Rendering errors on static meshes created with BSP

This happens with only a few of the static meshes I create using BSP, but it is a big problem because it messes up the mesh completely. There are stretches and other artifacts all over it, and the lighting doesn’t look right.

This is what one of the meshes looks like, you can see the artifacts on the building:

Is there a fix for this, or do I need to edit it in a modeling program?