Ash counter top over formica????

That’s weird…tile over plastic laminate!
For the best advice more detail is needed. Are you planning to use solid ash? How thick will the ash be? Does it go over the dishwasher? Yes, you will need air flow. And you should put the same finish on the underside you put on the top, just like veneering.
Now, making some assumptions in the interest of saving time and back and forth, I would think you could get by with 1/4” thick by say, 3/4” wide, sticker strips every 12 inches or so under a 3/4” thick ash top. You could drill large holes in the substrate counter to expedite air exchange for the solid ash. Screw up from the underside at the back of the counter and the rest of the screws have to go in oversize holes.
Good luck!
DanK

-- Dan Krager, Olney IL http://www.kragerwoodworking.weebly.com

thor4570, just for the heck of it, have you tried to see what is under the plastic laminate? Just rip out the old countertop, level out the top of the cabinets and install the ash, you’ll get your air flow from the top of the cabinet frames to the underside of the ash, while your leveling, place a 1/8” piece of sheet metal over the top of the dishwasher and fill under with fiberglass insulation. Just don’t pack the insulation, leave room for some air. Happy remodeling.

-- Mark, ”...NEWBEE: On the road to learning a lot; but; a lot more to learn…” ("My Granddad used to tell me, if you didn't learn something new today, it just wasn't worth getting out of bed")

Make sure you allow for wood movement. The ash will want to move; the formica is basically inert.

-- Dwight - "Free legal advice available - contact Dewey, Cheetam & Howe""