diff --git a/tests/repositories/epc/test_epc_round_trip.py b/tests/repositories/epc/test_epc_round_trip.py
index 5def5e7e8..d18a2bc0d 100644
--- a/tests/repositories/epc/test_epc_round_trip.py
+++ b/tests/repositories/epc/test_epc_round_trip.py
@@ -240,3 +240,45 @@ def test_building_part_wall_insulation_thickness_preserves_int(
     value = reloaded.sap_building_parts[0].wall_insulation_thickness
     assert value == 100
     assert isinstance(value, int)
+
+
+def test_rafter_insulation_thickness_round_trips(db_engine: Engine) -> None:
+    # RdSAP 10 §5.11.2 — a roof insulated AT RAFTERS lodges its insulation
+    # thickness in `rafter_insulation_thickness`, NOT `roof_insulation_thickness`
+    # (which stays null for rafter roofs, since rafters aren't loft joists).
+    # `heat_transmission.py:922` prefers it when `roof_insulation_location == 1`
+    # so the MEASURED Table 16 column (2) row applies. It had no DB column, so it
+    # was silently dropped on save and every at-rafters roof fell back to the
+    # Table 18 column (2) unknown-thickness default (persist != score).
+    #
+    # Real impact: property 749719 lodges "250mm" -> Table 16 col (2) U 0.23.
+    # Dropped, the roof bills at U 2.30 — a 10x error worth -9.8 SAP (lodged 73,
+    # calculated 63.2) that moved the dwelling a whole band, C -> D.
+    #
+    # Note this is an RdSAP INPUT (an insulation thickness feeding the standard
+    # cascade), not a lodged U-value. The sibling `wall/roof/floor_u_value` and
+    # `wall_is_basement` fields are full-SAP artefacts and are deliberately NOT
+    # persisted — see the `_UNPERSISTED_ALLOWLIST` note in
+    # test_epc_persistence_field_coverage.py.
+    from dataclasses import replace
+
+    # Arrange — a green fixture carrying a measured at-rafters thickness.
+    original = _load_epc("RdSAP-Schema-21.0.1")
+    assert original.sap_building_parts, "fixture must have a building part"
+    bp0 = replace(
+        original.sap_building_parts[0],
+        rafter_insulation_thickness="250mm",
+    )
+    original = replace(
+        original, sap_building_parts=[bp0, *original.sap_building_parts[1:]]
+    )
+
+    # Act
+    with Session(db_engine) as session:
+        epc_property_id = EpcPostgresRepository(session).save(original)
+        session.commit()
+    with Session(db_engine) as session:
+        reloaded = EpcPostgresRepository(session).get(epc_property_id)
+
+    # Assert — the measured rafter thickness survives, deep-equal.
+    assert reloaded == original