--- a/taskcluster/taskgraph/transforms/job/mozharness.py
+++ b/taskcluster/taskgraph/transforms/job/mozharness.py
@@ -263,16 +263,25 @@ def mozharness_on_generic_worker(config,
         ]
 
     hg_commands = checkout_repo(
         base_repo=env['GECKO_BASE_REPOSITORY'],
         head_repo=env['GECKO_HEAD_REPOSITORY'],
         head_rev=env['GECKO_HEAD_REV'],
         path='.\\build\\src')
 
+    if run['comm-checkout']:
+        hg_commands.extend(
+            checkout_repo(
+                base_repo=env['COMM_BASE_REPOSITORY'],
+                head_repo=env['COMM_HEAD_REPOSITORY'],
+                head_rev=env['COMM_HEAD_REV'],
+                path='.\\build\\src\\comm')
+        )
+
     worker['command'] = []
     if taskdesc.get('needs-sccache'):
         worker['command'].extend([
             # Make the comment part of the first command, as it will help users to
             # understand what is going on, and why these steps are implemented.
             dedent('''\
             :: sccache currently uses the full compiler commandline as input to the
             :: cache hash key, so create a symlink to the task dir and build from