Came here looking for an answer to the same conundrum in 2021!
There are some commentators who are absolutely convinced that “it’s always grooved side up” and... “why else would they groove the boards” ...and so on and so forth.
Trouble is it turns out that if you bought decking with grooves on one side only then it’s a bit of a problem because you actually NEED grooves on the underside for airflow between joists and over hard/concrete surfaces!
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