no thinset under backer board

[DJ]

my friend put is putting in a granite floor,he put backerboard down without thinset what will happen? i know nothing about tile setting i have seen it done bit by bit on jobsites with the tile setter there. i go by (the room where he setting tile) ) on the way to the carpeted rooms, i did see that he put down thinset before he put down the backer board.should he worry?should he pull it up? there is about a million nails holding it nowi told him i would research"backerboard without thinset "and found nothing

[Bud Cline]

DJ you must have stirred his curiosity and he went and read the instructions. All backerboard manufacturers recommend the use of thinset under the backerboard.

The thinset is not there to adhere the cement board to the substrate, it is there to fill the voids that are created when the fasteners are installed. When thinset IS NOT USED under the backerboard the small voids that exist will usually be enough to cause the floor to cush and cush means movement and movement means failure in a rigid tile installation.

Not very likely the install will survive.

I just finished a job where the original installer used a 3/16" "V" notch trowel to install the thinset under the backboard. The floor failed, too much cush.

typically a 1/4" X 1/4" X 1/4" square notch or U-notched trowel is the requirement.

[Floorguy]

With granite, he is doomed, for sure.

Your making a high tech mudbed with thinset and CBU panels. He just has panels, with the panels alone, there is movement, no matter how good he fastened it.

[DJ]

thanks all,i will give him the awful news

[Daniel Wachtel]

moral of the story:
find and read instructions

[Danny Ferguson]

Hey DJ, how thick is the subfloor? I would want at least 1/1/4" before laying hardi.

[Jerry Thomas]

DJ, while you breaking the bad news to your friend you may as well prepare him for another potential problem. Is he aware that the substrate needs to twice as stiff for granite than a normal ceramic tile job. Ceramic tiles call for L/360 deflection... natural stones require L/720 deflection. This is another common mistake people make.

[DJ]

does that mean the floor needs to be thicker? the tile is 12x12 1/2 inch thick

[Bud Cline]

DJ,

That could very-well mean the floor would have to be thicker. That is normally how it is done providing the floors structure meets certain minimum criteria to begin with.

One would have to know the size of the floor joists, the spacing of the floor joists, the unsupported span of the floor joists.

Under most circumstances if the joists are 2X10 and 16" o.c. and the span is around eleven feet and the primary subfloor is 3/4" t&g plywood nailed and glued to the joists, then the next thing to do would be to add an additional layer of 3/4" exterior-grade plywood. This top 3/4" layer of plywood would be installed perpendicular to the first layer and NOT glued to the first layer. The fasteners would be installed into the first layer and delibertly missing the floor joists.

Overall deflection is a main issue while deflection between the joists is also an issue. The double layer of 3/4 ply is generally enough to solve any deflection-between-the-joists issues. However it won't offer a lot of improvement if the overall deflection is excessive and the joists are overspanned to begin with.

Should overspanning be an issue there are ways to deal with it in most cases.

Hey, you asked.

[DJ]

the floor is 15x15 joists are 12x10 beams 3 of them one on each side of room and one in the middle with 2x10s on 16" centers.1 inch plywood for the floor. it was carpeted. the house was built by a logger/ carpenter. like i said, i know nothing about tile setting thanks

[Bud Cline]

Well there ya go!!!!!!

Sounds like deflection isn't going to be much of an issue. Who built that damned thing (?) Paul Bunyun?

[DJ]

i guess, its in the north cascade mountains