Quick answer: Unreal static lighting comes out black on some meshes after Build Lighting? Either lightmap UVs overlap, the lightmap resolution is too low, or the mesh doesn’t have a lightmap UV channel.
A pillar mesh renders pure black indoors after a successful Build Lighting. Its lightmap UVs have overlapping triangles — Lightmass couldn’t allocate a unique texel.
Lightmap UV Channel
Mesh editor → LOD → Lightmap Coordinate Index. Most meshes use UV1 for lightmaps. If UV1 doesn’t exist, set Generate Lightmap UVs = true and re-import.
Non-Overlapping UVs
Lightmap UVs must not overlap. Auto-generated UVs are usually OK; hand-authored UVs from artists may stack. Use the Lightmap Density viewmode to spot black spots before baking.
Lightmap Resolution
Too low resolution + complex shape = black islands. Bump OverriddenLightMapRes on the placed instance. Doubles texel count, baked lighting becomes detailed enough to fill.
Build Settings Quality
World Settings → Lightmass Settings. Production quality is necessary for shipping; Preview misses indirect bounces and may leave dark spots.
Verifying
Build Lighting at Production. All meshes show consistent indirect lighting. No black surfaces or harsh transitions across instance boundaries.
“Black bakes are UV problems. Generate lightmap UVs, raise resolution, ban overlaps.”
Build a checklist of meshes to validate (lightmap UV present, non-overlapping, resolution sane) and run it before every overnight bake.