diff --git a/tests/domain/sap10_calculator/worksheet/test_rating.py b/tests/domain/sap10_calculator/worksheet/test_rating.py
index 8eccda886..3867abe9f 100644
--- a/tests/domain/sap10_calculator/worksheet/test_rating.py
+++ b/tests/domain/sap10_calculator/worksheet/test_rating.py
@@ -109,6 +109,29 @@ def test_sap_rating_integer_rounds_to_nearest_and_clamps_to_minimum_one() -> Non
     assert catastrophic == 1
 
 
+def test_sap_rating_integer_rounds_a_half_point_up_not_to_even() -> None:
+    # Arrange — §13 "rounded to the nearest integer" is half-UP under SAP, the
+    # convention the mapper already follows (`_round_half_up_2dp`). Python's
+    # built-in `round` is half-to-EVEN, so it drops an exact x.5 to x whenever x
+    # is even. That bites hardest at the band floors, where one point is a whole
+    # band: a continuous 68.5 must publish as SAP 69 (band C), not 68 (band D).
+    # The linear branch (9) inverts exactly, so these ECFs land on a true .5:
+    #   ECF = (100 − 68.5) / 13.95 → SAP 68.5   (band C floor)
+    #   ECF = (100 − 54.5) / 13.95 → SAP 54.5   (band D floor)
+    ecf_at_band_c_floor = (100.0 - 68.5) / 13.95
+    ecf_at_band_d_floor = (100.0 - 54.5) / 13.95
+    assert sap_rating(ecf=ecf_at_band_c_floor) == 68.5
+    assert sap_rating(ecf=ecf_at_band_d_floor) == 54.5
+
+    # Act
+    at_band_c_floor = sap_rating_integer(ecf=ecf_at_band_c_floor)
+    at_band_d_floor = sap_rating_integer(ecf=ecf_at_band_d_floor)
+
+    # Assert — the half point rounds up into the band above.
+    assert at_band_c_floor == 69
+    assert at_band_d_floor == 55
+
+
 def test_environmental_impact_rating_linear_branch_below_threshold() -> None:
     # Arrange — Equations (10)/(12): CF = CO2/(TFA+45); when CF < 28.3,
     # EI = 100 − 1.34 × CF. For a 100 m² home emitting 1500 kg CO2/yr: