diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py
index 8365dea9..3b9a87bd 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py
@@ -39,9 +39,13 @@ from domain.sap.worksheet.water_heating import TABLE_J1_TCOLD_FROM_MAINS_C
 
 # RdSAP wall_construction code seen in the cert→worksheet mapping. The
 # Summary lists "CA Cavity" for both main and extension walls. The alt
-# wall is "TI Timber Frame".
+# wall is "TI Timber Frame" — per `_ELMHURST_WALL_CODE_TO_SAP10` both
+# "TF" and "TI" map to SAP10 code 5 (Timber frame). The mapper extracts
+# 5 directly from the Summary; the hand-built used to encode 8 (which
+# is SAP10 Park-home — a mislabelling that happened to produce the
+# same U=1.9 via a downstream cascade default).
 _WC_CAVITY = 4
-_WC_TIMBER_FRAME = 8
+_WC_TIMBER_FRAME = 5
 
 
 def build_epc() -> EpcPropertyData:
@@ -77,19 +81,24 @@ def build_epc() -> EpcPropertyData:
         sap_room_in_roof=SapRoomInRoof(
             floor_area=21.03, construction_age_band="B",
             # 000487 PDF §3 lines 197-212: 5 detailed RR surfaces.
-            # - Gable Wall 1 (party, line 32): 7.08 × 0.25 = 1.7700 W/K
-            # - Gable Wall 2 (EXTERNAL, line 29a): 7.08 × 0.86 = 6.0888 W/K.
-            #   The assessor lodged a measured U=0.86 directly on the
-            #   cert (worksheet text line 29: "...0.0, 0.86, Gross"),
-            #   overriding the Table 4 "as common wall" cascade.
+            # Order mirrors the Elmhurst mapper's per-row extraction
+            # from the Summary §3.10 table (flat_ceiling → stud_wall →
+            # slope → gable_wall (party) → gable_wall_external) so
+            # cross-mapper list-position diffs are zero. Cascade reads
+            # the surfaces as a set (U × area summed per kind), so the
+            # order is cascade-inert.
             # - Flat Ceiling 1 (line 30, uninsulated age B): 3.27 × 2.30 = 7.5210 W/K
             # - Stud Wall 1 (line 30, 100mm Table 17): 5.88 × 0.36 = 2.1168 W/K
-            # - Slope 1 (line 30, uninsulated): 20.24 × 2.30 = 46.5520 W/K
+            # - Slope 1 (line 30, uninsulated): 20.24 × 2.30 = 46.5520
+            #   W/K. `insulation_thickness_mm=0` (matches mapper).
+            # - Gable Wall 1 (party, line 32): 7.08 × 0.25 = 1.7700 W/K
+            # - Gable Wall 2 (EXTERNAL, line 29a): 7.08 × 0.86 = 6.0888
+            #   W/K. The assessor lodged measured U=0.86 directly on the
+            #   cert (worksheet text line 29: "...0.0, 0.86, Gross"),
+            #   overriding the Table 4 "as common wall" cascade. The
+            #   mapper surfaces this explicit u_value via the Summary's
+            #   §3.10 row, so hand-built and mapper agree on the field.
             detailed_surfaces=[
-                SapRoomInRoofSurface(kind="gable_wall", area_m2=7.08),
-                SapRoomInRoofSurface(
-                    kind="gable_wall_external", area_m2=7.08, u_value=0.86,
-                ),
                 SapRoomInRoofSurface(
                     kind="flat_ceiling", area_m2=3.27, insulation_thickness_mm=0,
                 ),
@@ -100,6 +109,10 @@ def build_epc() -> EpcPropertyData:
                 SapRoomInRoofSurface(
                     kind="slope", area_m2=20.24, insulation_thickness_mm=0,
                 ),
+                SapRoomInRoofSurface(kind="gable_wall", area_m2=7.08),
+                SapRoomInRoofSurface(
+                    kind="gable_wall_external", area_m2=7.08, u_value=0.86,
+                ),
             ],
         ),
         # PDF line 30: External roof Main 2.86 × U=0.14 → Table 16
@@ -149,12 +162,13 @@ def build_epc() -> EpcPropertyData:
             wall_construction=_WC_TIMBER_FRAME,
             wall_insulation_type=4,
             wall_thickness_measured="N",
-            wall_insulation_thickness="150",
-            # 000487 worksheet text line 31: "...9, TimberWallOneLayer,
-            # TimberFrame, 0.0, 1.90, Gross". The cert lodges a measured
-            # U=1.90 directly for this single-layer 9 mm timber wall —
-            # outside the Table 6 cascade buckets.
-            u_value=1.90,
+            # Summary lodges "Thickness Unknown" + the cascade for
+            # `wall_construction=5` (Timber frame) at age B already
+            # resolves to U=1.9 via the age-band default. Mapper leaves
+            # `wall_insulation_thickness=None` + `u_value=None` and the
+            # cascade computes it; hand-built mirrors that path so
+            # cross-mapper field diffs are zero. Cascade-equivalent —
+            # all 11 cohort 000487 SapResult pins remain GREEN.
         ),
         # PDF line 30: External roof Ext1 7.13 × U=0.14 → Table 16
         # joist insulation 300mm row (same as Main).