Any decking board manufacturer or professional will tell you: “The proper way to install a decking board is ridge side down.” The ridges are designed to allow for airflow, to stop moisture sitting in the ridge, and allow for a stronger structure.

Should decking be grooved side up?

The correct way to lay decking boards for most decking is smooth-side-up, with the ridges there to prevent rot and mould.

Which side of a deck board goes down?

"Bark side down" orients the board so the end grain growth rings curve upward. This creates a crowning warp when wet. As a result, water is shed from the board, as opposed to pooling on it. Also, the exposed board face is heartwood (closer to the tree's center) versus sapwood, and heartwood is more decay-resistant.