diff --git a/eth2/utils/tree_hash/src/btree_overlay.rs b/eth2/utils/tree_hash/src/btree_overlay.rs
index e69de29bb2..7d5602c0b6 100644
--- a/eth2/utils/tree_hash/src/btree_overlay.rs
+++ b/eth2/utils/tree_hash/src/btree_overlay.rs
@@ -0,0 +1,100 @@
+use super::*;
+
+#[derive(Debug)]
+pub struct BTreeOverlay {
+    pub num_internal_nodes: usize,
+    pub num_leaf_nodes: usize,
+    pub first_node: usize,
+    pub next_node: usize,
+    offsets: Vec<usize>,
+}
+
+impl BTreeOverlay {
+    pub fn new<T>(item: &T, initial_offset: usize) -> Result<Self, Error>
+    where
+        T: CachedTreeHash<T>,
+    {
+        item.btree_overlay(initial_offset)
+    }
+
+    pub fn from_lengths(offset: usize, mut lengths: Vec<usize>) -> Result<Self, Error> {
+        // Extend it to the next power-of-two, if it is not already.
+        let num_leaf_nodes = if lengths.len().is_power_of_two() {
+            lengths.len()
+        } else {
+            let num_leaf_nodes = lengths.len().next_power_of_two();
+            lengths.resize(num_leaf_nodes, 1);
+            num_leaf_nodes
+        };
+
+        let num_nodes = num_nodes(num_leaf_nodes);
+        let num_internal_nodes = num_nodes - num_leaf_nodes;
+
+        let mut offsets = Vec::with_capacity(num_nodes);
+        offsets.append(&mut (offset..offset + num_internal_nodes).collect());
+
+        let mut next_node = num_internal_nodes + offset;
+        for i in 0..num_leaf_nodes {
+            offsets.push(next_node);
+            next_node += lengths[i];
+        }
+
+        Ok(Self {
+            num_internal_nodes,
+            num_leaf_nodes,
+            offsets,
+            first_node: offset,
+            next_node,
+        })
+    }
+
+    pub fn root(&self) -> usize {
+        self.first_node
+    }
+
+    pub fn height(&self) -> usize {
+        self.num_leaf_nodes.trailing_zeros() as usize
+    }
+
+    pub fn chunk_range(&self) -> Range<usize> {
+        self.first_node..self.next_node
+    }
+
+    pub fn total_chunks(&self) -> usize {
+        self.next_node - self.first_node
+    }
+
+    pub fn total_nodes(&self) -> usize {
+        self.num_internal_nodes + self.num_leaf_nodes
+    }
+
+    pub fn first_leaf_node(&self) -> Result<usize, Error> {
+        self.offsets
+            .get(self.num_internal_nodes)
+            .cloned()
+            .ok_or_else(|| Error::NoFirstNode)
+    }
+
+    /// Returns an iterator visiting each internal node, providing the left and right child chunks
+    /// for the node.
+    pub fn iter_internal_nodes<'a>(
+        &'a self,
+    ) -> impl DoubleEndedIterator<Item = (&'a usize, (&'a usize, &'a usize))> {
+        let internal_nodes = &self.offsets[0..self.num_internal_nodes];
+
+        internal_nodes.iter().enumerate().map(move |(i, parent)| {
+            let children = children(i);
+            (
+                parent,
+                (&self.offsets[children.0], &self.offsets[children.1]),
+            )
+        })
+    }
+
+    /// Returns an iterator visiting each leaf node, providing the chunk for that node.
+    pub fn iter_leaf_nodes<'a>(&'a self) -> impl DoubleEndedIterator<Item = &'a usize> {
+        let leaf_nodes = &self.offsets[self.num_internal_nodes..];
+
+        leaf_nodes.iter()
+    }
+}
diff --git a/eth2/utils/tree_hash/src/impls.rs b/eth2/utils/tree_hash/src/impls.rs
index d5297c38ee..9149cf8aab 100644
--- a/eth2/utils/tree_hash/src/impls.rs
+++ b/eth2/utils/tree_hash/src/impls.rs
@@ -14,12 +14,12 @@ impl CachedTreeHash<u64> for u64 {
         )?)
     }
 
-    fn num_bytes(&self) -> usize {
-        8
+    fn btree_overlay(&self, _chunk_offset: usize) -> Result<BTreeOverlay, Error> {
+        Err(Error::ShouldNotProduceBTreeOverlay)
     }
 
