diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index bf29c169..432a5abe 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -1455,6 +1455,7 @@ class EpcPropertyDataMapper:
                         bp.roof_construction,
                         bp.roof_insulation_thickness,
                         bp.construction_age_band,
+                        bp.sloping_ceiling_insulation_thickness,
                     ),
                     sap_room_in_roof=_api_build_room_in_roof(
                         bp.sap_room_in_roof,
@@ -1730,6 +1731,7 @@ class EpcPropertyDataMapper:
                         bp.roof_construction,
                         bp.roof_insulation_thickness,
                         bp.construction_age_band,
+                        bp.sloping_ceiling_insulation_thickness,
                     ),
                     sap_room_in_roof=_api_build_room_in_roof(
                         bp.sap_room_in_roof,
@@ -3203,25 +3205,53 @@ def _api_resolve_wall_insulation_thickness(
     return wall_insulation_thickness
 
 
+def _api_thickness_is_numeric(value: Union[str, int, None]) -> bool:
+    """True when an insulation-thickness lodgement carries a measured value
+    (an int, or a string whose leading characters are digits, e.g. "100mm").
+    Categorical sentinels ("AB" As Built, "NI" Not Insulated) and None are
+    NOT numeric. Mirrors the cascade's `_parse_thickness_mm` digit-prefix
+    rule so the two agree on what counts as an observed thickness."""
+    if isinstance(value, int):
+        return True
+    return isinstance(value, str) and value.strip()[:1].isdigit()
+
+
 def _api_resolve_sloping_ceiling_thickness(
     roof_construction: Optional[int],
     roof_insulation_thickness: Union[str, int, None],
     age_band: Optional[str],
+    sloping_ceiling_insulation_thickness: Union[str, int, None] = None,
 ) -> Union[str, int, None]:
-    """Apply Slice 57's pre-1950 sloping-ceiling-roof rule to the API
-    path: when a "Pitched, sloping ceiling" roof carries no insulation
-    thickness lodgement on a pre-1950 dwelling (age bands A-D), set
-    the thickness to 0 mm so the cascade's `u_roof` returns the
-    uninsulated Table 16 row (U=2.30) rather than the age-band default
-    (e.g. U=0.40 for age C pitched-with-loft). Mirrors the Elmhurst
-    `_resolve_sloping_ceiling_thickness` for the API code-based path.
+    """Resolve the roof-insulation thickness the cascade should see for a
+    "Pitched, sloping ceiling" (`roof_construction == 8`) API building part.
 
-    Observed on cert 001479 Ext2: age C, roof_construction=8 (PS),
-    roof_insulation_thickness=None — worksheet U=2.30 (uninsulated PS
-    sloping ceiling); without this rule the cascade returns U=0.40."""
+    A code-8 roof's ceiling follows the slope, so its insulation is lodged
+    in the dedicated `sloping_ceiling_insulation_thickness` field, NOT
+    `roof_insulation_thickness` (which stays None — the loft-joist field is
+    meaningless for a slope-following ceiling). When that field carries a
+    NUMERIC thickness it wins: feeding e.g. "100mm" lets `u_roof` reach
+    Table 17 column (1a) "Insulated slope – sloping ceiling, mineral
+    wool/EPS" (RdSAP 10 §5.11.3 page 44 — 100 mm → U=0.40), instead of
+    treating the slope as uninsulated (U=2.30). Cert 9884-3059-9202-7506
+    (code 8, age B, sloping 100 mm) over-stated roof heat loss ~74% before
+    this preference. A categorical lodgement ("AB" As Built / "NI") is NOT
+    a measured thickness, so it falls through to the as-built rule below
+    (Table 18 column (3) age-band default via `is_pitched_sloping_ceiling`,
+    or the description signal) rather than masking it.
+
+    Otherwise the original Slice 57 rule applies: a code-8 roof with NO
+    thickness lodged anywhere on a pre-1950 dwelling (age bands A-D) gets
+    0 mm so `u_roof` returns the uninsulated Table 16 row (U=2.30) rather
+    than the age-band default. Observed on cert 001479 Ext2 (age C, code 8,
+    both thickness fields None) — worksheet U=2.30."""
+    if (
+        roof_construction == 8  # 8 = Pitched, sloping ceiling
+        and _api_thickness_is_numeric(sloping_ceiling_insulation_thickness)
+    ):
+        return sloping_ceiling_insulation_thickness
     if roof_insulation_thickness is not None:
         return roof_insulation_thickness
-    if roof_construction != 8:  # 8 = Pitched, sloping ceiling
+    if roof_construction != 8:
         return roof_insulation_thickness
     if age_band is None or age_band.upper() not in _PRE_1950_AGE_CODES:
         return roof_insulation_thickness
diff --git a/datatypes/epc/domain/tests/test_from_rdsap_schema.py b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
index c1ef3ad3..c9fe69b6 100644
--- a/datatypes/epc/domain/tests/test_from_rdsap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
@@ -765,6 +765,81 @@ class TestApiResolveWallInsulationThickness:
         assert resolved == lodged_thickness
 
