The 1x4 select pine is actually only 3/4x3.5 inches. The Luan is a 1/4 inch. If I stuck the side panels on the frame I would only have a door that is 1.25 inches thick. I've never seen a door knob this skinny before. Most door knobs will do 1 3/8 to 2 inches. I decided to put spacing on both sides which would give me an end result of 1.5 inch thick door. With the excess pieces I had left from cutting the side panels from the Luan, I ruffly cut, glued and used brad nails (5/8 inch) to secure them to the frame. I also left over hang on these spacers.