diff --git a/buildbot_nix/__init__.py b/buildbot_nix/__init__.py
index b795afe..fc58e34 100644
--- a/buildbot_nix/__init__.py
+++ b/buildbot_nix/__init__.py
@@ -259,8 +259,12 @@ def nix_eval_config(
             sshPrivateKey=gerrit_private_key
         ),
     )
+    # use one gcroots directory per worker. this should be scoped to the largest unique resource
+    # in charge of builds (ie, buildnumber is too narrow) to not litter the system with permanent
+    # gcroots in case of worker restarts.
+    # TODO perhaps we should clean the entire /drvs/ directory up too during startup.
     drv_gcroots_dir = util.Interpolate(
-        "/nix/var/nix/gcroots/per-user/buildbot-worker/%(prop:project)s/drvs/",
+        "/nix/var/nix/gcroots/per-user/buildbot-worker/%(prop:project)s/drvs/%(prop:workername)s/",
     )
 
     factory.addStep(
@@ -297,6 +301,7 @@ def nix_eval_config(
                 "-rf",
                 drv_gcroots_dir,
             ],
+            alwaysRun=True,
         ),
     )