0 ⇒ rir present per _part_geometry
-            walls += uw * (rr_common + rr_gable)
+            assert rir is not None  # rr_roof_area > 0 ⇒ rir present per _part_geometry
+            walls += uw * (rr_common_wall_area + rr_gable_area)
             # Deduct any "Roof of Room" rooflights piercing the RR shell
             # (see `rw_area_on_rr` rationale at the gross-roof block).
-            a_rr_final = max(0.0, rr_a_rr - rr_common - rr_gable - rw_area_on_rr)
+            rr_residual_roof_area = max(
+                0.0,
+                rr_roof_area - rr_common_wall_area - rr_gable_area - rw_area_on_rr,
+            )
             u_rr = u_rr_default_all_elements(
                 country=country, age_band=rir.construction_age_band,
             )
-            roof += u_rr * a_rr_final
+            roof += u_rr * rr_residual_roof_area
         elif part.sap_room_in_roof is not None and part.sap_room_in_roof.detailed_surfaces:
             # RdSAP10 §3.10 Detailed RR — iterate per-surface lodgement.
             # Slope / flat_ceiling / stud_wall route to roof (worksheet
@@ -1007,7 +1014,8 @@ def heat_transmission_from_cert(
             # band". Wall-going RIR surfaces (gable_wall, gable_wall_
             # external, common_wall) deduct from the simplified A_RR
             # to leave the residual area, mirroring the Simplified
-            # branch's `a_rr_final = rr_a_rr - rr_common - rr_gable`.
+            # branch's `rr_residual_roof_area = rr_roof_area -
+            # rr_common_wall_area - rr_gable_area`.
             # Roof-going surfaces (slope / flat_ceiling / stud_wall)
             # do NOT deduct — they sit inside the RR shell rather than
             # forming its perimeter walls.
@@ -1179,7 +1187,7 @@ def heat_transmission_from_cert(
             main_wall_area
             + (alt_walls_total_area - alt_window_area)
             + roof_area + floor_area_total
-            + w_area + d_area + rw_area_part + rr_a_rr + rr_detailed_area
+            + w_area + d_area + rw_area_part + rr_roof_area + rr_detailed_area
             + cantilever_area
         )
         total_external_area += part_external_area