diff --git a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
index ceff10a8..cf4ecb27 100644
--- a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
+++ b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
@@ -306,3 +306,24 @@ def test_summary_001479_ext2_floor_is_exposed_to_external_air() -> None:
     ext2 = epc.sap_building_parts[2]
     assert ext2.floor_type == "To external air"
     assert ext2.sap_floor_dimensions[0].is_exposed_floor is True
+
+
+def test_summary_001479_ext2_sloping_ceiling_roof_uninsulated_for_pre_1950() -> None:
+    # Arrange — cert 001479 Ext2 §8 lodges "Type: PS Pitched, sloping
+    # ceiling" + "Insulation Thickness: As Built" + age band C (1930-49).
+    # Original 1930s construction had no sloping-ceiling insulation;
+    # worksheet §3 `External roof Ext2 … 2.30` pins U=2.30 (uninsulated
+    # Table 16 row 0). Pre-slice the mapper passed thickness=None through,
+    # routing to `u_roof`'s pitched-roof Table 18 col 1 default (0.40 for
+    # age C, assumes loft-joist retrofit) — wrong geometry for PS.
+    # Ext1's PS roof at age M leaves thickness=None (modern build,
+    # cascade default U=0.15 matches worksheet).
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_001479_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert
+    assert epc.sap_building_parts[2].roof_insulation_thickness == 0
+    assert epc.sap_building_parts[1].roof_insulation_thickness is None
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index a9c7f94f..7e2b7083 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -1879,6 +1879,33 @@ def _elmhurst_wall_insulation_int(coded: str) -> Optional[int]:
 _UPPER_FLOOR_HEIGHT_ADD_M: float = 0.25
 
 
+# Pre-1950 age bands lodged with an "As Built" sloping-ceiling roof get
+# their original-construction U-value (uninsulated) — original 1930-49
+# construction had no sloping-ceiling insulation by default. RdSAP10
+# Table 18 col 1 (the pitched-roof default behind `u_roof`) assumes
+# joist insulation accessible from the loft, which doesn't apply to PS
+# sloping-ceiling roofs; without an explicit thickness override the
+# cascade would understate U for this geometry. Cert 001479 Ext2's
+# worksheet row `External roof Ext2 … 2.22, 2.30` pins U=2.30 at this
+# age + geometry combination.
+_PRE_1950_AGE_CODES: Final[frozenset[str]] = frozenset({"A", "B", "C", "D"})
+
+
+def _resolve_sloping_ceiling_thickness(
+    roof: ElmhurstRoofDetails, age_code: str,
+) -> Optional[int]:
+    """Map an Elmhurst sloping-ceiling roof (`PS Pitched, sloping ceiling`)
+    with no lodged thickness ("As Built") to an explicit 0 mm for pre-
+    1950 age bands. Other lodgements pass through unchanged."""
+    if roof.insulation_thickness_mm is not None:
+        return roof.insulation_thickness_mm
+    if not roof.roof_type.startswith("PS"):
+        return None
+    if age_code in _PRE_1950_AGE_CODES:
+        return 0
+    return None
+
+
 def _is_floor_exposed_to_unheated_space(location: Optional[str]) -> bool:
     """True when the floor sits above an unheated space OR is exposed
     directly to external air. Both lodgements route through
@@ -2118,9 +2145,10 @@ def _map_elmhurst_building_part(
     ]
     while len(alt_walls) < 2:
         alt_walls.append(None)
+    age_code = _leading_code(age_band)
     return SapBuildingPart(
         identifier=identifier,
-        construction_age_band=_leading_code(age_band),
+        construction_age_band=age_code,
         wall_construction=_elmhurst_wall_construction_int(walls.wall_type),
         wall_insulation_type=_elmhurst_wall_insulation_int(walls.insulation),
         wall_thickness_measured=not walls.thickness_unknown,
@@ -2128,7 +2156,9 @@ def _map_elmhurst_building_part(
         sap_floor_dimensions=floor_dims,
         wall_thickness_mm=walls.thickness_mm,
         roof_insulation_location=_strip_code(roof.insulation),
-        roof_insulation_thickness=roof.insulation_thickness_mm,
+        roof_insulation_thickness=_resolve_sloping_ceiling_thickness(
+            roof, age_code,
+        ),
         floor_type=_strip_code(floor.location),
         floor_construction_type=_strip_code(floor.floor_type),
         floor_insulation_type_str=_strip_code(floor.insulation),