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Instructions for installing a solid surface countertop call to use "one dab of silicone adhesive every 12" (305 mm) to 18" (457 mm) to secure". I have two questions:

  1. How big is a "dab"? I've seen references to "the size of a thumbnail." Is this a generally accepted definition?

  2. How thick is the Silicone layer between the countertop and plywood once installed?

I understand this could vary. I am looking for a general/typical range.

  • The context for the question: The countertop flush/level with an adjacent fixture top. The sets the counter height (i.e., I shouldn't increase or decrease the countertop height to adjust the clearance below).
  • I wish to understand the maximum realistic clearance below a 1/2" countertop + 3/4" plywood, assuming the work surface is 34" AFF.

Thanks

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    Silicon or most adhesives will add maybe the thickness of a paper sheet, most times less. Dabs or even weaves of thin ropes along the surface should work. Then you add weight or clamps to both surfaces to bond, so plywood, adhesive, counter top, blanket(protection, wood to cover and clamps to squeeze.
    – crip659
    Commented Dec 3, 2023 at 19:54
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    It really depends how flat the plywood is. If it's flat, the dabs can be small. If not flat, the deeper spots of the plywood will have a bigger gap to the bottom of the countertop, and the dab won't squish as much, and thus won't provide as much surface area.
    – Huesmann
    Commented Dec 4, 2023 at 13:59

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The last one I did I had spacers to set the exact height so the top was level from the cupboard framing / supports. These spacers were all between 2 or 3mm and some points needed none.

Then the dabs of adhesive inbetween all the spacers - the spacers had been stuck with a drop of glue already. Those dabs were about 1cm or 1/2" diameter and 5mm or 1/4" thick, they all got squashed...

Then the top was laid back on with an absolute minimum of sliding about.

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  • Spacers add another variable I hadn't considered and should, thank you. The dab sizes are helpful. I was thinking in terms of just silicone after being clamped . Without spacers, if the thickness is similar to a sheet of paper (per crip659 comment above), then there is no issue. I saw some section drawings (not to scale and not dimensioned) where the silicone looked greater than 1/8" to 1/4", which prompted the question. Commented Dec 3, 2023 at 20:51
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    It very much depends on how flat/true the top of the cabinet is. If that has twist or is otherwise not flat and true, you'll get gaps since the counter top is not going to flex much.
    – Ecnerwal
    Commented Dec 3, 2023 at 22:49
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    @Ecnerwal well, always found that the cabinets are never level, usually because the builder cannot do a flat floor... So spacers between floor and cabinet will also work. As long as there is sufficient contact then the top will sit and not move.
    – Solar Mike
    Commented Dec 4, 2023 at 6:42

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