[Jim Port]

The box needs to be flush or extend from a combustible surface.

 

[Jim Port]

Under the NEC, a box can be recessed up to 1/4" in a non combustible surface like drywall. In a combustible surface like wood it needs to be flush or proud of the surface to avoid exposing the combustible surface to an arc or spark.

 

[Jim Port]

The gap outside the box can be no larger than 1/8".