diff --git a/hosts/by-name/desko/fs.nix b/hosts/by-name/desko/fs.nix
index 7f22679af..22db863be 100644
--- a/hosts/by-name/desko/fs.nix
+++ b/hosts/by-name/desko/fs.nix
@@ -2,17 +2,10 @@
 
 {
   sane.persist.root-on-tmpfs = true;
-  # we need a /tmp for building large nix things.
+  # increase /tmp space (defaults to 50% of RAM) for building large nix things.
   # a cross-compiled kernel, particularly, will easily use 30+GB of tmp
-  fileSystems."/tmp" = {
-    device = "none";
-    fsType = "tmpfs";
-    options = [
-      "mode=777"
-      "size=64G"
-      "defaults"
-    ];
-  };
+  fileSystems."/tmp".options = [ "size=64G" ];
+
   fileSystems."/nix" = {
     # device = "/dev/disk/by-uuid/985a0a32-da52-4043-9df7-615adec2e4ff";
     device = "/dev/disk/by-uuid/0ab0770b-7734-4167-88d9-6e4e20bb2a56";
diff --git a/hosts/by-name/lappy/fs.nix b/hosts/by-name/lappy/fs.nix
index c26b5bd28..d741816ea 100644
--- a/hosts/by-name/lappy/fs.nix
+++ b/hosts/by-name/lappy/fs.nix
@@ -2,15 +2,6 @@
 
 {
   sane.persist.root-on-tmpfs = true;
-  # 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";
diff --git a/hosts/by-name/servo/fs.nix b/hosts/by-name/servo/fs.nix
index e58d05bb8..f41d927d1 100644
--- a/hosts/by-name/servo/fs.nix
+++ b/hosts/by-name/servo/fs.nix
@@ -2,15 +2,6 @@
 
 {
   sane.persist.root-on-tmpfs = true;
-  # we need a /tmp for building large nix things
-  fileSystems."/tmp" = {
-    device = "none";
-    fsType = "tmpfs";
-    options = [
-      "mode=777"
-      "defaults"
-    ];
-  };
 
   fileSystems."/nix" = {
     device = "/dev/disk/by-uuid/cc81cca0-3cc7-4d82-a00c-6243af3e7776";
diff --git a/hosts/modules/roles/default.nix b/hosts/modules/roles/default.nix
index a668caeb6..9f16952e3 100644
--- a/hosts/modules/roles/default.nix
+++ b/hosts/modules/roles/default.nix
@@ -1,4 +1,4 @@
-{ ... }:
+{ config, lib, ... }:
 {
   imports = [
     ./ac.nix
@@ -6,4 +6,13 @@
     ./client
     ./dev-machine.nix
   ];
+
+  fileSystems."/tmp" = lib.mkIf (config.sane.roles.build-machine.enable || config.sane.roles.dev-machine) {
+    device = "none";
+    fsType = "tmpfs";
+    options = [
+      "mode=777"
+      "defaults"
+    ];
+  };
 }