Fix: Unity ProBuilder UV Stretching After Merge

Here is how to fix UV stretching after merging faces in Unity ProBuilder. You build a wall out of several quads, merge them into one face for cleanliness, and the texture goes from crisp brickwork to a smeared mess. The geometry looks correct, the normals are fine, but the UVs are completely wrong. ProBuilder does not re-unwrap after a merge, so you have to do it yourself. Here is the full process.

The Symptom

After using ProBuilder’s Merge Faces tool, one or more of these occur:

• Texture stretches across the merged face
• Texture appears at the wrong scale (too large or too small)
• Texture rotates or skews on the merged face
• Lightmap baking produces black spots or light bleeding on merged geometry

Undoing the merge and looking at the individual faces shows correct UVs. The merge operation itself is the cause.

What Causes This

UV preservation without re-projection. When you merge faces, ProBuilder takes the UV coordinates from each source face and keeps them. But the vertex topology changes — shared edges are removed, the face outline changes — and the old UVs no longer map correctly to the new geometry. The UV coordinates are technically valid per-vertex, but the interpolation across the new larger polygon produces stretching.

Mixed UV modes. If some source faces were using Auto UV and others Manual UV, the merge produces a face with inconsistent UV data. ProBuilder defaults the merged face to whatever mode the first selected face had, but the coordinates are from all sources.

Non-planar merges. Merging faces that are not coplanar produces a single face with vertices at different angles. Auto UV projects along one normal direction, which cannot correctly map a non-planar surface. The projection works for one part of the face and stretches on the rest.

The Fix

Step 1: Select the merged face. Enter Face selection mode (keyboard shortcut: the number 3 key with ProBuilder toolbar open). Click the merged face. It should highlight as a single selection.

Step 2: Re-apply Auto UV. With the face selected, open the ProBuilder UV Editor (Tools > ProBuilder > UV Editor). Click Auto to switch the face to Auto UV mode. ProBuilder will re-project the UVs based on the face normal.

// If scripting ProBuilder, re-apply auto UVs after merge:
using UnityEngine.ProBuilder;
using UnityEngine.ProBuilder.MeshOperations;

ProBuilderMesh mesh = GetComponent<ProBuilderMesh>();
Face mergedFace = mesh.faces[targetIndex];

// Reset to auto UV with default settings
mergedFace.uv = new AutoUnwrapSettings();
mergedFace.manualUV = false;
mesh.RefreshUV(new Face[] { mergedFace });
mesh.ToMesh();
mesh.Refresh();

Step 3: Adjust Auto UV settings. In the UV Editor with Auto mode active, tune these settings:

• Offset: shifts the UV origin to align texture patterns
• Rotation: rotates the projection to match the desired texture direction
• Tiling: scales the UV to match surrounding faces
• Anchor: which point the projection is anchored to

Match the tiling values to adjacent faces so the texture appears seamless across the geometry.

Step 4: Use Manual UV for complex shapes. If the merged face is not planar or Auto UV cannot produce acceptable results, switch to Manual UV mode. This opens a 2D UV editor where you can move individual UV vertices, pin edges, and sew UV islands. Select the face, click Manual in the UV Editor, then use the projection tools (Box, Planar, Cylindrical) to get a starting layout and refine from there.

Lightmap UVs After Merge

ProBuilder generates UV2 (lightmap UVs) separately from texture UVs. After a merge, UV2 coordinates may overlap, which causes baked lighting artifacts.

To fix: select the ProBuilder mesh, open the ProBuilder toolbar, and click Generate UV2. Or in the MeshRenderer Inspector, click Generate Lightmap UVs. This regenerates non-overlapping lightmap UV coordinates. Rebake lighting afterward.

If you see dark patches or light bleeding after baking, overlapping UV2 is almost always the cause. Check the UV2 channel in the UV Editor to confirm no islands overlap.

Preventing the Problem

Before merging faces, decide if the merge is necessary. Merged faces reduce polygon count but create UV complexity. For textured geometry, keeping faces separate often produces better visual results with no performance penalty — the GPU does not care about face count the way it cares about draw calls and vertex count.

If you do merge, immediately re-unwrap. Make it a habit: merge, then Auto UV, then adjust tiling. Doing it right after the merge takes seconds. Finding stretched UVs hours later in a complex scene takes much longer to diagnose.

“ProBuilder keeps your old UVs after a merge because it does not want to destroy manual work. But when Auto UV was good enough before, it is good enough to re-apply after.”

Related Issues

For texture tiling mismatches between ProBuilder objects and standard meshes, check that material tiling settings are consistent. For lightmap seams at ProBuilder mesh boundaries, increase lightmap padding in Lighting Settings. For ProBuilder mesh export issues, use Export Mesh to create a standard .mesh asset and re-import with correct UV settings.

Merge faces, then re-unwrap. Auto UV for planar. Manual UV for complex. Regenerate UV2 for lightmaps.