diff --git a/hosts/monitoring-3/rules.yml b/hosts/monitoring-3/rules.yml
index 71a4d26..05c0a4b 100644
--- a/hosts/monitoring-3/rules.yml
+++ b/hosts/monitoring-3/rules.yml
@@ -16,3 +16,11 @@ groups:
     annotations:
       summary: "Uptime of {{ $labels.instance }} more than 90 days"
       description: "The last reboot of {{ $labels.instance }} was more than 90 days ago"
+  - alert: BackupStorageFull
+    expr: ((node_filesystem_avail_bytes{instance="clerie-backup.mon.clerie.de:9100", job="node-exporter", mountpoint="/mnt/clerie-backup"} / node_filesystem_size_bytes{instance="clerie-backup.mon.clerie.de:9100", job="node-exporter", mountpoint="/mnt/clerie-backup"}) * 100) < 5
+    for: 30m
+    labels:
+      severity: critical
+    annotations:
+      summary: "Storage for backup is nearly full"
+      description: "Storage for backups is nearly full"