diff --git a/app/zoom.py b/app/zoom.py
index f77e5d4c0bbd98c49311a9c96a5b58f163db2534..5a9e8edcfa02cb77303eb27038c7d113d1b6e04d 100644
--- a/app/zoom.py
+++ b/app/zoom.py
@@ -188,7 +188,7 @@ def main_page():
                 "x": row_offset + i * room_box_width + 4 + 10*int(coll),
                 "y": int(start / 3600. * room_hour_height),
                 "w": room_box_width - 7 - 10*int(coll),
-                "h": int((end - start) / 3600 * room_hour_height),
+                "h": int((end - start) / 3600 * room_hour_height) - 1,
                 "start": time.strftime("%H:%M", time.localtime(start_t)),
                 "end": time.strftime("%H:%M", time.localtime(end_t)),
                 "topic": r.topic,