Roughness textures stretch on 3D models in Substance Painter mainly due to problematic UV unwrapping—like uneven scaling, overlapping islands, or poor seam placement.
UVs act as a 2D "map" for 3D surfaces; if they’re stretched or misaligned, any texture (including roughness) distorts when applied. Common issues include overly stretched UV islands (from manual errors or bad tools), overlapping UVs (mixing texture data), or seams in visible areas (e.g., curves) causing unnatural stretching.
To fix it, adjust UVs in 3D software (e.g., Blender)—ensure even scaling, no overlaps, and seams along hidden edges. Use Substance Painter’s "UV Checker" to spot stretching before painting.