Don’t Rely on Plywood for Structural Support

Question:
Is it always necessary to install plywood tops on cabinets?

Answer:
The answer really depends on whether the concrete countertops are being cast in place or if they are precast. If they are cast in place, then the answer is yes. The plywood for cast-in-place concrete countertops is a permanent portion of the formwork, serving as a form bottom to keep concrete out of the cabinets.

Precast concrete countertops do not require installing plywood on top of open cabinets. However, plywood can make installing large, awkward or heavy pieces easier, since the slab can be slid along the smooth top without catching on a cabinet wall. If plywood is installed, it’s best to recess the plywood so the height of the cabinet remains unchanged. Simply screwing plywood to the top of the cabinet raises the working height of the countertop and requires additional efforts to hide the plywood edge.

Whether the countertop is precast or cast in place, you should never rely on plywood as structural support for an improperly reinforced countertop. Properly designed and fabricated concrete countertops – even at a thickness of only 1 ½ inches – are structural beams fully capable of supporting well over 300 pounds once installed. Plywood of any practical thickness (including 3/4-inch material) is far more flexible than the concrete placed above it, and thus provides little or no structural support.