diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 2a61bebb..caf09b6f 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2433,11 +2433,28 @@ _API_FLOOR_CONSTRUCTION_TO_STR: Dict[int, Optional[str]] = {
 # are observed on cert 001479; the wider RdSAP10 roof-construction
 # enum (1=Flat, 3=Pitched no-access, 5=Vaulted, etc.) is mapped as
 # best-effort against SAP10 nomenclature.
-_API_ROOF_CONSTRUCTION_TO_STR: Dict[int, str] = {
+#
+# Codes 6 and 7 → None. This field is read ONLY for the sloping-ceiling
+# inclined factor; the base roof U-value comes from the global
+# roofs[].description, so a non-sloping code carries no information the
+# cascade consumes here, and None correctly avoids the cos(30°) false-
+# trigger:
+#   6 = "Thatched, with additional insulation" — its U is set by the
+#       global description; not a sloping ceiling.
+#   7 = "(same dwelling above)" / "(another dwelling above)" — an
+#       internal ceiling with no roof heat loss (the roof-side analogue
+#       of floor_construction code 0). Heat loss is governed by the
+#       roof_heat_loss / description path, not this field.
+# Empirically inert: roof W/K is identical whether 6/7 map to None or to
+# an explicit pitched string across all code-6/7 certs in the 2026
+# sample (were raising UnmappedApiCode, blocking the cert).
+_API_ROOF_CONSTRUCTION_TO_STR: Dict[int, Optional[str]] = {
     1: "Flat",
     3: "Pitched (slates/tiles), no access to loft",
     4: "Pitched (slates/tiles), access to loft",
     5: "Pitched (vaulted ceiling)",
+    6: None,
+    7: None,
     8: "Pitched, sloping ceiling",
 }
 
diff --git a/datatypes/epc/domain/tests/test_from_rdsap_schema.py b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
index b0711d9f..5a4796a5 100644
--- a/datatypes/epc/domain/tests/test_from_rdsap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
@@ -879,3 +879,49 @@ class TestDefaultMissingPostTown:
 
         # Assert
         assert result["post_town"] == "BARNSTAPLE"
+
+
+class TestApiRoofConstructionCode:
+    """`_api_roof_construction_str` maps the GOV.UK API integer
+    roof_construction code to the string the cascade reads ONLY for the
+    "sloping ceiling" cos(30°) inclined-surface factor (Slice 89). Codes
+    6 and 7 are neither sloping ceilings nor base-U drivers (the roof
+    U-value comes from the global roofs[].description), so both map to
+    None: code 6 = "Thatched" (its U is set by the description, not this
+    field) and code 7 = "(same/another dwelling above)" — an internal
+    ceiling with no roof heat loss, the roof-side analogue of
+    floor_construction code 0. Empirically inert: roof W/K is identical
+    whether 6/7 map to None or to an explicit pitched string across all
+    code-6/7 certs in the 2026 sample."""
+
+    def test_code_7_same_dwelling_above_maps_to_none(self) -> None:
+        # Arrange
+        from datatypes.epc.domain.mapper import _api_roof_construction_str  # pyright: ignore[reportPrivateUsage]
+
+        # Act
+        result = _api_roof_construction_str(7)
+
+        # Assert — None: no sloping-ceiling signal (avoids the cos(30°)
+        # false-trigger); the internal ceiling has no roof heat loss.
+        assert result is None
+
+    def test_code_6_thatched_maps_to_none(self) -> None:
+        # Arrange
+        from datatypes.epc.domain.mapper import _api_roof_construction_str  # pyright: ignore[reportPrivateUsage]
+
+        # Act
+        result = _api_roof_construction_str(6)
+
+        # Assert — None: thatched is not a sloping ceiling; its U-value is
+        # carried by the global roof description, not roof_construction_type.
+        assert result is None
+
+    def test_code_8_still_maps_to_sloping_ceiling(self) -> None:
+        # Arrange — regression guard: the sloping-ceiling code is unchanged.
+        from datatypes.epc.domain.mapper import _api_roof_construction_str  # pyright: ignore[reportPrivateUsage]
+
+        # Act
+        result = _api_roof_construction_str(8)
+
+        # Assert
+        assert result == "Pitched, sloping ceiling"