diff --git a/swmmanywhere/geospatial_utilities.py b/swmmanywhere/geospatial_utilities.py
index c1184cf2..53fdb3ea 100644
--- a/swmmanywhere/geospatial_utilities.py
+++ b/swmmanywhere/geospatial_utilities.py
@@ -760,7 +760,7 @@ def derive_rc(subcatchments: gpd.GeoDataFrame,
     )
 
     # Store as impervious area in subcatchments
-    subcatchments["impervious_area"] = 0
+    subcatchments["impervious_area"] = 0.0
     subcatchments.loc[areas.index, "impervious_area"] = areas
     subcatchments["rc"] = subcatchments["impervious_area"] / \
         subcatchments.geometry.area * 100