Installing Tile Over Engineered Wood Flooring Systems
To start, we define some of these systems as they are being used in today's
I-Joists, as their name suggest, look like an "I" beam
when viewed from the end. The top and bottom pieces (flanges) are
generally made from 2x4's. The inner web (the "I") connecting
the two flanges is generally made from plywood. I-Joists replace 2x10 joists
and vary in length, depth, and load carrying capacity.
Open Web Trusses are used to replace joists. They are formed
from 2x4's which act as the flanges and the webs. The webs appear to join
the trusses at a 45-degree angle, spaced at regular intervals.
Laminated Veneer Lumber (LVL) resembles plywood. It is
actually a sandwich lamination of various thickness veneers selected from
any suitable stock. It is built up in thicknesses and layers for a specific
purpose and is usually used to replace girders or headers.
GLULAM (glued, laminated, structural wood) is another version
of LVL which appears as a sandwich of 2x4's bonded together.
Builders enjoy the use of these new flooring systems because they reduce
cost while offering flexibility in floor framing methods. The problem for
tile contractors is that these new systems introduce a new set of design
parameters which, if not properly addressed, can result in callbacks and
failures. Specifically, tiles may crack or loosen, and grout will quite
often crack or splinter out of joints. Most of us in the industry are all
familiar with this.
What do you look for in anticipating problems?
First, check the o.c. (on-center spacing) of the joint-truss
system. Although they may be designed to carry the necessary load at
24" o.c., this is not acceptable for tile installations. Wide o.c.
spacing may be suitable for rug or vinyl installations, but not for ceramic
tile. The large deflections (bounce) in these floors will loosen your grout,
if not your tile.
Second, check the plywood. There must be two layers with a minimum
of 1-1/4" combined thickness (that's a minimum, not a maximum). For
a reliable installation, we prefer a combined thickness of 1-1/2" or
more. The plywood must be properly installed, with screw nails
at the proper spacing. Plywood requirements are
spelled out in detail on our packages of One Step
and Super-Lastic mortars.
Finally, check the span. These new framing systems are made to
order for the builder and can be ordered in excessive lengths. Naturally,
the longer the span, the greater the sag at midpoint. Support columns may
be needed at interim points.
Do not install ceramic tile, granite,
or marble when any of the above conditions exist.
Advise the builder of your concerns and ask for
it to be corrected. These corrections inevitably call for stiffening the
floor in some manner. How that is acheived depends on the particular case.
Some methods include: adding an additional layer of plywood (or better yet,
Wonderboard®), adding supports, adding cats to prevent joist twisting,
etc. If the builder refuse to correct the framing, consult your attorney.
In order to continue the job with faulty preparation, have the builder sign
a release acknowledging the shortfall and leave him, not you, responsible
for future damages.