responds: "When" and "why"aren’t the
only questions to ask — how about "where"
and "what"? In particular, what kind of
underlayment and surfacing materials have been
selected for this installation? OSB should never be
considered for any tile installation: Its
expansion/contraction rate is so different from
that of tile that this alone would rule out its use
as a setting bed for tile. Besides that, in many
controlled tests, OSB has proven to be so
inadequate as a tile substrate that an industry
installation standard has never been written for
Having said that, I realize that OSB has been
used with varying degrees of success. This often
hinges on the condition of the OSB, the other
materials involved in the installation, and "where"
the tile is installed. If the OSB panels have been
recently installed in a dry, stable environment
over a sturdy plywood subfloor, and the boards have
been protected with a thin-bed ceramic tile
membrane laminated to the surface using a low
moisture adhesive, the installation may have a
better chance of surviving than if the tiles are
set directly over OSB panels using a moisture-laden
As a consultant, my first advice would be to
walk away from installing tiles on this surface.
Perhaps another kind of floor covering whose
installation requirements are more forgiving would
be a better choice than to risk covering an already
problematic situation with a material that needs a
stable setting bed.
One of the reasons plywood has been used
successfully as a tile substrate is that, within
limits, its ability to support weight is fairly
reliable. OSB, however, has proven itself
unreliable when used for this purpose. Remember
that the weight of the tile floor and its contents
are ultimately supported by the strength of the
subflooring. Your idea to install underlayment over
the OSB may work for a while, but once the floor is
put into use and begins to flex, the tiles or grout
joints are likely to show signs of cracking.
OSB-based installation systems, in addition to
being severely affected by moisture, are usually
unable to meet the L/360 industry standard for
If your customer insists on using tile, get a
disclaimer in writing to protect yourself and the
subs connected with the installation, and improve
your chances of success by finding an adhesive or
membrane system formulated for use with OSB (I am
not familiar with any). Another alternative would
be to use 100% epoxy mortar to attach a cement
backer board underlayment, a sheet membrane, and
the tiles. There is no water in this adhesive that
could be absorbed by the OSB and cause it to swell.
(There will be some swelling, but not nearly as
much as with water-based adhesives.)
Finally, as extra insurance, make certain that
the floor is surrounded by an expansion joint that
meets the criteria found in the Handbook for
Ceramic Tile Installation, published by the
Tile Council of America (TCA, 100 Clemson Research
Blvd., Anderson, SC 29625; 864/646-8453). Expansion
joints are required on all ceramic tile
installations, but in this situation they become
even more critical.
For your customer’s sake, weigh the
costs of jury-rigging the existing situation
against removing the OSB and starting from scratch
with more appropriate subflooring materials. This
is the course I would suggest.
Michael Byrne is the director of the Ceramic
Tile Education Foundation and a contributing editor
to the Journal of Light Construction.