diff --git a/src/c3nav/mapdata/render/data.py b/src/c3nav/mapdata/render/data.py
index 117d06e2..5e5c507a 100644
--- a/src/c3nav/mapdata/render/data.py
+++ b/src/c3nav/mapdata/render/data.py
@@ -598,7 +598,7 @@ class LevelGeometries:
         self.walls_bottom = HybridGeometry(self.walls.geom, self.walls.faces)
 
         self.walls.build_polyhedron(self._create_polyhedron,
-                                    lower=vertex_altitudes,
+                                    lower=vertex_altitudes - int(0.7 * 1000),
                                     upper=vertex_wall_heights)
 
         if interpolator is not None: