feat(modelling): roof dispatcher insulates a flat roof

Slice 3 (ADR-0021). The dispatcher gains a flat-roof branch: a "flat"
roof_construction_type with no lodged thickness (uninsulated → None on the
Elmhurst path) gets a single flat_roof_insulation Option whose overlay raises
roof_insulation_thickness to 200 mm — tested before the loft fallback so a flat
roof's None doesn't trip the loft trigger. Pinned against the Elmhurst
before→after cert at 1e-4. Golden cohort roof firing unchanged (none across 57).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

domain/modelling/generators/roof_recommendation.py

@@ -32,6 +32,9 @@ _ROOF_UNINSULATED_MM = 0
 _RECOMMENDED_LOFT_THICKNESS_MM = 300
 # Recommended sloping-ceiling depth (mm); Elmhurst re-lodges 100 mm (ADR-0021).
 _RECOMMENDED_SLOPING_CEILING_THICKNESS_MM = 100
+_FLAT_ROOF_MEASURE_TYPE = "flat_roof_insulation"
+# Recommended flat-roof depth (mm); Elmhurst re-lodges 200 mm (ADR-0021).
+_RECOMMENDED_FLAT_ROOF_THICKNESS_MM = 200
 
 
 def recommend_roof_insulation(
@@ -65,6 +68,19 @@ def recommend_roof_insulation(
             thickness_mm=_RECOMMENDED_SLOPING_CEILING_THICKNESS_MM,
         )
 
+    if "flat" in roof_type:
+        # A flat roof lodges no thickness when uninsulated ("As Built" → None
+        # on the Elmhurst path); a lodged thickness means it's already done.
+        if main.roof_insulation_thickness is not None:
+            return None
+        return _roof_recommendation(
+            epc,
+            products,
+            measure_type=_FLAT_ROOF_MEASURE_TYPE,
+            description="Flat-roof insulation",
+            thickness_mm=_RECOMMENDED_FLAT_ROOF_THICKNESS_MM,
+        )
+
     if "no access" in roof_type:
         return None  # the roof void can't be reached to insulate it
 

tests/domain/modelling/test_elmhurst_cascade_pins.py

@@ -392,6 +392,32 @@ def test_roof_generator_insulates_a_thatched_roof_as_loft_pinning_its_after() ->
     )
 
 
+def test_roof_generator_insulates_a_flat_roof_pinning_its_after() -> None:
+    # Arrange — a flat roof, uninsulated (As Built → None on the Elmhurst path);
+    # the re-lodged after raises it to 200 mm (ADR-0021).
+    before: EpcPropertyData = parse_recommendation_summary(
+        "flat_roof_001431_before.pdf"
+    )
+    after: EpcPropertyData = parse_recommendation_summary(
+        "flat_roof_001431_after.pdf"
+    )
+
+    # Act
+    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 flat-roof Option whose overlay reproduces the after.
+    assert set(options) == {"flat_roof_insulation"}
+    _assert_overlay_reproduces_after(
+        before, after, options["flat_roof_insulation"].overlay
+    )
+
+
 def test_solid_floor_overlay_reproduces_the_relodged_after() -> None:
     # Arrange
     before: EpcPropertyData = parse_recommendation_summary(