If you need more than 1/4 inch of spacer or need spacing at all with a combustible finished wall you have violation of 314.20. If using Arlington's non metallic box extenders (BE1 specifically) to correct this violation, often the flange of the extender is big enough to seat against the finished wall and the extender is also the needed spacer.
I use these also where you have a receptacle in a cabinet like the micro-mate over the range, almost everyone one installs the receptacle flush with the drywall and never realizes that code requires the box to be flush with the back of the cabinet, but for the cost I been using those green spacers that you zigzag together how many needed to space the receptacle when you get plaster of wall board over cut and the box is back a little, I don't think plaster is considered combustible? wall board, such as dry wall, I have heard it said it was combustible, but well I have never seen any that you could get to burn, the paper glued right against the rock just will not burn as the rock takes the heat right out of the paper, loose ends will burn but it will go right out as soon as it gets to the gypsum.
I have used the Arlington extenders for box's set way to far back like where someone just installed new wall board right over the existing and they work great for that.
But what gets me is the code has never really address ceiling lights, that the open back of the canopy is right against the ceiling combustible or not, even though the box is flush with the ceiling, why doesn't UL require ceiling light to be enclosed with only an opening as large as the box?
