Mystery of the hollow tile ???

[George K]

Only the area around the wood stove sounds hollow. Why?
I laid my last tile in June of 08'.
Wood stove fired up mid November 08'.
I started grouting just this past week.

I test each area that I am about to grout by dropping a golf ball on each tile.
Of the 1300 sq. ft., ONLY the area immediately around the wood stove sounds hollow (marked by pieces of blue tape). I have not yet grouted this area.

Layers are as follows: Scarified concrete floor, unmodified thinset, DITRA, unmodified thinset, ceramic tile.

[duneslider]

I have noticed that ditra floors some times sound a little hollow.

Is it possible your thinset skinned over before you got the ditra over it? Were any of these tiles set wet, or stepped on to soon after setting them?

[George K]

The tiles were not set wet nor were they stepped on soon thereafter.
That end of the basement has the first strip of DITRA that I put down.
The strip runs parallel to the wall behind the wood stove. The hollow sounds seem to be tile specific rather than area specific which make me suspect a tile to DITRA bond problem, since the centers of the larger tiles sound more solid than some edges. How would I determine if the problem is the slab to Ditra or the DITRA to tile bond? The blue tape is over a hollow sounding area.
If no tape is visible, that part of the tile sounds solid.

We are dealing with about 20 to 25 sq.ft. of hollow sounding tile, or about 2% of the floor surface. I worked with very small batches of thin set. No more than 1/2 bag at a time and laid no more than 8 to ten pieces of each of the two tile sizes at a time. No time for skimming over of the thinset during tiling. When I laid the DITRA, I would spread about 4 foot of thinset, then roll and press down the DITRA. If it was a skimming over problem, I would have expected it to occur at the end of the rolled strip.

That area of the floor was tiled about 1/2 way through the whole project since I tiled from the center of the basement to that end and then from the center of the basement to the other end. My tile laying technique would have been advanced by the time I laid that section.

Each tile was individually scrubbed (to get rid of the kiln wash) and washed and allowed to dry. Each tile was back buttered first before being laid. No hollow sounding spots in the other 98% of the field , not even under or near the point loads of the water heater, basement steps, or heat pump / air conditioner.

I have not checked the area directly under the stainless steel sheet that the wood stove is resting on. That might give a clue. Right now, I'm thinking local thermal expansion. If it is, than it would be fruitless to tear the tile up and reset new tile, because the same thing will happen again.

[mossypath]

Will Ditra stand up to the heat of that stove. Without grout I would thinkit would allow even more heat ? I don't know Just asking?

[duneslider]

Have you been running that stove?

I don't know if it is appropriate to put a stove over tiled ditra like you have done. I would want to check stove building codes. I am pretty sure in my area you have to have it built up a little under the stove plate.

If you fired that stove before the thinset could cure that could be the problem. Most stuff I read says you should wait 28 days for the full cure before running the heat. Probably don't need the full 28 days but if you ran the stove a few days after setting I could see it causing problems.

[George K]

The stove was not turned on until 5 months after the last tile was laid.
The basement floor is cool concrete below grade. While the tile surface near the stove is warm to the bare feet, it is no where close to being called hot.
The stove is up on four 6" steel legs. The actual fire box is above that even more because of the ash drawer in the bottom of the stove. There is a sheet of highly reflective stainless steel placed on the tile surface that those 4 legs are resting on.

DITRA on concrete, that is then covered with tile, could hardly be considered flammable, because if nothing else, there is no source of oxygen. I do not recall seeing anything in the DITRA literature that would imply that DITRA over concrete that is then covered with tile would be considered a flammable surface. Over a wooden floor, maybe additional insulation would be required.

Even though the floor is not yet grouted, thinset is present in all the grout lines to a depth of at least 1/3 the height of the tile. There is no DITRA showing between the tiles.

[duneslider]

Sounds like all is good. If you don't want to try and pull one up I would say get some extra tile (enough to cover that area) and save it in case you have issues down the road.

[George K]

Good suggestion, and I might take the gamble, but I'm using Spectra Lock PRO epoxy grout and a down-the- road problem would be a big problem to remove that kind of grout. It's tough stuff.

[Trask]

Golf Ball =accoustic impact tester

ditra often does sound hollow.. but isolated areas might be odd.

How soon was the stove moved onto the tile after install? What was present in that spot prior to the stove/tile install? Where did the tile come from and was it shuffled before it was set or did you pull from boxes as you went along?

[The Kid]

Did you test all 1300 sqft with a golfball?

Was it the same golf ball? Some are solid core and some are wound rubber bands, they could give off different sounds.

[George K]

The golf ball really does work great. I like the fact that you do not have to bend over. You can test a large area just as fast as you can bounce and catch it. You can't miss the change in sound.

It was probably a couple of months after I tiled that end of the basement before I moved the stove from the center of the slab down to it's final position. No hollow tiles along the path that I rolled it on the dolly.

Prior to the stove/tile install, nothing was present in that spot accept the original asbestos tile and stove installed in the identical place. I had a few bird baths to fill, so there was a small shallow SLC pour down that end of the basement, but I believe it was further to the right of the stove. There were also two other similar shallow SLC pours in other parts of the basement, but the tile is solid in those areas.

The tile came from Home Depot. The color was very consistent and from the same dye lot ( 4 full pallets special ordered ). While I did NOT pull from several boxes and mix up the tiles, each tile was inspected for chips, glaze flaws, and warping before I scrubbed off the Kiln wash and leaned the tiles against the wall to dry for the next days setting.

[George K]

Yes it was the same golf ball. I was testing each section just prior to grouting that section, until I found the hollow area. Then I proceeded to test the entire 1300 to see if I had a larger problem. I didn't.

[The Kid]

Whats stopping ya from popin one up?

[George K]

What is the best way to pop one up without destroying the DITRA or adjacent tiles or destroying the evidence?

[gueuzeman]

Angle grinder with a diamond blade. Chase the blade with a shop[ vac hose. Cut 2- 3" strips in a piecestick a 5 in 1 tool in and pry up the strips. It may or may not pull the tile with thinset out of the lugs without uncoupling the plastic from the fleecing. I have done it once or twice, but only on new (2-3 day old) installs.

The ditra can always be cut out and patched in under a single tile if needed.

gueuze