new floorboards

[Patsie]

Experienced advice please? I am laying new redwood T&G wooden floorboards on wooden joists over a damp-proof membrane on a concrete subfloor in a brick workshop.
I was told by a guy at the timber merchants that I need to varnish the boards on both sides, and also to varnish the tongues and grooves. He said that this is to prevent the boards from curling when they absorb moisture from the atmosphere on the underside but are unable to absorb moisture on the upperside as it will be varnished.
I have never heard of this requirement and wonder if it is necessary? It seems to me that it might be difficult to fit the boards if I have varnished over the tongues and grooves before fitting.

 

[WoodYouLike]

Never heard of it! (And we install a lot of sq m unfinished Oak flooring, never had to go back to a customer because of that specific problem).
All pre-finished solid floors are also only finished on the top-side, no problems there.

Perhaps your guy at the timber merchant has more experience with flooring for outside?