Although the construction of a Murphy bed box is a simple part of the project, be sure to cut all pieces to exact dimensions – a tight fit is essential for proper operation.
Once you've determined the measurements for your Murphy bed, rip two 1x12 poplar side-boards to the size needed to allow your mattress to sit slightly above the bed box.
Next, cut the shorter 1x12 end-boards that will form the top and bottom of the bed box. In this example, the top and bottom boards each measure 54-1/2".
With the top and bottom pieces cut, cut the 1x12 boards that will make up the longer sides. Use a square to mark the ends before making the cuts. In this case, the side pieces are cut to 77-1/2".
With the pieces cut, use a router to cut rabbets on the top inside edge of all boards (Image 1).
Here, the depth is set to 3/4". On the long boards, stop the rabbets short of the ends so they are not visible at the finished corners (Image 2).
Cut the bottom board from medium density fiberboard, or MDF, which is a good choice for interior projects such as this. It takes paint well and provides a smooth surface appearance when the bed is stored away.
Finally, cut a 2x8 to create the ledger board that will connect the bed box to the wall. Here, the ledger board is cut to just under 57" long.