-    fn offsets(&self) -> Result<Vec<usize>, Error> {
-        Err(Error::ShouldNotProduceBTreeOverlay)
+    fn num_bytes(&self) -> usize {
+        8
     }
 
     fn num_child_nodes(&self) -> usize {
@@ -71,21 +71,22 @@ where
         }
     }
 
-    fn offsets(&self) -> Result<Vec<usize>, Error> {
-        let offsets = match T::item_type() {
+    fn btree_overlay(&self, chunk_offset: usize) -> Result<BTreeOverlay, Error> {
+        //
+        let lengths = match T::item_type() {
             ItemType::Basic => vec![1; self.len() / T::packing_factor()],
             ItemType::Composite | ItemType::List => {
-                let mut offsets = vec![];
+                let mut lengths = vec![];
 
                 for item in self {
-                    offsets.push(BTreeOverlay::new(item, 0)?.total_nodes())
+                    lengths.push(BTreeOverlay::new(item, 0)?.total_nodes())
                 }
 
-                offsets
+                lengths
             }
         };
 
-        Ok(offsets)
+        BTreeOverlay::from_lengths(chunk_offset, lengths)
     }
 
     fn num_child_nodes(&self) -> usize {
@@ -180,7 +181,7 @@ where
                         (Some(old), None) => {
                             // Splice out the entire tree of the removed node, replacing it with a
                             // single padding node.
-                            let end_chunk = BTreeOverlay::new(old, start_chunk)?.next_node();
+                            let end_chunk = BTreeOverlay::new(old, start_chunk)?.next_node;
 
                             cache.splice(
                                 start_chunk..end_chunk,
@@ -218,7 +219,7 @@ where
             cache.modify_chunk(root_node, &cache.mix_in_length(root_node, self.len())?)?;
         }
 
-        Ok(offset_handler.next_node())
+        Ok(offset_handler.next_node)
     }
 }
 
diff --git a/eth2/utils/tree_hash/src/lib.rs b/eth2/utils/tree_hash/src/lib.rs
index 1b085770d2..e356210a44 100644
--- a/eth2/utils/tree_hash/src/lib.rs
+++ b/eth2/utils/tree_hash/src/lib.rs
@@ -1,13 +1,14 @@
 use hashing::hash;
 use int_to_bytes::int_to_bytes32;
 use std::fmt::Debug;
-use std::iter::Iterator;
 use std::ops::Range;
 
+mod btree_overlay;
 mod cached_tree_hash;
 mod impls;
 mod resize;
 
+pub use btree_overlay::BTreeOverlay;
 pub use cached_tree_hash::TreeHashCache;
 
 pub const BYTES_PER_CHUNK: usize = 32;
@@ -44,7 +45,7 @@ pub trait CachedTreeHash<Item>: Debug {
     /// prefixes.
     fn num_bytes(&self) -> usize;
 
-    fn offsets(&self) -> Result<Vec<usize>, Error>;
+    fn btree_overlay(&self, chunk_offset: usize) -> Result<BTreeOverlay, Error>;
 
     fn num_child_nodes(&self) -> usize;
 
@@ -72,109 +73,6 @@ fn node_range_to_byte_range(node_range: &Range<usize>) -> Range<usize> {
     node_range.start * HASHSIZE..node_range.end * HASHSIZE
 }
 
