Have you checked the subfloor rigidity for the deflection rating? Do you know which way the joists run? How level is it? Have you used a 10ft level to determine the issues that need resolution?

Who's doing the installation? Technically you can use an uneven T-molding at the tile/wood interface....but your height variation is more than what most uneven T's offer. You might have to look at a reducer. You will need to purchase such a thing from the engineered hardwood manufacturer.

Of course the EASIEST way to deal with this = 3/4" thick engineered. Almost all tile installs have a height of 3/4" - 1.5". Your 3/8" (which is very thin) hardwood doesn't even make up the thickness of the backer board....let alone the height of the tile. You are missing 5/8" (15mm) of floor height. That's A LOT.

The solution to both the leveling and the floor height variation = 1/2" (12mm) plywood with shims used to offer the flatness level required by the hardwood.

If you were FLOATING the hardwood you could use 1/2" cork underlay (12mm) to make up most of that height. The cork would 'float' (no adhesive) and then the wood wood 'float' over top (adhesive only on the edges) = done like dinner.

But you are gluing so the plywood is the option I would go with. Of course your well trained flooring installer could walk you through this quite easily.

Thank you for all the help SJ. Much appreciated.

I have walked around the floor and it seems sturdy in most areas. The joists run across the room according to the nail pattern in the picture. I have installed a ton of screws to make sure the flooring is secure.

I have looked at the floor with a 6 foot level and there are some uneven areas. I'm wondering if I should sand the plywood, then put some sort of a barrier then use a self leveling compound to make the floor flat. I'm thinking I may need a barrier to prevent the self leveler from leaking through any cracks in the tongue and groove plywood. Have you ever done this? Any products you could recommend?

As far as the drop off from the tile to the engineered hardwood. I'll likely use a reducer. I'm not sure it would be worth the cost to put plywood on my entire second floor.

I might just go with a reducer instead of installing another layer of plywood

Better check the joist distances...and make sure you know how to use self leveling compound (SLC). First thing you need to know: it is not self leveling. Second thing you need to know is you do not sand down plywood subfloors except at the edges (if one panel sits proud, go ahead and knock it down). The third thing you need to know = no barrier of ANY kind. Wood does NOT ALLOW a vapour barrier to be used over top.

You can use a primer that is SPECIFICALLY made to WORK with the adhesive you are about to use (yep...another $0.50/sf for the primer)....that will get what you want done.

Have you ever used SLC before? Do you have a sanding machine? Do you know what glue you will be using (should be about $1/sf)?

Yes, just sanding the edges of the plywood that stick up. Any primer you have used that you could recommend? Several of the installers recommended Mapei ECO 980 glue, have you ever used it?

Mapei ECO line is really good. They make their own primers. Primer won't hurt and it will certainly help....but you need to get the level stuff figured out. I'm not hands on...but I know every floor needs to be within 3mm over 3 meter for flatness rating. Especially if you are going to use glue.