diff --git a/yarn-project/circuit-types/src/l2_block.ts b/yarn-project/circuit-types/src/l2_block.ts
index f3d570d3705..2a8efb85a3d 100644
--- a/yarn-project/circuit-types/src/l2_block.ts
+++ b/yarn-project/circuit-types/src/l2_block.ts
@@ -8,6 +8,7 @@ import {
   MAX_NEW_NULLIFIERS_PER_TX,
   MAX_PUBLIC_DATA_UPDATE_REQUESTS_PER_TX,
   NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP,
+  NUM_BYTES_PER_SHA256,
   PartialStateReference,
   STRING_ENCODING,
   StateReference,
@@ -223,7 +224,7 @@ export class L2Block {
        */
       newContractData: ContractData[];
       /**
-       * The L1 to L2 messages to be inserted into the L2 toL2 message tree.
+       * The L1 to L2 messages to be inserted into the L1 to L2 message tree.
        */
       newL1ToL2Messages: Fr[];
       /**
@@ -357,7 +358,7 @@ export class L2Block {
     );
     const state = new StateReference(endL1ToL2MessageTreeSnapshot, partial);
     // TODO(#3938): populate bodyHash
-    const header = new Header(startArchiveSnapshot, [Fr.ZERO, Fr.ZERO], state, globalVariables);
+    const header = new Header(startArchiveSnapshot, Buffer.alloc(NUM_BYTES_PER_SHA256), state, globalVariables);
 
     return L2Block.fromFields(
       {
diff --git a/yarn-project/circuits.js/src/structs/global_variables.ts b/yarn-project/circuits.js/src/structs/global_variables.ts
index 7fd59476772..2a569fb984c 100644
--- a/yarn-project/circuits.js/src/structs/global_variables.ts
+++ b/yarn-project/circuits.js/src/structs/global_variables.ts
@@ -53,6 +53,7 @@ export class GlobalVariables {
   }
 
   static getFields(fields: FieldsOf<GlobalVariables>) {
+    // Note: The order here must match the order in the HeaderDecoder solidity library.
     return [fields.chainId, fields.version, fields.blockNumber, fields.timestamp] as const;
   }
 
diff --git a/yarn-project/circuits.js/src/structs/header.test.ts b/yarn-project/circuits.js/src/structs/header.test.ts
new file mode 100644
index 00000000000..4cc4e60d8b5
--- /dev/null
+++ b/yarn-project/circuits.js/src/structs/header.test.ts
@@ -0,0 +1,12 @@
+import { makeHeader } from '../tests/factories.js';
+import { Header } from './header.js';
+
+describe('Header', () => {
+  it(`serializes to buffer and deserializes it back`, () => {
+    const randomInt = Math.floor(Math.random() * 1000);
+    const expected = makeHeader(randomInt, undefined);
+    const buffer = expected.toBuffer();
+    const res = Header.fromBuffer(buffer);
+    expect(res).toEqual(expected);
+  });
+});
diff --git a/yarn-project/circuits.js/src/structs/header.ts b/yarn-project/circuits.js/src/structs/header.ts
index 13e518a7306..12aebcf39af 100644
--- a/yarn-project/circuits.js/src/structs/header.ts
+++ b/yarn-project/circuits.js/src/structs/header.ts
@@ -1,25 +1,30 @@
-import { Fr } from '@aztec/foundation/fields';
 import { BufferReader, serializeToBuffer } from '@aztec/foundation/serialize';
 
-import { NUM_FIELDS_PER_SHA256 } from '../constants.gen.js';
 import { GlobalVariables } from './global_variables.js';
 import { AppendOnlyTreeSnapshot } from './rollup/append_only_tree_snapshot.js';
 import { StateReference } from './state_reference.js';
 
