Bathroom flooring over wood boards

[Bill Smith]

Daughter wants to lay tiles on her old, wooden-board bathroom floor. My elderly DIY book says leven first, using hardboard. Would MDF be a suitable alternative, or would plywood be better? What thickness? Also, what treatment to give the MDF/plywood before laying floor tiles? TIA, Bill

[BigWallop]

You're actually better with something like 6 or 10 mm Plywood sheet. Hardboard and MDF are softer, so warp much more than Plywood, and so stays more rigid under tiles.