Cabinets or other supports for countertops must be level to within ⅛” over a 10 foot length. The portion of the top that is above the cabinets needs support using strips of wood or plywood that are 2-1/2” to 3” wide and ¾” to 1” thick. These strips are placed front to back every 18” or less and can be affixed to the top of the cabinets with screws, glue or daubs of silicone, as shown in Figure 19a.
Do not use full sheets of plywood for support instead of wooden strips. The solid surface top needs to dissipate heat from the underside to prevent damage to the top or cabinets from expansion or contraction.
Figure 19a shows wooden strip support for the portion of the top that is directly above the cabinets (as well as a COUNTERBalance™ IslandBrackets™).
Hold the countertop in place using thumb sized dabs of silicone sealant placed on the strips about every 12”. Alternatively, the strips can be affixed under the surface of the top with the dabs of silicone sealant. Then, the top (with the strips attached) can be affixed to the cabinet tops with dabs of silicone sealant. Silicone sealant is used to allow for expansion and contraction of the top. These 1” thick strips serve a secondary purpose. If you place the countertop on top of the cabinets without these 1” spacers, the 1” dropped edge on the front of the countertop could prevent the drawers near the top of the cabinets from opening.