QUESTION

What is the proper way to install tile over a wood subfloor assembly? The joists are Douglas Fir 9 inch x 1.5 inch. The walls are poured cement. There is a metal I-beam going the length of the house from under the garage of spancrete to under the hall and bedrooms common wall east to west. The Depths of the house is about 27 feet. The measure from the back wall to the metal beam is about 15 feet.

Between the joists at about 7 feet from the back wall the joists are supported by crossed lumber between each joist and the adjacent one. For deflector rating to we use the 7 feet from the back wall to the crossed lumber between the joists or do we measure to the I Steel Beam? to get the deflecto ratin?

If we build a wall under the kitchen floor for another room in the basement it seams it would greatly improve this deflection rating. So what measurement is for the length of the joist from where to where?

The OSB is 1/2 inch. If we put screwed and glued down a 1/2 inch cement board above it and red guard for water resistance would that be sufficient for 12 x 24 tiled flooring. Was told needed at least 3/4 OSB for using drita one to of the OSB. But what is the 1 1/4 inch floor depth mean? How much of it can be added in cement board? Or can SLC or self leveling cement be used instead?

One guy was really upset with had only 1/2 OSB under the concrete board above it.
The OSB says on it stamped L360.

If not good enough how would putting a room with two walls in the lower level do to support the joists and upper tiling if needed? So that the span of the joists would be much shorter. Or can 2x4 strips of wood be secured perpendicular to the joists to reduce deflection which is bad for inflexible tiles.

ANSWER

ANSWER - Per the Tile Council of America Handbook for the installation of tile detail F144-15 it recommends a minimum 19/32” exterior-glue tongue and grove plywood with 1/8” gap between sheets as base over 16” oc floor joists.  Then apply the ½” thick cementitious backer board properly installed.

Bottom line is that the floor joists and the span between joists can’t have deflection greater than 1/360 for ceramic tile and not more than 1/720 for natural stone.  You can install bracing to stiffen the floor.  The joists have to be installed correctly.  The subfloor has to be installed with gaps and nailed properly. The backer board has to be glued and nailed correctly, and must be installed in a staggered off-set pattern to itself and to the subfloor sheets.  The joints have to be properly filled and taped.  You need to installed movement joints at perimeters and within the field of tile every 20 to 25 feet.

There is a free download on our website that provides installation recommendations that you should read and follow.  It is located on our website under Expert Answers Installations.