+export const NUM_BYTES_PER_SHA256 = 32;
+
 /** A header of an L2 block. */
 export class Header {
   constructor(
     /** Snapshot of archive before the block is applied. */
     public lastArchive: AppendOnlyTreeSnapshot,
     /** Hash of the body of an L2 block. */
-    public bodyHash: [Fr, Fr],
+    public bodyHash: Buffer,
     /** State reference. */
     public state: StateReference,
     /** Global variables of an L2 block. */
     public globalVariables: GlobalVariables,
-  ) {}
+  ) {
+    if (bodyHash.length !== 32) {
+      throw new Error('Body hash buffer must be 32 bytes');
+    }
+  }
 
   toBuffer() {
+    // Note: The order here must match the order in the HeaderDecoder solidity library.
     return serializeToBuffer(this.lastArchive, this.bodyHash, this.state, this.globalVariables);
   }
 
@@ -27,7 +32,7 @@ export class Header {
     const reader = BufferReader.asReader(buffer);
     return new Header(
       reader.readObject(AppendOnlyTreeSnapshot),
-      reader.readArray(NUM_FIELDS_PER_SHA256, Fr) as [Fr, Fr],
+      reader.readBytes(NUM_BYTES_PER_SHA256),
       reader.readObject(StateReference),
       reader.readObject(GlobalVariables),
     );
diff --git a/yarn-project/circuits.js/src/structs/rollup/root_rollup.ts b/yarn-project/circuits.js/src/structs/rollup/root_rollup.ts
index 5a4b34df659..2a839e6eda6 100644
--- a/yarn-project/circuits.js/src/structs/rollup/root_rollup.ts
+++ b/yarn-project/circuits.js/src/structs/rollup/root_rollup.ts
@@ -95,23 +95,6 @@ export class RootRollupPublicInputs {
     return new RootRollupPublicInputs(...RootRollupPublicInputs.getFields(fields));
   }
 
-  /**
-   * Returns the sha256 hash of the calldata.
-   * @returns The sha256 hash of the calldata.
-   */
-  public sha256CalldataHash(): Buffer {
-    const high = this.header.bodyHash[0].toBuffer();
-    const low = this.header.bodyHash[1].toBuffer();
-
-    const hash = Buffer.alloc(32);
-    for (let i = 0; i < 16; i++) {
-      hash[i] = high[i + 16];
-      hash[i + 16] = low[i + 16];
-    }
-
-    return hash;
-  }
-
   /**
    * Deserializes a buffer into a `RootRollupPublicInputs` object.
    * @param buffer - The buffer to deserialize.
diff --git a/yarn-project/circuits.js/src/structs/state_reference.ts b/yarn-project/circuits.js/src/structs/state_reference.ts
index 88b649f601b..21c9359434f 100644
--- a/yarn-project/circuits.js/src/structs/state_reference.ts
+++ b/yarn-project/circuits.js/src/structs/state_reference.ts
@@ -15,6 +15,7 @@ export class StateReference {
   ) {}
 
   toBuffer() {
+    // Note: The order here must match the order in the HeaderDecoder solidity library.
     return serializeToBuffer(this.l1ToL2MessageTree, this.partial);
   }
 
diff --git a/yarn-project/circuits.js/src/tests/factories.ts b/yarn-project/circuits.js/src/tests/factories.ts
index 93439eea2bf..6586d23ceb1 100644
--- a/yarn-project/circuits.js/src/tests/factories.ts
+++ b/yarn-project/circuits.js/src/tests/factories.ts
@@ -3,6 +3,8 @@ import { AztecAddress } from '@aztec/foundation/aztec-address';
 import { EthAddress } from '@aztec/foundation/eth-address';
 import { numToUInt32BE } from '@aztec/foundation/serialize';
 
+import { randomBytes } from 'crypto';
+
 import { SchnorrSignature } from '../barretenberg/index.js';
 import {
   ARCHIVE_HEIGHT,
@@ -101,7 +103,7 @@ import {
   WitnessedPublicCallData,
 } from '../index.js';
 import { GlobalVariables } from '../structs/global_variables.js';
-import { Header } from '../structs/header.js';
+import { Header, NUM_BYTES_PER_SHA256 } from '../structs/header.js';
 
