diff --git a/polymath_code_standard/yaml_format.py b/polymath_code_standard/yaml_format.py
index 0eec21a..f063aaf 100644
--- a/polymath_code_standard/yaml_format.py
+++ b/polymath_code_standard/yaml_format.py
@@ -323,7 +323,10 @@ def format_yaml(source: str, explicit_start: bool = True) -> str:
     if not source:
         return source
     matrices = detect_matrices(source)
-    fixed = fix_code(source, _yamlfix_config(explicit_start=explicit_start))
+    source_has_explicit_start = source.startswith('---\n')
+    fixed = fix_code(source, _yamlfix_config(explicit_start=True))
+    if not explicit_start and not source_has_explicit_start and fixed.startswith('---\n'):
+        fixed = fixed[4:]
     if matrices:
         fixed = apply_matrices(fixed, matrices)
     return _strip_trailing_whitespace(fixed)
diff --git a/tests/test_yaml_format.py b/tests/test_yaml_format.py
index 5d80989..3b11f5a 100644
--- a/tests/test_yaml_format.py
+++ b/tests/test_yaml_format.py
@@ -104,6 +104,15 @@ def test_adds_document_start(self):
         result = format_yaml('key: value\n')
         assert result.startswith('---\n')
 
+    def test_no_explicit_start_preserves_root_sequence_indentation(self):
+        source = textwrap.dedent("""\
+            - name: first
+              enabled: true
+            - name: second
+              enabled: false
+        """)
+        assert format_yaml(source, explicit_start=False) == source
+
     def test_true_normalised(self):
         result = format_yaml('enabled: True\n')
         assert 'enabled: true' in result