The window still is two layers of 3/4" plywood with no measurable
deflection. Normally cement backer board would be used, but the window
will not be walked on. The wood surface should be clean and free of
surface imperfections. Use a hand scraper or planner to ensure that the
mastic will adhere to the clean surface.
With the surface scraped,
vacuum the area and start the layout. First check the
area to ensure that the sill is a uniform width. Next, run a
string line to see how straight the sill is. It is best to
identify any imperfections before trimming the tile. There is
probably no need for a chalk line for this particular project.
We purchased tiles in one foot
square lots with caulk tabs separating each tile. Depending on your experience level,
you may wish to trim off the last row of tiles and cut these tiles separately. We choose to trim off the last row as there are
several more cut outs required for the center window