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Is Spot Bonding Tile an Acceptable Installation Method and will it result in a Failure?
QUESTION
We had 8”x48” large format porcelain tile installed floor to ceiling on 2 walls in our home for a total of 270SF. Our ceilings are 9ft. The tile was installed directly onto painted (not newly painted) drywall. Work being done on our home now a year later caused a tile to have to be removed. It turns out these tiles were spot set (bonded). Each tile weighs over 12 lbs & each tile appears to have 3-4 blops of white adhesive holding it on the wall. The adhesive is sort of dry & caulk-like in its dry state. I have been told spot setting was prohibited except in 4 fairly specialized installations. Is this correct? Does the TCNA handbook specifically say it’s prohibited or do they just not list it as the accepted manor in which to install tiles such as these? Is this a failure waiting to happen? What steps do I take from here to have this corrected? Thank you for any help you can give me with this.ANSWER
ANSWER - Adhering a tile using a spot bonded method , where the installer typically applies a spot of cementitious thin-set adhesive in the four corners of the tile and at the center of the tile, or less, is not an acceptable installation method unless they are using an epoxy adhesive on an interior application.Spot boding in effect reduces the amount of surface area that is bonded; thus the potential bond strength is reduced by that percentage. So if only 50% of the tile is bonded then the potential bond strength is reduced by 50%. The industry standards state that tiles should be bonded so that at least 80% of the tile is bonded for interior dry applications. For interior wet applications or exterior applications 95% contact is required. In both cases the contact has to provide uniform contact and full support at tile edges and corners.
The TCNA Handbook does say spot bonding with cementitious adhesives is not acceptable, but they do show a spot bonded wall method using an epoxy adhesive, which has about 5 times more bond strength than a cementitious thin-set adhesive. There are not floor spot bonded methods allowed.
Spot bonded methods reduce the potential bond strength, it leaves voids that if subjected to heavy loads or impacts can crack or crush, and excessive voids allow moisture to collect behind the tile that can lead to various issues.
Because the cementitious thin-sets provide a lot of bond strength, even if the tile is 50% bonded (if properly bonded) it is still substantially attached. So it doesn't mean the tile will fail if it is spot bonded.
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On a tile that is 4 ft long there were 3 or 4 blobs of adhesive about 6″ in diameter . So if I understand you although this is not an acceptable or even preferred method of installation because my application is on in interior wall & is not in a bathroom the tile is most likely bonded well enough to last for years & years as it typically would had it been installed correctly? When we removed more tile ( had to be removed for work being done inside that wall) some of them came off easily with a spatula behind the tile. Some were not as easy. All the tiles came off in one piece. So it is ok if I leave them as they are? I don’t want the grout to crack through as it didn’t go back between the tiles enough to touch the wall as the space behind the tiles is probably 3/8th inch at least.
If you have a 8″ x 48″ = 384 in2 tile and only 4×36″ = 144 in2 is bonded then you only have 37.5% of the tile bonded. That means that 62.5% of the tile is not bonded, which means the potential bond strength has been reduced by that percentage.
Installing over a painted drywall is not a great surface to bond to. We don’t know what the compatibility is between the paint and the adhesive. We know that the tensile strength of the drywall is relatively low; meaning its resistance to stress is low.
We don’t know how well the tile is bonded to the wall unless we test it. Normally tiles bonded to drywall when forced to fail will fail cohesively in the drywall, since that is the weakest link. So having the tile come off in one piece might not be unusual. How the tile fails in terms of at what point of the tile assembly and whether it is cohesive or adhesive failure is significant.
They should have grouted the tile so the grout joints were completely filled with grout to the underlying substrate.
The only way to determine if your situation is critical or not is to have a professional evaluate it , but that might not be practical.