 /**
  * Creates an arbitrary side effect object with the given seed.
@@ -882,7 +884,7 @@ export function makeRootRollupPublicInputs(
 export function makeHeader(seed = 0, globalVariables: GlobalVariables | undefined): Header {
   return new Header(
     makeAppendOnlyTreeSnapshot(seed + 0x100),
-    [new Fr(5n), new Fr(6n)],
+    randomBytes(NUM_BYTES_PER_SHA256),
     makeStateReference(seed + 0x200),
     globalVariables ?? makeGlobalVariables((seed += 0x100)),
   );
diff --git a/yarn-project/foundation/src/serialize/free_funcs.test.ts b/yarn-project/foundation/src/serialize/free_funcs.test.ts
new file mode 100644
index 00000000000..e3be26dd2d8
--- /dev/null
+++ b/yarn-project/foundation/src/serialize/free_funcs.test.ts
@@ -0,0 +1,18 @@
+import { randomBytes } from '../crypto/index.js';
+import { from2Fields, to2Fields } from './free_funcs.js';
+
+describe('buffer to fields and back', () => {
+  it('should correctly serialize and deserialize a buffer', () => {
+    // Generate a random 32-byte buffer
+    const originalBuffer = randomBytes(32);
+
+    // Serialize the buffer to two fields
+    const [field1, field2] = to2Fields(originalBuffer);
+
+    // Deserialize the fields back to a buffer
+    const reconstructedBuffer = from2Fields(field1, field2);
+
+    // Check if the original buffer and reconstructed buffer are identical
+    expect(reconstructedBuffer).toEqual(originalBuffer);
+  });
+});
diff --git a/yarn-project/foundation/src/serialize/free_funcs.ts b/yarn-project/foundation/src/serialize/free_funcs.ts
index 6165657f0f9..a9f9dcdfc1e 100644
--- a/yarn-project/foundation/src/serialize/free_funcs.ts
+++ b/yarn-project/foundation/src/serialize/free_funcs.ts
@@ -126,3 +126,22 @@ export function to2Fields(buf: Buffer): [Fr, Fr] {
 
   return [Fr.fromBuffer(buf1), Fr.fromBuffer(buf2)];
 }
+
+/**
+ * Reconstructs the original 32 bytes of data from 2 field elements.
+ * @param field1 - First field element
+ * @param field2 - Second field element
+ * @returns 32 bytes of data as a Buffer
+ */
+export function from2Fields(field1: Fr, field2: Fr): Buffer {
+  // Convert the field elements back to buffers
+  const buf1 = field1.toBuffer();
+  const buf2 = field2.toBuffer();
+
+  // Remove the padding (first 16 bytes) from each buffer
+  const originalPart1 = buf1.slice(16, 32);
+  const originalPart2 = buf2.slice(16, 32);
+
+  // Concatenate the two parts to form the original buffer
+  return Buffer.concat([originalPart1, originalPart2]);
+}
diff --git a/yarn-project/noir-protocol-circuits/src/crates/rollup-lib/src/root.nr b/yarn-project/noir-protocol-circuits/src/crates/rollup-lib/src/root.nr
index 0fff41cc8fe..e44769c9719 100644
--- a/yarn-project/noir-protocol-circuits/src/crates/rollup-lib/src/root.nr
+++ b/yarn-project/noir-protocol-circuits/src/crates/rollup-lib/src/root.nr
@@ -65,11 +65,6 @@ impl RootRollupInputs {
             0
         );
 
-        let zeroed_out_snapshot = AppendOnlyTreeSnapshot {
-            root : 0,
-            next_available_leaf_index : 0
-        };
-
         let header = Header {
             last_archive: left.constants.last_archive,
             body_hash: components::compute_calldata_hash(self.previous_rollup_data),
diff --git a/yarn-project/noir-protocol-circuits/src/type_conversion.ts b/yarn-project/noir-protocol-circuits/src/type_conversion.ts
index 260d5946f8e..9b598111b4a 100644
--- a/yarn-project/noir-protocol-circuits/src/type_conversion.ts
+++ b/yarn-project/noir-protocol-circuits/src/type_conversion.ts
@@ -72,7 +72,7 @@ import {
   TxContext,
   TxRequest,
 } from '@aztec/circuits.js';
-import { Tuple, mapTuple } from '@aztec/foundation/serialize';
+import { Tuple, from2Fields, mapTuple } from '@aztec/foundation/serialize';
 
