How would I add tile to a raised deck?
To install in an unconditioned space over outdoor wood, you will need two things and these aren't really negotiable - although you can certainly choose your &quot;manufacturer&quot;.

You will need cement board installed to specifications on the deck floor.   Depending on your deck screwing these down will probably be OK but there could be a requirement of using mortar then screw - that will be messy to get that level... in both senses.  It may also need gapping to account for wood movement.

You will certainly need a membrane like Ditra that spans the area.   There are a few manufacturers that make membranes for these applications, I have only used Ditra (kerdi).   You will need to mortar between concrete board and membrane for sure.


The other option is to install pure vinyl (rubber) click lock planks or squares.   Some of these high end ones come really really close to looking like tile.   And for a deck they might not be as slippery when wet and will probably be much more comfortable without the grout maintenance (that's if you don't have cracking issues).  You are probably laying down some plywood here but that's a simple job.  Only hard part here is trimming the outside so it looks good.
Either way - tile or fake tile - the biggest issue with doing a flooring like this on a deck is &quot;where does the water go.   If you get the slope wrong or don't provide drainage tiles it is a really hard process to go back and fix it without ripping a lot of finished things out.