Currently I have 3/4 " particle board on top of floor joists in my bathroom. I want to lay tile. I was told to lay 1/4" luan plywood ontop of particle board and then apply backerboard. Can anyone help as to if this is correct subfloor before laying tile? Do I need to fill in the gaps of the luan before applying thin set/backerboard. Help!!!
not correct. For tile you need 1 1/8" of sub floor. you now have 3/4" so add exterior grade 1/2" Luan is an underlayment and has no structural strength. 1/4" CBU next with thinset and screws. The joints on the 1/2" should not be over the 3/4" ply joints. Overlap joints by 12" or so. You leave 1/8" - 1/4" gaps in the plywood and tape over them.
Are you sure it's particle board. This has no structural use what so ever. It's used in cheap furniture. Are you sure it's not OSB?
No point in putting time and money into a project if the bones are crap.