diff --git a/system/common.nix b/system/common.nix
index 35b9194..fd48359 100644
--- a/system/common.nix
+++ b/system/common.nix
@@ -70,6 +70,13 @@
     # optimize the store
     optimise.automatic = true;
 
+    # auto garbage collect old generations
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 30d";
+    };
+
     # enable flakes!
     settings.experimental-features = [
       "nix-command"