It's hard to tell how much it may expand due to the actual water content in the wood to begin with. If you build the rack in a humid area(90%) and you bring it in to a dry area (10%) I doubt any more than 1/16th of an inch. But I use 1/2 melamine and it has been good, and 3/4 is what you stated you may use and that may expand a little more as there is more wood content.
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Jimmy Tintle

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