diff --git a/docs/userguide/icu_data/buildtool.md b/docs/userguide/icu_data/buildtool.md
index da4c859fa7..4262e414b0 100644
--- a/docs/userguide/icu_data/buildtool.md
+++ b/docs/userguide/icu_data/buildtool.md
@@ -222,12 +222,17 @@ There are two concrete differences between additive and subtractive mode:
 |                         | Additive    | Subtractive |
 |-------------------------|-------------|-------------|
 | Default Feature Filter  | `"exclude"` | `"include"` |
-| Default Resource Filter | `"-/"`      | `"+/"`      |
+| Default Resource Filter | `"-/"`, `"+/%%ALIAS"`, `"+/%%Parent"` | `"+/"` |
 
 To enable additive mode, add the following setting to your filter file:
 
     strategy: "additive"
 
+**Caution:** If using `"-/"` or similar top-level exclusion rules, be aware of
+the fields `"+/%%Parent"` and `"+/%%ALIAS"`, which are required in locale tree
+resource bundles. Excluding these paths may cause unexpected locale fallback
+behavior.
+
 #### Filter Types
 
 You may list *filters* for each category in the *featureFilters* section of
diff --git a/icu4c/source/python/icutools/databuilder/filtration.py b/icu4c/source/python/icutools/databuilder/filtration.py
index 9150eb28f2..0eff5b9c57 100644
--- a/icu4c/source/python/icutools/databuilder/filtration.py
+++ b/icu4c/source/python/icutools/databuilder/filtration.py
@@ -319,9 +319,15 @@ class ResourceFilterInfo(object):
             )
         ]
         if self.strategy == "additive":
-            self.rules_by_file = [["-/"] for _ in range(len(files))]
+            self.rules_by_file = [
+                [r"-/", r"+/%%ALIAS", r"+/%%Parent"]
+                for _ in range(len(files))
+            ]
         else:
-            self.rules_by_file = [["+/"] for _ in range(len(files))]
+            self.rules_by_file = [
+                [r"+/"]
+                for _ in range(len(files))
+            ]
 
     def add_rules(self, file_filter, rules):
         for file, rule_list in zip(self.input_files, self.rules_by_file):