We needa bit more info and really the best to get this info is the place you got the FP and is there a reason to have a mantel at all on ano outdoor FP.

The information you need regarding the mantel clearance would be in the installation manual of the fireplace. If you do not have the manual, there is a label inside the fireplace that has the brand and model number. Google that to find a copy of the manual online, or just post a picture of the label and I'll see if I have it. Post some pictures of the fireplace so we can provide advice on adding tile.