A subtop is a supporting layer used under a freestanding table or peninsula counter top that uses a heavy top material (like marble, granite or aggregate material) to provide a base to attach your table legs or table base or bases to. This subtop is usually a piece of plywood or metal that ideally would have a smaller overall dimension than your top, usually glued to the bottom using a construction adhesive, allowing overhang of the top with edges withdrawn enough to not be seen from the sides. Different table or counter top materials have different recommended lengths with which they can be unsupported and overhanging the subtop without risking being broken off. This overhang allows an easthetic appearance and the ability for people to put their fingers under the edge of the top (ie, to scoot themselves up the the table) without touching the subtop. Subtops can be made from plywood, solid wood planks, MDF (medium density fiberboard) or even custom steel plating.
Minimum thickness for marble and granite tops is 3/4” but many cabinet shops recommend 1” to 1.5” for maximum rigidity and support. MDF and solid wood boards/planks can be edged with a router or other tools to provide a more esthetically pleasing edge. For marble, stone and aggregate tops they are glued to the bottom of the top because you do not want to drill holes in these materials as cracking and breaking can result. One recommended construction adhesive is LIQUID NAILS® Brand Heavy Duty Construction Adhesive or similar adhesive to attach the subtop to the stone top. Once the subtop is in place and the adhesive is set, you can attach your legs or base very easily.
Always remember that the larger the dimensions of your top and the higher it is (ie, Dining or Table Height (28″- 30″), Counter Height (34″- 36″) or Bar Height (40″ – 42″)), the stronger and broader the table base you will need to safely support it! Cast iron table bases and Stainless steel table bases are an excellent choice for projects using heavy material tops and connect easily to your subtop!