diff --git a/machines/renge/services/prometheus.nix b/machines/renge/services/prometheus.nix
index b054194..8f91fa4 100644
--- a/machines/renge/services/prometheus.nix
+++ b/machines/renge/services/prometheus.nix
@@ -169,6 +169,12 @@ in
                 expr = "sum by (instance) (qBittorrent_torrent_connected_leechs) == 0";
                 description = "qBittorrent instance {{ $labels.instance }} has no peers. There might be a network connectivity problem";
               }
+              # <40% is to account for /boot being full (which causes ugly errors on rebuild)
+              {
+                name = "DiskFull";
+                expr = ''node_filesystem_free_bytes{fstype!~"ramfs|tmpfs", mountpoint!~"/nix/store"} / node_filesystem_size_bytes{fstype!~"ramfs|tmpfs", mountpoint!~"/nix/store"} < 0.4 and node_filesystem_free_bytes{fstype!~"ramfs|tmpfs", mountpoint!~"/nix/store"} < 4*1024^3'';
+                description = "Device {{ $labels.device }} on {{ $labels.instance }}:{{ $labels.mountpoint }} has less than 4GiB free space while being used over 40%";
+              }
             ];
           };
         })