The front and back frames should have an upper horizontal member. Attach the upper horizontal members using 3″ finishing nails and carpenters glue.
Figure 20a - Attaching upper horizontal support to back and front of wall cabinets
Figure 20b - Side view of wall cabinet showing base and supports
Figure 21 - Installing cross member supports for wall cabinets
The next step is to add, using finishing nails and carpenters glue, top and cross supports as shown in Figure 21.
For best support, the cross members, one at each end or every 4', if the cabinet exceeds 4' in length, should run from the top of the front to the bottom of the back.
If the wall cabinets are not going to terminate at the ceiling, a top should added, as shown in Figure 22. The top can be plywood, particle board or Masonite.
Note, the top of the front and back frame assemblies must have a rabbit as shown in Figure 22, in order to hide the front edge of the material used and provide horizontal support for the top of the wall cabinet. Use 1″ finishing nails and carpenters glue to fasten the top to the frame.
Figure 22 - Position of rabbits on top of front and back frame supports for top of wall cabinets
Figure 23b - Shelf bracket and standard
Shelves can be added to the wall cabinets by using shelf standards with brackets (Figure 23b) mounted to the back of the wall, which gives you complete flexibility in shelf location or you can mount strips of wood between the back and front frame at suitable heights as shown in Figure 23.
Figure 23 - Installation of shelf supports in wall cabinets