 import {
   BlockHeader as BlockHeaderNoir,
@@ -137,6 +137,8 @@ import {
   AppendOnlyTreeSnapshot as AppendOnlyTreeSnapshotNoir,
   BaseOrMergeRollupPublicInputs as BaseOrMergeRollupPublicInputsNoir,
   ConstantRollupData as ConstantRollupDataNoir,
+  Field,
+  FixedLengthArray,
   GlobalVariables as GlobalVariablesNoir,
   Header as HeaderNoir,
   PartialStateReference as PartialStateReferenceNoir,
@@ -678,6 +680,15 @@ export function mapTupleFromNoir<T, N extends number, M>(
   return Array.from({ length }, (_, idx) => mapper(noirArray[idx])) as Tuple<M, N>;
 }
 
+/**
+ * Maps a SHA256 hash from noir to the parsed type.
+ * @param hash - The hash as it is represented in Noir (2 fields).
+ * @returns The hash represented as a 32 bytes long buffer.
+ */
+export function mapSha256HashFromNoir(hash: FixedLengthArray<Field, 2>): Buffer {
+  return from2Fields(mapFieldFromNoir(hash[0]), mapFieldFromNoir(hash[1]));
+}
+
 /**
  * Maps optionally revealed data from noir to the parsed type.
  * @param optionallyRevealedData - The noir optionally revealed data.
@@ -1318,7 +1329,7 @@ export function mapRootRollupPublicInputsFromNoir(
 export function mapHeaderFromNoir(header: HeaderNoir): Header {
   return new Header(
     mapAppendOnlyTreeSnapshotFromNoir(header.last_archive),
-    mapTupleFromNoir(header.body_hash, 2, mapFieldFromNoir),
+    mapSha256HashFromNoir(header.body_hash),
     mapStateReferenceFromNoir(header.state),
     mapGlobalVariablesFromNoir(header.global_variables),
   );
diff --git a/yarn-project/sequencer-client/src/block_builder/solo_block_builder.test.ts b/yarn-project/sequencer-client/src/block_builder/solo_block_builder.test.ts
index 5a390b36fa6..a0e03bb5afc 100644
--- a/yarn-project/sequencer-client/src/block_builder/solo_block_builder.test.ts
+++ b/yarn-project/sequencer-client/src/block_builder/solo_block_builder.test.ts
@@ -250,11 +250,7 @@ describe('sequencer/solo_block_builder', () => {
       newUnencryptedLogs,
     });
 
-    const callDataHash = l2Block.getCalldataHash();
-    const high = Fr.fromBuffer(callDataHash.slice(0, 16));
-    const low = Fr.fromBuffer(callDataHash.slice(16, 32));
-
-    rootRollupOutput.header.bodyHash = [high, low];
+    rootRollupOutput.header.bodyHash = l2Block.getCalldataHash();
 
     return txs;
   };
diff --git a/yarn-project/sequencer-client/src/block_builder/solo_block_builder.ts b/yarn-project/sequencer-client/src/block_builder/solo_block_builder.ts
index d8958a6d409..72a4fbf741e 100644
--- a/yarn-project/sequencer-client/src/block_builder/solo_block_builder.ts
+++ b/yarn-project/sequencer-client/src/block_builder/solo_block_builder.ts
@@ -144,11 +144,11 @@ export class SoloBlockBuilder implements BlockBuilder {
       newUnencryptedLogs,
     });
 
-    if (!l2Block.getCalldataHash().equals(circuitsOutput.sha256CalldataHash())) {
+    if (!l2Block.getCalldataHash().equals(circuitsOutput.header.bodyHash)) {
       throw new Error(
-        `Calldata hash mismatch, ${l2Block.getCalldataHash().toString('hex')} == ${circuitsOutput
-          .sha256CalldataHash()
-          .toString('hex')} `,
+        `Calldata hash mismatch, ${l2Block
+          .getCalldataHash()
+          .toString('hex')} == ${circuitsOutput.header.bodyHash.toString('hex')} `,
       );
     }