I'm making some 16" tall plastic laminate (formica) back splashes for the walls around my kitchen counters. I don't want to glue directly to the new sheetrock and mud joints. So I thought about srewing down 1/4" masonite as a substrate,  then gluing the laminate to the masonite with contact cement. Would it be better to glue to the smooth side or the rough side? 

You might consider a construction adhesive. Bothe LocTite and TiteBond make excellent products.

I've glued laminate with contact cement to particle board and plywood, but not masonite.  Just a guess, but I'd think gluing to the smooth side would maximize the contact between the surfaces.  Sounds like it's time for a couple of tests with some scraps, then you'll be the expert. 

If you google "contact cement aging" there is a comment that the rubber is affected by time.  If you only need the laminate to hold a couple years, the smooth side would do the trick.  Longer term, I'd get "real" adhesive as Lew suggested.

Edited April 30, 20251 yr by PeteM

FWIW, I've got a kitchen counter that was glued with contact cement 35 years ago and still hasn't let go. but yes, glue to the smooth side if the masonite.