Gaps between granite countertop and plywood base

Question

I recently had a 3cm granite countertop installed over a plywood base (approx. 72" x 24"). As is often the case, the plywood thickness varies throughout by 0.010" to 0.050". Thus, even though the base is quite level in both dimensions, there are random gaps between it and the granite. I'm concerned about the potential for cracks in the future, esp. if someone puts something heavy on the granite (or climbs up). My biggest concern is one end of the granite, where the last 12" or so floats over the plywood by 0.010-0.030". For now I shimmed this area with some thin metal plates.

Of course, this does nothing for any other hidden gaps; eliminating these would probably require a very thin layer of thinset or similar, then bed the granite on top while wet. After checking various resources, I haven't found anyone who actually does this (other than tile).

This is my first experience with granite. Am I being overly-concerned, and the granite will likely flex without issue if pressed? Or is this a common problem that most people ignore & hope for the best?

Thanks!

• John

Answer 1

You can support overhangs of granite with braces that are 16 - 20" apart and have lips that are unsupported out to 2-3". Fractions of an inch are irrelevant, unless maybe you are driving a truck on it...

Answer 2

I have a similar situation ; 2 cm granite, 60" X 32" , sitting on a wood cabinet. I was going to set it on wet silicone /RTV caulk 22 years ago but never got around to it. Just now I roughly measured the gap; it is very close most places but one corner has a gap of about 0.02" for at least a foot along each leg. Otherwise there are a few areas where i see a small gap of < 0.01". I have stood on it a few times but I will be more careful in the future . However , it has done just fine, granite is strong but it will not "flex " for 0.02" over a one foot span . I have no cantilevers, the wood cabinet is about 1.5" from the edge of the granite. But I am now going with metal shims as you mention.