The grooves are non-slip. You'll see a lot of people use grooves if they have a ramp or steps off their deck. However, the grooves are actually there for air circulation underneath the boards and manufacturers recommend you install your boards groove-side down. - Jackson
It should be grooves down. You might even void the manufacturer's warranty if you install them upside down. If you need a non-slip surface, then use an appropriate sealant or textured strips. The grooves should be face down as they are there for airflow and to stop moisture build up. If they are face up they will soon be full of grit and grime and algae etc and will become even more slippery. - AHoy
Just for some anecdotal evidence (the best kind), my brother-in-law (carpenter) has his ramp groove-side up and it isn't slippery at all. He has had no issues with rot or algae, and older relatives enjoy the extra grip. - Jackson
We recommend installing the smooth side up so that the rain runs off easily, helping to prolong the life of the deck and making it easier to keep clean. - stupidfish
