diff --git a/backup/src/delta.rs b/backup/src/delta.rs
index 6bdff88..dde906b 100644
--- a/backup/src/delta.rs
+++ b/backup/src/delta.rs
@@ -1,11 +1,15 @@
-use std::{borrow::Borrow, fmt};
+use std::{
+    borrow::Borrow,
+    fmt,
+    path::{Path, PathBuf},
+};
 
 use serde::{Deserialize, Serialize};
 
 use super::State;
 
 /// Represents the changes relative to the previous backup
-#[derive(Debug, Serialize, Deserialize, Clone, Default)]
+#[derive(Debug, Serialize, Deserialize, Clone, Default, PartialEq, Eq)]
 pub struct Delta {
     /// What files were added/modified in each part of the tarball.
     pub added: State,
@@ -19,7 +23,6 @@ pub struct Delta {
 impl Delta {
     /// Returns whether the delta is empty by checking whether both its added and removed state
     /// return true for their `is_empty`.
-    #[allow(dead_code)]
     pub fn is_empty(&self) -> bool {
         self.added.is_empty() && self.removed.is_empty()
     }
@@ -143,6 +146,58 @@ impl Delta {
 
         contributions
     }
+
+    /// Append the given files to the directory's list of added files
+    pub fn append_added<I>(&mut self, dir: impl Into<PathBuf>, files: I)
+    where
+        I: IntoIterator,
+        I::Item: Into<PathBuf>,
+    {
+        let dir: PathBuf = dir.into();
+        let files = files.into_iter().map(Into::into);
+
+        if let Some(dir_files) = self.added.get_mut(&dir) {
+            dir_files.extend(files);
+        } else {
+            self.added.insert(dir, files.collect());
+        }
+    }
+
+    /// Wrapper around the `append_added` method for a builder-style construction of delta's
+    pub fn with_added<I>(mut self, dir: impl Into<PathBuf>, files: I) -> Self
+    where
+        I: IntoIterator,
+        I::Item: Into<PathBuf>,
+    {
+        self.append_added(dir, files);
+        self
+    }
+
+    /// Append the given files to the directory's list of removed files
+    pub fn append_removed<I>(&mut self, dir: impl Into<PathBuf>, files: I)
+    where
+        I: IntoIterator,
+        I::Item: Into<PathBuf>,
+    {
+        let dir: PathBuf = dir.into();
+        let files = files.into_iter().map(Into::into);
+
+        if let Some(dir_files) = self.removed.get_mut(&dir) {
+            dir_files.extend(files);
+        } else {
+            self.removed.insert(dir, files.collect());
+        }
+    }
+
+    /// Wrapper around the `append_removed` method for a builder-style construction of delta's
+    pub fn with_removed<I>(mut self, dir: impl Into<PathBuf>, files: I) -> Self
+    where
+        I: IntoIterator,
+        I::Item: Into<PathBuf>,
+    {
+        self.append_removed(dir, files);
+        self
+    }
 }
 
 impl fmt::Display for Delta {
@@ -153,3 +208,44 @@ impl fmt::Display for Delta {
         write!(f, "+{}-{}", added_count, removed_count)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_union_disjunct_dirs() {
+        let a = Delta::default()
+            .with_added("dir_added_1", ["file1", "file2"])
+            .with_removed("dir_removed_1", ["file1", "file2"]);
+        let b = Delta::default()
+            .with_added("dir_added_3", ["file1", "file2"])
+            .with_removed("dir_removed_3", ["file1", "file2"]);
+
+        let expected = Delta::default()
+            .with_added("dir_added_1", ["file1", "file2"])
+            .with_added("dir_added_3", ["file1", "file2"])
+            .with_removed("dir_removed_1", ["file1", "file2"])
+            .with_removed("dir_removed_3", ["file1", "file2"]);
+
+        assert_eq!(expected, a.union(&b));
+        assert_eq!(expected, b.union(&a));
+    }
+
+    #[test]
+    fn test_union_disjunct_files() {
+        let a = Delta::default()
+            .with_added("dir_added_1", ["file1", "file2"])
+            .with_removed("dir_removed_1", ["file1", "file2"]);
+        let b = Delta::default()
+            .with_added("dir_added_1", ["file3", "file4"])
+            .with_removed("dir_removed_1", ["file3", "file4"]);
+
+        let expected = Delta::default()
+            .with_added("dir_added_1", ["file1", "file2", "file3", "file4"])
+            .with_removed("dir_removed_1", ["file1", "file2", "file3", "file4"]);
+
+        assert_eq!(expected, a.union(&b));
+        assert_eq!(expected, b.union(&a));
+    }
+}