When distributing your Unreal Engine game via Steam, optimizing update size is essential for reducing download times and improving the user experience. However, by default, Unreal Engine does not align its chunk data with the chunking system used by Steam’s content delivery system, SteamPipe. This misalignment can lead to unnecessarily large update downloads, even when only a few assets change.
The Problem: Misaligned Chunk Boundaries
SteamPipe calculates delta updates based on 1MB block boundaries. Unfortunately, Unreal Engine typically uses a much smaller default alignment for asset padding inside .pak or .ucas files often 2KB or 2048 bytes. This discrepancy means that even small changes in the game can cause large portions of pak files to shift, resulting in cascading diffs and massive update sizes.
This is particularly problematic when:
- You’re using pak file compression.
- Assets are frequently modified or moved.
- You’re releasing frequent, incremental updates.
The Solution: Align Unreal Padding to Steam’s 1MB Chunk Size
By aligning Unreal’s internal asset padding and compression block size to 1MB (1048576 bytes)—the same as SteamPipe—you can drastically reduce unnecessary binary diffs and optimize your patch sizes.
There are two main ways to apply this fix: one that requires modifying the engine, and another that works with a configuration file if you’re using IoStore.
Option 1: Modify the Engine (Pak-Based Builds)
If you’re still using traditional .pak files and are comfortable modifying the engine, update the packaging logic directly.
Edit the file:
Engine/Source/Programs/AutomationTool/Win/WinPlatform.Automation.cs
In the function GetPlatformPakCommandLine, look for the line:
string PakParams = " -patchpaddingalign=2048";
Replace it with:
string PakParams = " -patchpaddingalign=1048576 -blocksize=1048576";
This forces Unreal to align padding and block sizes to match SteamPipe’s 1MB chunk boundaries, which leads to far smaller deltas on update.
Option 2: Use a Config File (IoStore-Based Builds)
If you prefer not to modify the engine, and you are using Unreal’s IoStore format (enabled by default in UE5), you can apply a similar fix via configuration:
In your DefaultGame.ini, add:
[/Script/UnrealEd.ProjectPackagingSettings]
PackageAdditionalCompressionOptions=-iostorepatchpaddingalign=1048576
This ensures IoStore chunk data also respects the 1MB alignment, which is just as important for minimizing update sizes on Steam.
Final Thoughts
Aligning Unreal’s asset chunking to SteamPipe’s 1MB block structure is a simple but powerful optimization. It can lead to significantly smaller patch sizes and a smoother experience for your players—especially if you’re shipping frequent updates. Whether you patch .pak or .ucas files, adopting this alignment strategy is a low-effort, high-impact fix for most Unreal projects distributed via Steam.