What do you need to put on top of a wooden porch floor before putting porcelain tile on it?

First, make sure it's level and very sturdy. The materials going down add up to a lot of extra weight, so make sure your porch was build to support it.

Next, you'll want to lay backer board. I prefer hardi-backer 1/4 inch. There are several other brand that will work as well, but hardi backer is light weight and easier to work with. Make sure to hit all your mounting points with the correct type of screws (backer-on are made for this, but you can just as easily use another).

Tape up your seams with backer board tape and your ready for thinset