diff --git a/domain/modelling/generators/roof_recommendation.py b/domain/modelling/generators/roof_recommendation.py
index 6b689733..8bbbd34b 100644
--- a/domain/modelling/generators/roof_recommendation.py
+++ b/domain/modelling/generators/roof_recommendation.py
@@ -18,12 +18,19 @@ from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
 from repositories.product.product_repository import ProductRepository
 
 _LOFT_MEASURE_TYPE = "loft_insulation"
+_SLOPING_CEILING_MEASURE_TYPE = "sloping_ceiling_insulation"
 # RdSAP 10 Table 16: 0 mm lodged roof insulation is an uninsulated loft.
 _ROOF_UNINSULATED_MM = 0
+# A roof is uninsulated when its lodged insulation thickness is 0 or absent —
+# the Elmhurst mapper resolves "As Built" to 0 for pitched/sloping roofs and to
+# None for flat roofs (ADR-0021).
+_ROOF_UNINSULATED_THICKNESSES: tuple[Optional[int], ...] = (None, 0)
 # Recommended loft-insulation depth (mm). Elmhurst re-lodges a loft-insulation
 # measure at 300 mm; pinning the before→after cascade (000490/001431) requires
 # the overlay to match that depth exactly (see test_elmhurst_cascade_pins).
 _RECOMMENDED_LOFT_THICKNESS_MM = 300
+# Recommended sloping-ceiling depth (mm); Elmhurst re-lodges 100 mm (ADR-0021).
+_RECOMMENDED_SLOPING_CEILING_THICKNESS_MM = 100
 
 
 def recommend_loft_insulation(
@@ -62,3 +69,63 @@ def recommend_loft_insulation(
         material_id=product.id,
     )
     return Recommendation(surface="Roof", options=(option,))
+
+
+def recommend_roof_insulation(
+    epc: EpcPropertyData, products: ProductRepository
+) -> Optional[Recommendation]:
+    """Return the single roof-insulation Recommendation for the MAIN roof,
+    dispatched by roof type (ADR-0021): a sloping ceiling is insulated at the
+    rafters to 100 mm. Returns None when the roof type has no applicable measure
+    or the roof is already insulated."""
+    main = next(
+        part
+        for part in epc.sap_building_parts
+        if part.identifier is BuildingPartIdentifier.MAIN
+    )
+    roof_type: str = (main.roof_construction_type or "").lower()
+
+    if "sloping ceiling" in roof_type:
+        if main.roof_insulation_thickness not in _ROOF_UNINSULATED_THICKNESSES:
+            return None
+        return _roof_recommendation(
+            epc,
+            products,
+            measure_type=_SLOPING_CEILING_MEASURE_TYPE,
+            description="Sloping-ceiling insulation (insulate at the rafters)",
+            thickness_mm=_RECOMMENDED_SLOPING_CEILING_THICKNESS_MM,
+        )
+    return None
+
+
+def _roof_recommendation(
+    epc: EpcPropertyData,
+    products: ProductRepository,
+    *,
+    measure_type: str,
+    description: str,
+    thickness_mm: int,
+) -> Recommendation:
+    """Build a single-Option "Roof" Recommendation: the measure's insulation
+    overlay (raising `roof_insulation_thickness` to the recommended depth)
+    priced at the roof area."""
+    product = products.get(measure_type)
+    area: float = roof_area(epc, BuildingPartIdentifier.MAIN)
+    cost = Cost(
+        total=area * product.unit_cost_per_m2,
+        contingency_rate=product.contingency_rate,
+    )
+    option = MeasureOption(
+        measure_type=measure_type,
+        description=description,
+        overlay=EpcSimulation(
+            building_parts={
+                BuildingPartIdentifier.MAIN: BuildingPartOverlay(
+                    roof_insulation_thickness=thickness_mm
+                )
+            }
+        ),
+        cost=cost,
+        material_id=product.id,
+    )
+    return Recommendation(surface="Roof", options=(option,))
diff --git a/tests/domain/modelling/fixtures/sloping_ceiling_001431_after.pdf b/tests/domain/modelling/fixtures/sloping_ceiling_001431_after.pdf
new file mode 100644
index 00000000..4d506a80
Binary files /dev/null and b/tests/domain/modelling/fixtures/sloping_ceiling_001431_after.pdf differ
diff --git a/tests/domain/modelling/fixtures/sloping_ceiling_001431_before.pdf b/tests/domain/modelling/fixtures/sloping_ceiling_001431_before.pdf
new file mode 100644
index 00000000..9e346c34
Binary files /dev/null and b/tests/domain/modelling/fixtures/sloping_ceiling_001431_before.pdf differ
diff --git a/tests/domain/modelling/test_elmhurst_cascade_pins.py b/tests/domain/modelling/test_elmhurst_cascade_pins.py
index 5821b5c1..6bffc3e2 100644
--- a/tests/domain/modelling/test_elmhurst_cascade_pins.py
+++ b/tests/domain/modelling/test_elmhurst_cascade_pins.py
@@ -27,7 +27,10 @@ from domain.modelling.scoring.package_scorer import PackageScorer, Score
 from domain.modelling.product import Product
 from domain.modelling.recommendation import Recommendation
 from domain.modelling.generators.floor_recommendation import recommend_floor_insulation
-from domain.modelling.generators.roof_recommendation import recommend_loft_insulation
+from domain.modelling.generators.roof_recommendation import (
+    recommend_loft_insulation,
+    recommend_roof_insulation,
+)
 from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
 from domain.modelling.generators.wall_recommendation import recommend_cavity_wall
 from domain.geospatial.planning_restrictions import PlanningRestrictions
@@ -336,6 +339,33 @@ def test_loft_overlay_reproduces_the_relodged_after() -> None:
     )
 
 
+def test_roof_generator_insulates_a_sloping_ceiling_pinning_its_after() -> None:
+    # Arrange — a pitched roof with an uninsulated sloping ceiling; the re-lodged
+    # after raises its insulation from As Built to 100 mm (ADR-0021).
+    before: EpcPropertyData = parse_recommendation_summary(
+        "sloping_ceiling_001431_before.pdf"
+    )
+    after: EpcPropertyData = parse_recommendation_summary(
+        "sloping_ceiling_001431_after.pdf"
+    )
+
+    # Act — the dispatcher detects "sloping ceiling" and offers the sloping
+    # measure (not loft).
+    recommendation: Recommendation | None = recommend_roof_insulation(
+        before, _AnyProduct()
+    )
+    assert recommendation is not None
+    options: dict[str, MeasureOption] = {
+        option.measure_type: option for option in recommendation.options
+    }
+
+    # Assert — one sloping-ceiling Option whose overlay reproduces the after.
+    assert set(options) == {"sloping_ceiling_insulation"}
+    _assert_overlay_reproduces_after(
+        before, after, options["sloping_ceiling_insulation"].overlay
+    )
+
+
 def test_solid_floor_overlay_reproduces_the_relodged_after() -> None:
     # Arrange
     before: EpcPropertyData = parse_recommendation_summary(