diff --git a/src/htmlcmp/compare_output_cli.py b/src/htmlcmp/compare_output_cli.py
index 159412f..7560134 100644
--- a/src/htmlcmp/compare_output_cli.py
+++ b/src/htmlcmp/compare_output_cli.py
@@ -57,21 +57,19 @@ def __init__(self, rel: Path, kind: str, reason: str):
 
 
 def collect_tasks(
-    a: Path, b: Path, root: Path = None, diff_output: Path = None, retries: int = 1
+    a: Path, b: Path, rel: Path = Path(), diff_output: Path = None, retries: int = 1
 ) -> tuple[list[Task], list[Failure]]:
     """Walk both trees once and return (comparable tasks, structural failures).
 
     Structural failures are files/directories present on only one side; they are
-    known to differ up-front and never get submitted to the executor.
+    known to differ up-front and never get submitted to the executor. `rel` is
+    the path of `a` and `b` below the roots of the walk.
     """
     if not isinstance(a, Path) or not isinstance(b, Path):
         raise TypeError("Both arguments must be of type Path")
     if not a.is_dir() or not b.is_dir():
         raise FileNotFoundError("Both arguments must be directories")
 
-    if root is None:
-        root = a
-
     tasks: list[Task] = []
     failures: list[Failure] = []
 
@@ -86,11 +84,10 @@ def collect_tasks(
     }
 
     for name in sorted(left_files | right_files):
-        rel = (a / name).relative_to(root)
         if name in left_files and name in right_files:
             tasks.append(
                 Task(
-                    rel,
+                    rel / name,
                     a / name,
                     b / name,
                     None if diff_output is None else diff_output / name,
@@ -98,9 +95,9 @@ def collect_tasks(
                 )
             )
         elif name in left_files:
-            failures.append(Failure(rel, "missing", "missing in monitored (B)"))
+            failures.append(Failure(rel / name, "missing", "missing in monitored (B)"))
         else:
-            failures.append(Failure(rel, "missing", "missing in reference (A)"))
+            failures.append(Failure(rel / name, "missing", "missing in reference (A)"))
 
     left_dirs = {name for name in left if (a / name).is_dir()}
     right_dirs = {name for name in right if (b / name).is_dir()}
@@ -109,7 +106,7 @@ def collect_tasks(
         sub_tasks, sub_failures = collect_tasks(
             a / name,
             b / name,
-            root=root,
+            rel=rel / name,
             diff_output=None if diff_output is None else diff_output / name,
             retries=retries,
         )
@@ -117,13 +114,9 @@ def collect_tasks(
         failures.extend(sub_failures)
 
     for name in sorted(left_dirs - right_dirs):
-        failures.append(
-            Failure((a / name).relative_to(root), "missing", "missing in monitored (B)")
-        )
+        failures.append(Failure(rel / name, "missing", "missing in monitored (B)"))
     for name in sorted(right_dirs - left_dirs):
-        failures.append(
-            Failure((b / name).relative_to(root), "missing", "missing in reference (A)")
-        )
+        failures.append(Failure(rel / name, "missing", "missing in reference (A)"))
 
     return tasks, failures
 
diff --git a/tests/test_collect_tasks.py b/tests/test_collect_tasks.py
new file mode 100644
index 0000000..734f93c
--- /dev/null
+++ b/tests/test_collect_tasks.py
@@ -0,0 +1,30 @@
+"""A file or directory on one side only is a failure, named below the roots."""
+
+from pathlib import Path
+
+from htmlcmp.compare_output_cli import collect_tasks
+
+
+def write(path: Path):
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text("<html></html>")
+
+
+def test_one_sided_entries_are_named_below_the_roots(tmp_path):
+    a, b = tmp_path / "a", tmp_path / "b"
+    write(a / "both" / "page.html")
+    write(b / "both" / "page.html")
+    write(a / "both" / "only_a.html")
+    write(b / "both" / "only_b.html")
+    write(a / "dir_a" / "page.html")
+    write(b / "dir_b" / "page.html")
+
+    tasks, failures = collect_tasks(a, b)
+
+    assert [task.rel for task in tasks] == [Path("both/page.html")]
+    assert sorted((str(f.rel), f.reason) for f in failures) == [
+        ("both/only_a.html", "missing in monitored (B)"),
+        ("both/only_b.html", "missing in reference (A)"),
+        ("dir_a", "missing in monitored (B)"),
+        ("dir_b", "missing in reference (A)"),
+    ]