diff --git a/machines/lappy/fs.nix b/machines/lappy/fs.nix
index af97910f..0aba756c 100644
--- a/machines/lappy/fs.nix
+++ b/machines/lappy/fs.nix
@@ -10,6 +10,15 @@
       "defaults"
     ];
   };
+  # we need a /tmp of default size (half RAM) for building large nix things
+  fileSystems."/tmp" = {
+    device = "none";
+    fsType = "tmpfs";
+    options = [
+      "mode=777"
+      "defaults"
+    ];
+  };
 
   fileSystems."/nix" = {
     device = "/dev/disk/by-uuid/75230e56-2c69-4e41-b03e-68475f119980";