--- a/taskcluster/ci/desktop-test/tests.yml
+++ b/taskcluster/ci/desktop-test/tests.yml
@@ -53,17 +53,20 @@ crashtest:
         extra-options:
             - --reftest-suite=crashtest
 
 external-media-tests:
     description: "External Media Test run"
     suite: external-media-tests
     treeherder-symbol: tc-VP(b-m)
     e10s: false
-    tier: 2
+    tier:
+        by-test-platform:
+            win.*: 3
+            default: 2
     max-run-time: 5400
     mozharness:
         script: firefox_media_tests_taskcluster.py
         config:
             by-test-platform:
                 win.*:
                     - mediatests/taskcluster_windows_config.py
                 default:

--- a/taskcluster/taskgraph/transforms/tests/all_kinds.py
+++ b/taskcluster/taskgraph/transforms/tests/all_kinds.py
@@ -40,16 +40,18 @@ def set_tier(config, tests):
     for test in tests:
         # only override if not set for the test
         if 'tier' not in test:
             if test['test-platform'] in ['linux64/debug',
                                          'linux64-asan/opt',
                                          'android-4.3-arm7-api-15/debug',
                                          'android-x86/opt']:
                 test['tier'] = 1
+            elif test['test-platform'].startswith('win'):
+                test['tier'] = 3
             else:
                 test['tier'] = 2
         yield test
 
 
 @transforms.add
 def set_expires_after(config, tests):
     """Try jobs expire after 2 weeks; everything else lasts 1 year.  This helps
@@ -84,16 +86,17 @@ def resolve_keyed_by(config, tests):
     """Resolve fields that can be keyed by platform, etc."""
     fields = [
         'instance-size',
         'max-run-time',
         'chunks',
         'e10s',
         'suite',
         'run-on-projects',
+        'tier',
     ]
     for test in tests:
         for field in fields:
             test[field] = get_keyed_by(item=test, field=field, item_name=test['test-name'])
         test['mozharness']['config'] = get_keyed_by(item=test,
                                                     field='mozharness',
                                                     subfield='config',
                                                     item_name=test['test-name'])

--- a/taskcluster/taskgraph/transforms/tests/test_description.py
+++ b/taskcluster/taskgraph/transforms/tests/test_description.py
@@ -58,17 +58,20 @@ test_description_schema = Schema({
     # projects on which this task should be included in the target task set.
     # See the attributes documentation for details.
     Optional('run-on-projects', default=['all']): Any(
         [basestring],
         {'by-test-platform': {basestring: [basestring]}},
     ),
 
     # the sheriffing tier for this task (default: set based on test platform)
-    Optional('tier'): int,
+    Optional('tier'): Any(
+        int,
+        {'by-test-platform': {basestring: int}},
+    ),
 
     # number of chunks to create for this task.  This can be keyed by test
     # platform by passing a dictionary in the `by-test-platform` key.  If the
     # test platform is not found, the key 'default' will be tried.
     Required('chunks', default=1): Any(
         int,
         {'by-test-platform': {basestring: int}},
     ),