On Oct 22, 10:46 am, "Sammy bin Snoozin"
wrote:

I see. My building vocabulary is not that good. It already has 2x6 boards on the joists, if that's what underlayment is. They are in good shape.

If you do not understand the terminology you will have a tougher time
asking questions and deciphering what is meant by the answers. Google
'construction glossary' and you'll find a good place to start to fill
in the blanks.

Underlayment is a plywood (or approved equal) layer on top of the
subfloor. In a typical tiling situation the house might have 3/4"
plywood or OSB as the subfloor. The underlayment is meant to smooth
out the surface in preparation for the tile. If your subfloor is
strong enough you don't need an additional underlayment - you can bond
the cement board directly to the subfloor.

I don't understand how you could have 2x6s as a subfloor unless your
joists are spaced far greater than the typical 16". Tile
installations live and die by how much the floor will deflect under
load. If you subfloor or joist spacing is unusual the normal advice
doesn't apply.

Post pictures on one of the free hosting sites and link to them in
your next post.

R