If possible, I would add an underlayment (concrete backer board is commonly used)- it is fairly inexpensive and can be bought at local hardware or big borg stores. Since this is outside, I would use the water resistent version – usually a light green color.
The other issues of using tile on furniture is that it must lay flat and not have gaps or voids underneath it to create flex or torsion – this will cause the tile to crack or the grout to separate (allowing leaks and damage to your underlayment). Underlayment (either concrete backer board – or at least some exterior grade plywood) helps keep the tile flat and helps reduce motion pressure on the tile.
I would also recommend you use a sanded grout (has more flex) and make sure your tile is sealed – or you should seal it with a good water resistent sealer – there are some good products out there to seal and protect the tile from moisture related damage.
-- Woodworking.....My small slice of heaven!
