diff --git a/eth2/utils/ssz/src/cached_tree_hash.rs b/eth2/utils/ssz/src/cached_tree_hash.rs
index ce90afd337..be3fe98de4 100644
--- a/eth2/utils/ssz/src/cached_tree_hash.rs
+++ b/eth2/utils/ssz/src/cached_tree_hash.rs
@@ -44,6 +44,39 @@ impl Into<Vec<u8>> for TreeHashCache {
 }
 
 impl TreeHashCache {
+    pub fn new(mut leaves_and_subtrees: Vec<u8>, offset_handler: OffsetHandler) -> Option<Self> {
+        if leaves_and_subtrees.len() % BYTES_PER_CHUNK != 0 {
+            return None;
+        }
+
+        // Allocate enough bytes to store the internal nodes and the leaves and subtrees, then fill
+        // all the to-be-built internal nodes with zeros and append the leaves and subtrees.
+        let internal_node_bytes = offset_handler.num_internal_nodes * BYTES_PER_CHUNK;
+        let mut cache = Vec::with_capacity(internal_node_bytes + leaves_and_subtrees.len());
+        cache.resize(internal_node_bytes, 0);
+        cache.append(&mut leaves_and_subtrees);
+
+        // Concat all the leaves into one big byte array, ready for `merkleize`.
+        let mut leaves = vec![];
+        for leaf_chunk in offset_handler.iter_leaf_nodes() {
+            let start = leaf_chunk * BYTES_PER_CHUNK;
+            let end = start + BYTES_PER_CHUNK;
+
+            leaves.extend_from_slice(cache.get(start..end)?);
+        }
+
+        // Merkleize the leaves, then split the leaf nodes off them. Then, replace all-zeros
+        // internal nodes created earlier with the internal nodes generated by `merkleize`.
+        let mut merkleized = merkleize(leaves);
+        merkleized.split_off(internal_node_bytes);
+        cache.splice(0..internal_node_bytes, merkleized);
+
+        Some(Self {
+            chunk_modified: vec![false; cache.len() / BYTES_PER_CHUNK],
+            cache,
+        })
+    }
+
     pub fn from_bytes(bytes: Vec<u8>) -> Option<Self> {
         if bytes.len() % BYTES_PER_CHUNK > 0 {
             return None;
diff --git a/eth2/utils/ssz/src/cached_tree_hash/tests.rs b/eth2/utils/ssz/src/cached_tree_hash/tests.rs
index 9db0a5906c..9cb012c794 100644
--- a/eth2/utils/ssz/src/cached_tree_hash/tests.rs
+++ b/eth2/utils/ssz/src/cached_tree_hash/tests.rs
@@ -13,28 +13,18 @@ impl CachedTreeHash for Inner {
     type Item = Self;
 
     fn build_cache_bytes(&self) -> Vec<u8> {
-        let cache_a = self.a.build_cache_bytes();
-        let cache_b = self.b.build_cache_bytes();
-        let cache_c = self.c.build_cache_bytes();
-        let cache_d = self.d.build_cache_bytes();
+        let mut leaves_and_subtrees = vec![];
 
-        let mut leaves = vec![];
-        leaves.extend_from_slice(&cache_a[0..32].to_vec());
-        leaves.extend_from_slice(&cache_b[0..32].to_vec());
-        leaves.extend_from_slice(&cache_c[0..32].to_vec());
-        leaves.extend_from_slice(&cache_d[0..32].to_vec());
-
-        // TODO: fix unwrap
-        let mut cache = TreeHashCache::from_bytes(merkleize(leaves)).unwrap();
+        leaves_and_subtrees.append(&mut self.a.build_cache_bytes());
+        leaves_and_subtrees.append(&mut self.b.build_cache_bytes());
+        leaves_and_subtrees.append(&mut self.c.build_cache_bytes());
+        leaves_and_subtrees.append(&mut self.d.build_cache_bytes());
 
         // TODO: fix unwrap
         let offset_handler = self.offset_handler(0).unwrap();
-        let mut iter = offset_handler.iter_leaf_nodes();
 
-        cache.single_chunk_splice(*iter.next().unwrap(), cache_a);
-        cache.single_chunk_splice(*iter.next().unwrap(), cache_b);
-        cache.single_chunk_splice(*iter.next().unwrap(), cache_c);
-        cache.single_chunk_splice(*iter.next().unwrap(), cache_d);
+        // TODO: fix unwrap
+        let cache = TreeHashCache::new(leaves_and_subtrees, offset_handler).unwrap();
 
         cache.into()
     }
@@ -111,25 +101,17 @@ impl CachedTreeHash for Outer {
     type Item = Self;
 
     fn build_cache_bytes(&self) -> Vec<u8> {
-        let cache_a = self.a.build_cache_bytes();
-        let cache_b = self.b.build_cache_bytes();
-        let cache_c = self.c.build_cache_bytes();
+        let mut leaves_and_subtrees = vec![];
 
-        let mut leaves = vec![];
-        leaves.extend_from_slice(&cache_a[0..32].to_vec());
-        leaves.extend_from_slice(&cache_b[0..32].to_vec());
-        leaves.extend_from_slice(&cache_c[0..32].to_vec());
-
-        // TODO: fix unwrap
-        let mut cache = TreeHashCache::from_bytes(merkleize(leaves)).unwrap();
+        leaves_and_subtrees.append(&mut self.a.build_cache_bytes());
+        leaves_and_subtrees.append(&mut self.b.build_cache_bytes());
+        leaves_and_subtrees.append(&mut self.c.build_cache_bytes());
 
         // TODO: fix unwrap
         let offset_handler = self.offset_handler(0).unwrap();
-        let mut iter = offset_handler.iter_leaf_nodes();
 
-        cache.single_chunk_splice(*iter.next().unwrap(), cache_a);
-        cache.single_chunk_splice(*iter.next().unwrap(), cache_b);
-        cache.single_chunk_splice(*iter.next().unwrap(), cache_c);
+        // TODO: fix unwrap
+        let cache = TreeHashCache::new(leaves_and_subtrees, offset_handler).unwrap();
 
         cache.into()
     }