How far apart should sleepers be for subfloor over concrete?

Q: I am installing a new hardwood floor. I leveled a low spot on the concrete, put 15 lb. tar paper, and I am putting 1 by 4 strips of wood for a subfloor. How far apart should these pieces be?

And do I need to adhere them to the concrete or even nail them, or not?

A: I would install the sleepers as if they were joists, 12 or 16′ centers. I think I would fasten them to the concrete. Otherwise, you may end up with some bounce. You may have to do some shimming if the floor is not level.