"grooved" tongue and groove

[Gabbe1]

Yesterday I got hold of a little over a km of 120x21 ändspont.
I noticed that on one side there are lots of small "räfflor" planed.
Does it matter if I turn these räfflor up or down?
My thought was to have them facing up to get a good grip when I walk on the roof.

 

[Chinaski]

Right thinking with the grooves up for grip, that's what I did and it worked brilliantly!

 

[nimhed]

I believe that's a result of the manufacturing. The board fits better against the machine, but that's just a guess.

 

[Gabbe1]

I don't know why these grooves are made, but as I see it, there are only advantages whether the surface is to be painted or covered with paper.

Thank you for the answers!