 
+class TestApiResolveSlopingCeilingThickness:
+    """A "Pitched, sloping ceiling" (`roof_construction == 8`) lodges its
+    insulation in the dedicated `sloping_ceiling_insulation_thickness`
+    field, NOT `roof_insulation_thickness` (which stays None — the loft-
+    joist field is meaningless for a slope-following ceiling). The cascade
+    must read the sloping-ceiling field so it reaches Table 17 column (1a)
+    (RdSAP 10 §5.11.3 page 44) — e.g. 100 mm → U=0.40 — rather than the
+    uninsulated 2.30. Cert 9884-3059-9202-7506 lodges code 8 / age B /
+    sloping_ceiling 100 mm; before this fix the pre-1950 None-fallback
+    forced 0 mm (U=2.30) and over-stated roof heat loss ~74%."""
+
+    def test_sloping_ceiling_thickness_used_for_code_8(self) -> None:
+        # Arrange
+        from datatypes.epc.domain.mapper import (
+            _api_resolve_sloping_ceiling_thickness,  # pyright: ignore[reportPrivateUsage]
+        )
+
+        # Act — code 8, no loft-joist thickness, age B (pre-1950), but the
+        # sloping ceiling carries a lodged 100 mm.
+        resolved: object = _api_resolve_sloping_ceiling_thickness(
+            8, None, "B", "100mm"
+        )
+
+        # Assert — the lodged sloping-ceiling thickness wins over the
+        # pre-1950 None → 0 mm fallback.
+        assert resolved == "100mm"
+
+    def test_pre_1950_none_fallback_unchanged_without_sloping_field(
+        self,
+    ) -> None:
+        # Arrange
+        from datatypes.epc.domain.mapper import (
+            _api_resolve_sloping_ceiling_thickness,  # pyright: ignore[reportPrivateUsage]
+        )
+
+        # Act — code 8, no thickness anywhere, pre-1950 age.
+        resolved: object = _api_resolve_sloping_ceiling_thickness(
+            8, None, "C", None
+        )
+
+        # Assert — existing Slice 57 behaviour preserved: 0 mm (U=2.30).
+        assert resolved == 0
+
+    def test_as_built_sloping_field_falls_through_to_pre_1950_zero(self) -> None:
+        # Arrange
+        from datatypes.epc.domain.mapper import (
+            _api_resolve_sloping_ceiling_thickness,  # pyright: ignore[reportPrivateUsage]
+        )
+
+        # Act — code 8, age B (pre-1950), sloping lodged "AB" (As Built —
+        # categorical, NOT a measured thickness).
+        resolved: object = _api_resolve_sloping_ceiling_thickness(
+            8, None, "B", "AB"
+        )
+
+        # Assert — "AB" is not a numeric thickness, so it must NOT win; the
+        # Slice 57 pre-1950 None → 0 mm (U=2.30) rule still applies.
+        assert resolved == 0
+
+    def test_sloping_field_ignored_for_non_code_8(self) -> None:
+        # Arrange
+        from datatypes.epc.domain.mapper import (
+            _api_resolve_sloping_ceiling_thickness,  # pyright: ignore[reportPrivateUsage]
+        )
+
+        # Act — code 5 (vaulted) is not a sloping-ceiling code-8; the
+        # sloping field must not be consumed here.
+        resolved: object = _api_resolve_sloping_ceiling_thickness(
+            5, "200mm", "C", "100mm"
+        )
+
+        # Assert — the regular roof_insulation_thickness passes through.
+        assert resolved == "200mm"
+
+
 # ---------------------------------------------------------------------------
 # Glazing-type label cleaning — pdftotext gap-column wrap
 # ---------------------------------------------------------------------------
diff --git a/datatypes/epc/schema/rdsap_schema_21_0_0.py b/datatypes/epc/schema/rdsap_schema_21_0_0.py
index 6db6fa50..da2125be 100644
--- a/datatypes/epc/schema/rdsap_schema_21_0_0.py
+++ b/datatypes/epc/schema/rdsap_schema_21_0_0.py
@@ -254,6 +254,13 @@ class SapBuildingPart:
     wall_insulation_thermal_conductivity: Optional[Union[str, int]] = None
     floor_insulation_thickness: Optional[str] = None
     flat_roof_insulation_thickness: Optional[Union[str, int]] = None
+    # Lodged insulation thickness (e.g. "100mm") for a "Pitched, sloping
+    # ceiling" roof (roof_construction == 8), whose ceiling follows the
+    # slope so the insulation is NOT at the loft joists. Previously
+    # undeclared → dropped by `from_dict`, leaving the cascade to treat
+    # the slope as uninsulated (Table 16 / Table 18 fallback). Consumed by
+    # `_api_resolve_sloping_ceiling_thickness` → Table 17 column (1a).
+    sloping_ceiling_insulation_thickness: Optional[Union[str, int]] = None
 
 
 @dataclass
diff --git a/datatypes/epc/schema/rdsap_schema_21_0_1.py b/datatypes/epc/schema/rdsap_schema_21_0_1.py
index e508c161..c5f456de 100644
--- a/datatypes/epc/schema/rdsap_schema_21_0_1.py
+++ b/datatypes/epc/schema/rdsap_schema_21_0_1.py
@@ -292,6 +292,13 @@ class SapBuildingPart:
     wall_insulation_thermal_conductivity: Optional[Union[str, int]] = None
     floor_insulation_thickness: Optional[str] = None
     flat_roof_insulation_thickness: Optional[Union[str, int]] = None
+    # Lodged insulation thickness (e.g. "100mm") for a "Pitched, sloping
+    # ceiling" roof (roof_construction == 8), whose ceiling follows the
+    # slope so the insulation is NOT at the loft joists. Previously
+    # undeclared → dropped by `from_dict`, leaving the cascade to treat
+    # the slope as uninsulated (Table 16 / Table 18 fallback). Consumed by
+    # `_api_resolve_sloping_ceiling_thickness` → Table 17 column (1a).
+    sloping_ceiling_insulation_thickness: Optional[Union[str, int]] = None
 
 
 @dataclass