diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 32765df1..83a0a9eb 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -1634,11 +1634,17 @@ class EpcPropertyDataMapper:
                 for w in schema.sap_windows
                 if not _api_is_roof_window(w)
             ],
+            # Empty → None (not []) so "no roof windows" has the single
+            # canonical representation the domain field defaults to
+            # (`Optional[List] = None`), matching the 21.0.0 path and the
+            # persistence round-trip (roof windows aren't yet stored — doc
+            # §2.4 — so a reloaded cert always reads None here).
             sap_roof_windows=[
                 _api_sap_roof_window(w)
                 for w in schema.sap_windows
                 if _api_is_roof_window(w)
-            ],
+            ]
+            or None,
             # SAP energy source
             sap_energy_source=SapEnergySource(
                 mains_gas=es.mains_gas == "Y",