I hope that is 20” above grade . I looked on google it seems it can be tiled so I guess the next is to get a tiler to check it out and give you the pros and cons. I am curious about why geofoam? It looks cool for many uses I guess since it is used in pool contruction and roof top patios to name a couple. I would wonder how it stands up over time.

You should have no problems tiling over Geofoam. However, Geofoam expands and contracts more then ceramic tile, so, I would first put down a decoupling membrane like Ditra so any expansion/contraction of the Geofoam is not telegraphed through to the tile possibly causing grout separation and tile cracking over time.

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Greg, how did this project work out for you? I have recently started planning a raised patio and have been considering using a geofoam base with 3/4 inch porcelain pavers. I was wondering if you ended up using the Ditra as a decoupling membrane? Did you run into any other unexpected problems?