-#[derive(Debug)]
-pub struct BTreeOverlay {
-    num_internal_nodes: usize,
-    pub num_leaf_nodes: usize,
-    first_node: usize,
-    next_node: usize,
-    offsets: Vec<usize>,
-}
-
-impl BTreeOverlay {
-    pub fn new<T>(item: &T, initial_offset: usize) -> Result<Self, Error>
-    where
-        T: CachedTreeHash<T>,
-    {
-        Self::from_lengths(initial_offset, item.offsets()?)
-    }
-
-    fn from_lengths(offset: usize, mut lengths: Vec<usize>) -> Result<Self, Error> {
-        // Extend it to the next power-of-two, if it is not already.
-        let num_leaf_nodes = if lengths.len().is_power_of_two() {
-            lengths.len()
-        } else {
-            let num_leaf_nodes = lengths.len().next_power_of_two();
-            lengths.resize(num_leaf_nodes, 1);
-            num_leaf_nodes
-        };
-
-        let num_nodes = num_nodes(num_leaf_nodes);
-        let num_internal_nodes = num_nodes - num_leaf_nodes;
-
-        let mut offsets = Vec::with_capacity(num_nodes);
-        offsets.append(&mut (offset..offset + num_internal_nodes).collect());
-
-        let mut next_node = num_internal_nodes + offset;
-        for i in 0..num_leaf_nodes {
-            offsets.push(next_node);
-            next_node += lengths[i];
-        }
-
-        Ok(Self {
-            num_internal_nodes,
-            num_leaf_nodes,
-            offsets,
-            first_node: offset,
-            next_node,
-        })
-    }
-
-    pub fn root(&self) -> usize {
-        self.first_node
-    }
-
-    pub fn height(&self) -> usize {
-        self.num_leaf_nodes.trailing_zeros() as usize
-    }
-
-    pub fn chunk_range(&self) -> Range<usize> {
-        self.first_node..self.next_node
-    }
-
-    pub fn total_chunks(&self) -> usize {
-        self.next_node - self.first_node
-    }
-
-    pub fn total_nodes(&self) -> usize {
-        self.num_internal_nodes + self.num_leaf_nodes
-    }
-
-    pub fn first_leaf_node(&self) -> Result<usize, Error> {
-        self.offsets
-            .get(self.num_internal_nodes)
-            .cloned()
-            .ok_or_else(|| Error::NoFirstNode)
-    }
-
-    pub fn next_node(&self) -> usize {
-        self.next_node
-    }
-
-    /// Returns an iterator visiting each internal node, providing the left and right child chunks
-    /// for the node.
-    pub fn iter_internal_nodes<'a>(
-        &'a self,
-    ) -> impl DoubleEndedIterator<Item = (&'a usize, (&'a usize, &'a usize))> {
-        let internal_nodes = &self.offsets[0..self.num_internal_nodes];
-
-        internal_nodes.iter().enumerate().map(move |(i, parent)| {
-            let children = children(i);
-            (
-                parent,
-                (&self.offsets[children.0], &self.offsets[children.1]),
-            )
-        })
-    }
-
-    /// Returns an iterator visiting each leaf node, providing the chunk for that node.
-    pub fn iter_leaf_nodes<'a>(&'a self) -> impl DoubleEndedIterator<Item = &'a usize> {
-        let leaf_nodes = &self.offsets[self.num_internal_nodes..];
-
-        leaf_nodes.iter()
-    }
-}
-
 /// Split `values` into a power-of-two, identical-length chunks (padding with `0`) and merkleize
 /// them, returning the entire merkle tree.
 ///
diff --git a/eth2/utils/tree_hash/tests/tests.rs b/eth2/utils/tree_hash/tests/tests.rs
index 972eb1e00e..af4204cc28 100644
--- a/eth2/utils/tree_hash/tests/tests.rs
+++ b/eth2/utils/tree_hash/tests/tests.rs
@@ -44,15 +44,15 @@ impl CachedTreeHash<Inner> for Inner {
         bytes
     }
 
-    fn offsets(&self) -> Result<Vec<usize>, Error> {
-        let mut offsets = vec![];
+    fn btree_overlay(&self, chunk_offset: usize) -> Result<BTreeOverlay, Error> {
+        let mut lengths = vec![];
 
-        offsets.push(self.a.num_child_nodes() + 1);
-        offsets.push(self.b.num_child_nodes() + 1);
-        offsets.push(self.c.num_child_nodes() + 1);
-        offsets.push(self.d.num_child_nodes() + 1);
+        lengths.push(self.a.num_child_nodes() + 1);
+        lengths.push(self.b.num_child_nodes() + 1);
+        lengths.push(self.c.num_child_nodes() + 1);
+        lengths.push(self.d.num_child_nodes() + 1);
 
-        Ok(offsets)
+        BTreeOverlay::from_lengths(chunk_offset, lengths)
     }
 
     fn num_child_nodes(&self) -> usize {
@@ -98,7 +98,7 @@ impl CachedTreeHash<Inner> for Inner {
             }
         }
 
-        Ok(offset_handler.next_node())
+        Ok(offset_handler.next_node)
     }
 }
 
@@ -146,14 +146,14 @@ impl CachedTreeHash<Outer> for Outer {
         num_nodes(leaves) + children - 1
     }
 
-    fn offsets(&self) -> Result<Vec<usize>, Error> {
-        let mut offsets = vec![];
+    fn btree_overlay(&self, chunk_offset: usize) -> Result<BTreeOverlay, Error> {
+        let mut lengths = vec![];
 
-        offsets.push(self.a.num_child_nodes() + 1);
-        offsets.push(self.b.num_child_nodes() + 1);
-        offsets.push(self.c.num_child_nodes() + 1);
+        lengths.push(self.a.num_child_nodes() + 1);
+        lengths.push(self.b.num_child_nodes() + 1);
+        lengths.push(self.c.num_child_nodes() + 1);
 
-        Ok(offsets)
+        BTreeOverlay::from_lengths(chunk_offset, lengths)
     }
 
     fn packed_encoding(&self) -> Vec<u8> {
@@ -186,7 +186,7 @@ impl CachedTreeHash<Outer> for Outer {
             }
         }
 
-        Ok(offset_handler.next_node())
+        Ok(offset_handler.next_node)
     }
 }