Some options for tile underlayment include a concrete slab, backer board or mortar slab. The underlayment must be flat and solid in order for the tile to sit on the floor correctly.
A concrete slab is a common underlayment, and if a floor already has one, the homeowner can use a straight edge to measure the high and low spots. To get a flat concrete slab, the homeowner simply files down the high spots.
A backer board is a concrete slab that has a layer of fiberglass on both sides. If hiring a professional, he will most likely use a mortar bed that consists of roofing felt, then wire mesh and finally mortar.