diff --git a/domain/epc/property_overlays/roof_type_overlay.py b/domain/epc/property_overlays/roof_type_overlay.py
index aadcc3878..a2ba1811b 100644
--- a/domain/epc/property_overlays/roof_type_overlay.py
+++ b/domain/epc/property_overlays/roof_type_overlay.py
@@ -27,6 +27,8 @@ from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
 from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
 
 _LOFT_MM = re.compile(r"(\d+)\+?\s*mm loft insulation")
+# A flat roof carries no loft, so its depth reads "N mm insulation" (no "loft").
+_FLAT_MM = re.compile(r"(\d+)\+?\s*mm insulation")
 
 
 def roof_overlay_for(
@@ -49,8 +51,18 @@ def _overlay_for(roof_type_value: str) -> Optional[BuildingPartOverlay]:
     if match is not None:
         return BuildingPartOverlay(roof_insulation_thickness=int(match.group(1)))
     if roof_type_value.startswith("Flat,"):
-        # Flat roof: U-value is the age-band default; flag the shape and let the
-        # age-band overlay drive `_FLAT_ROOF_BY_AGE` (ADR-0033).
+        flat_depth = _FLAT_MM.search(roof_type_value)
+        if flat_depth is not None:
+            # A flat roof with a known depth: RdSAP 10 Table 16 col (1) ("joists at
+            # ceiling level and flat roof") scores it by thickness, so carry the
+            # depth (the calculator routes flat + thickness through Table 16, not
+            # the age-band default). ADR-0041.
+            return BuildingPartOverlay(
+                roof_construction_type="Flat",
+                roof_insulation_thickness=int(flat_depth.group(1)),
+            )
+        # Flat roof with no stated depth: U-value is the age-band default; flag the
+        # shape and let the age-band overlay drive `_FLAT_ROOF_BY_AGE` (ADR-0033).
         return BuildingPartOverlay(roof_construction_type="Flat")
     if roof_type_value == "Pitched, Unknown loft insulation":
         # Unknown loft depth: U-value is the pitched age-band default. Assert the