PVA and tiling on plywood base

Pva is now recognised as an inferior bonding agent on which to tile. By doing this you are basically asking a thin coat of multi purpose glue to hold your tiles in place. You need to ensure that the plywood is fixed to the substrate extremely well then use a latex based primer followed by flexible adhesive and flexible grout. Cement based boards are now accepted as the best substrate on which to tile but a correctly prepared plywood substrate shouldn’t give you any problems

I would recommend to use ditra matting on plywood before put tiles.

Ensure floor is well secured and no screws nails etc are protruding. Prime floor with 2 coats of a good quality SBR such as Bal SBR. Not too thick so you don't create a thick skin - refer to back for mixing ratios. Use a flexible adhesive with 6mm notch trowel and lay anti cracking matting making sure all air pockets are dispersed. Use back of trowel in sweeping motion. I prefer Durabase but there are a few to choose from. I prefer rapid set adhesive for this part as you can get in to tile shortly after. Ensure you leave a sufficient gap around edges and inbetween matting joins . Again refer to matting manufacture advice. You may need to use a tape strip down the joints. These tapes are sold with the matting as they are specific for that matting. Once tiling begins fill the holes as this aids the flexible working properties to absorb any movement. Continue to tile. Your client should enjoy a crack free finish to their tiles for years to come if the process is correctly followed.

Most tilers these days dont even tile on plywood floors anymore,,cement broards are the future.