diff --git a/backend/onboarders/base.py b/backend/onboarders/base.py
index e69de29b..12ef9c94 100644
--- a/backend/onboarders/base.py
+++ b/backend/onboarders/base.py
@@ -0,0 +1,7 @@
+class OnboarderBase:
+
+    def read(self):
+        pass
+
+    def write(self):
+        pass
diff --git a/backend/onboarders/parity.py b/backend/onboarders/parity.py
index a77e76a8..a215d444 100644
--- a/backend/onboarders/parity.py
+++ b/backend/onboarders/parity.py
@@ -380,22 +380,27 @@ roof_mapping = {
 }
 
 
-def classify_flat_roof(age_band: EpcConstructionAgeBand):
-    # # flat roof, which if there is observed insulation is just "flat, insulated", however there is a
-    #     # different efficiency rating depending on insulation thickness
-    #     # categories:
-    #     # 12mm = very poor & has limited insulation description
-    #     # 25, 50 = poor & has limited insulation description
-    #     # 75, 100, 125mm = average (Flat, insulated)
-    #     # 150, 175, 200, 225, 250mm = good (Flat, insulated)
-    #     # 270mm+ = very good (Flat, insulated)
-    #     # As built 2023 = Flat, insulated, Very good
-    #     # 2003 - 2006, up to 2012-2022 = Flat insulated, Good
-    #     # 1983-1990, 1996-2002 = Flat, insulated, Average
-    #     # 1976-1982 = Flat, limited insulation, poor
-    #     # 1967 - 1975 = Flat, limited insulation, Very Poor
-    #     # 1950-1966 and earlier bands = flat, no insulation, very poor
-    raise NotImplementedError("Flat roof classification not implemented yet")
+def classify_flat_roof(age_band: EpcConstructionAgeBand) -> EpcRoofDescriptions:
+    """
+    For a flat, as built roof, these are the breakdowns:
+
+    2023 onwards               → Flat, insulated
+    2003–2022                  → Flat, insulated
+    1983–2002                  → Flat, insulated
+    1976–1982                  → Flat, limited insulation
+    1967–1975                  → Flat, limited insulation
+    1950–1966 and earlier      → Flat, no insulation
+    """
+
+    year = age_band.start_year()
+
+    if year >= 1983:
+        return EpcRoofDescriptions.flat_insulated
+
+    if year >= 1967:
+        return EpcRoofDescriptions.flat_limited_insulation
+
+    return EpcRoofDescriptions.flat_no_insulation
 
 
 def classify_pitched_loft_unknown(age_band: EpcConstructionAgeBand):
@@ -421,7 +426,7 @@ AS_BUILT_ROOF_CLASSIFIERS = {
 
 def fill_roof_as_built(row):
     # Already resolved
-    if row.landlord_roof_description is not None:
+    if not pd.isnull(row.landlord_roof_description):
         return row.landlord_roof_description
 
     roof_type = row["Roof Construction"]
@@ -435,7 +440,13 @@ def fill_roof_as_built(row):
             f"Missing age band for roof classification ({roof_type})"
         )
 
-    return classifier(row.construction_age_band)
+    output = classifier(row.construction_age_band)
+    if output is None:
+        raise NotImplementedError(
+            f"Roof classification returned None for roof type '{roof_type}'"
+        )
+
+    return output
 
 
 data["landlord_roof_description"] = (