diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index fe947bf9c..4143a54ae 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -741,11 +741,32 @@ class EpcPropertyDataMapper:
                         bp, "wall_insulation_thermal_conductivity", None
                     ),
                     floor_heat_loss=bp.floor_heat_loss,
-                    floor_insulation_thickness=None,
+                    # 17.0 lodges these three building-part fabric fields but the
+                    # mapper hardcoded them to None / omitted them, unlike the
+                    # 19.0/20.0/21.x paths. "NI" (no measured thickness) → None so
+                    # u_floor reaches its age-band default rather than the
+                    # explicit-zero path (see the 19.0 note). roof_insulation_
+                    # thickness is left raw here (the sloping-ceiling resolve
+                    # helper is a separate, roof-U-moving follow-up).
+                    floor_insulation_thickness=(
+                        None
+                        if bp.floor_insulation_thickness == "NI"
+                        else bp.floor_insulation_thickness
+                    ),
+                    flat_roof_insulation_thickness=bp.flat_roof_insulation_thickness,
                     roof_construction=bp.roof_construction,
                     roof_insulation_location=bp.roof_insulation_location,
                     roof_insulation_thickness=bp.roof_insulation_thickness,
-                    sap_room_in_roof=None,
+                    sap_room_in_roof=(
+                        SapRoomInRoof(
+                            floor_area=_measurement_value(
+                                bp.sap_room_in_roof.floor_area
+                            ),
+                            construction_age_band=bp.sap_room_in_roof.construction_age_band,
+                        )
+                        if bp.sap_room_in_roof
+                        else None
+                    ),
                 )
                 # RdSAP 10 §6.1 — exclude the glazed conservatory BP from the
                 # fabric loop; carried as `sap_conservatory` below. Mirrors the
diff --git a/tests/datatypes/epc/domain/test_mapper_building_part_fabric_17_0.py b/tests/datatypes/epc/domain/test_mapper_building_part_fabric_17_0.py
new file mode 100644
index 000000000..1a929b28f
--- /dev/null
+++ b/tests/datatypes/epc/domain/test_mapper_building_part_fabric_17_0.py
@@ -0,0 +1,66 @@
+"""Regression: the RdSAP-Schema-17.0 mapper path must carry the lodged
+building-part fabric fields, like the 19.0/20.0/21.x paths.
+
+PRD #1385 (mapper normalization), building-part fabric family. 17.0 hardcoded
+`floor_insulation_thickness` / `sap_room_in_roof` to None and omitted
+`flat_roof_insulation_thickness`, all of which the 17.0 schema lodges and the
+sibling paths read. (17.1 is NOT affected — its SapBuildingPart schema has none
+of these fields, so its None there is correct.) `roof_insulation_thickness` is
+intentionally still passed raw; the sloping-ceiling resolve helper is a separate
+roof-U-moving follow-up.
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from typing import Any
+
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+
+# Lodges a room-in-roof (floor_area 48.83, age band K).
+_FIXTURE_RIR = Path("tests/fixtures/epc_prediction/BD24JG/cert-75aa70bdd22a.json")
+# Lodges a flat-roof insulation thickness code ("AB").
+_FIXTURE_FLAT_ROOF = Path("tests/fixtures/epc_prediction/BD24JG/cert-01f1488000e8.json")
+
+
+def _load(path: Path) -> dict[str, Any]:
+    data: dict[str, Any] = json.loads(path.read_text())
+    assert data["schema_type"] == "RdSAP-Schema-17.0"
+    return data
+
+
+def test_17_0_maps_lodged_room_in_roof() -> None:
+    epc = EpcPropertyDataMapper.from_api_response(_load(_FIXTURE_RIR))
+
+    rirs = [bp.sap_room_in_roof for bp in epc.sap_building_parts if bp.sap_room_in_roof]
+    assert rirs, "room-in-roof was dropped (mapped to None)"
+    assert rirs[0].floor_area == 48.83
+    assert rirs[0].construction_age_band == "K"
+
+
+def test_17_0_maps_lodged_flat_roof_insulation_thickness() -> None:
+    epc = EpcPropertyDataMapper.from_api_response(_load(_FIXTURE_FLAT_ROOF))
+
+    vals = [
+        bp.flat_roof_insulation_thickness
+        for bp in epc.sap_building_parts
+        if bp.flat_roof_insulation_thickness is not None
+    ]
+    assert "AB" in vals
+
+
+def test_17_0_floor_insulation_thickness_flows_and_ni_maps_to_none() -> None:
+    # A measured thickness carries through (schema lodges None on the real certs;
+    # inject to prove the field is no longer hardcoded to None).
+    data = _load(_FIXTURE_RIR)
+    data["sap_building_parts"][0]["floor_insulation_thickness"] = "150"
+    epc = EpcPropertyDataMapper.from_api_response(data)
+    assert epc.sap_building_parts[0].floor_insulation_thickness == "150"
+
+    # The "NI" sentinel (no measured thickness) maps to None so u_floor reaches
+    # its age-band default, matching the 19.0 path.
+    data_ni = _load(_FIXTURE_RIR)
+    data_ni["sap_building_parts"][0]["floor_insulation_thickness"] = "NI"
+    epc_ni = EpcPropertyDataMapper.from_api_response(data_ni)
+    assert epc_ni.sap_building_parts[0].floor_insulation_thickness is None