A mortar base is the best choice for leveling an irregular subfloor.
Although structurally suitable for tiling, this diagonally laid plank subfloor presented irregularities in plane that would telegraph through plywood underlayment and interfere with tile placement. A mortar base, or mud job, evens out the high and low spots, making an ideal tile bed.
After covering the floor with 15-pound asphalt felt to retain curing moisture in the mud bed, the author staples reinforcing galvanized wire mesh over the entire surface.
The mud mix, or "screed" -- a combination of Portland cement, sand, and water -- is mixed fairly dry, so that it holds its shape when compressed.
The mix is first dumped around the perimeter of the room to establish screeding points for leveling the general floor area.
The primary "boss line" of screed establishes the mud bed level between critical points -- in this case opposing door openings.
Subordinate lines are then extended and leveled to establish the overall floor plane.
The author floats the floor by adding mix between lines and leveling it with a straightedge.
The mud is packed and floated in one two-fisted action: A wood trowel levels and fills the base, followed by a steel float for a smooth finish.
After curing for 48 hours, the mud base is ready for a standard tile installation. The author uses latex-modified thinset to adhere the tile.