If you want to install tile over decking and prevent failure this would be the best way to go

And the SLOPE of the deck must be maintained. The slatted wood decking is designed to allow rain/moisture to drip down between the planks. It is a form of 'water evacuation' that is different from solid deck surface (such as a tile install).

In THEORY your wood deck has a slope (decks are supposed to slope away from the foundation of a home) but one's designed like yours *may not have that slope. Now you have to add in the slope if you want to create a solid surface. Sigh.

As you can see GN Builders has already put down a technique that should work....but there are too many variables to say what is the best approach for your situation.

First things first: check for a run-off slope for water evacuation AWAY from the foundation of the home. Go from there.

Thank you