[stringblade]

is it nessesary to spread thinset then lay backer board over it?
or is this only nessesary when your floor is out of level or has dips?

[R&D Tile]

Yes, it is needed, unmodified is fine for this, it's there to fill any voids, if the floor is out of level, this should be fixed after the board is down, thinset under the boards won't help there, the boards will follow any contour of the floor when screwed down.

Just to add, if you plan on doing any leveling of the floor prior to using a cbu, SLCs or any other poured products, CBUs shouldn't be installed over it.

You can use a SLC over plywood and tile directly over that and not use a CBU,

All CBUs have to be installed over plywood or OSB.

[Mike Finley]

This thinset under CBU seems to go around and around...

RD in your opinion is the thinset needed if the sub-floor is perfectly level and has no voids?

[R&D Tile]

You can't see or feel them, but they are there, you also void any warrantees, it's required by all CBU manufacturers.

Alot of consumers who have not used thinset under the boards have written in about cracked grout and loose tile over time, there is no going back once this happens to fix it, not worth taking a chance, unmodified thinset is cheap, only $7 a bag, it's not there to adhere the boards down, just to fill the voids, use Modified over them to set your tile.

Hope this all makes sense.

[Mike Finley]

Originally Posted by R&D Tile

You can't see or feel them, but they are there, you also void any warrantees, it's required by all CBU manufacturers.

Alot of consumers who have not used thinset under the boards have written in about cracked grout and loose tile over time, there is no going back once this happens to fix it, not worth taking a chance, unmodified thinset is cheap, only $7 a bag, it's not there to adhere the boards down, just to fill the voids, use Modified over them to set your tile.

Hope this all makes sense.

It makes sense, no doubt. I have just heard this debated so many times, wondered what you thought. If this was one of those "isn't required under all circumstances but if you do it every time you will never run into a problem type things."

[Zero Punch]

I've been bedding CBU with thinset for 15yrs and never had a problem, cheap insurance, we shoot it down with 1 1/4 roofing nails.

As far as thinset not really holding, we had one job where we did not get a change order and I had to remove 150 foot of durock. Not fun! Thank goodness for Milwaukee tools.

[Bill_Vincent]

Originally Posted by Mike Finley

It makes sense, no doubt. I have just heard this debated so many times, wondered what you thought. If this was one of those "isn't required under all circumstances but if you do it every time you will never run into a problem type things."

There's a real easy way to end ALL debate-- read the manufacturer's recomendations. EVery manufacturer, without fail, states clearly that if the CBU is NOT bedded in thinset, all warranties are null and void.