Why does my Direction Light cause lines on my mesh?

Directional light creates lines on meshes. It’s most noticeable on flat surfaces like this cube. When light hits it from an angle lines show up.

If I’m correct, this was an issue with cascaded shadow maps all the way back in UDK. when shadows hit a certain angle on geometry.
It seems much better in UE4 as there are more options to control shadow rendering. I can elimate this entirely by adjusting shadow properties. Play with the numbers for things like Light Angle, number of shadows and try turning on Distance Field Shadows and playing with those settings too. The lines will disappear and the resolution of your shadows including softness etc can all be tweaked.

Ok i’ll try

You can have a look at our Wiki Lighting Troubleshooting and Tips guide in the Dynamic Lighting section.