What are the allowable size of voids under porcelain tiles?


Hello - I recently had my the ground floor of my house retiled with large (48X8) Mediterrania porcelain tiles. I am very disappointed in the installation and have found 36 voids. I only counted the ones that are greater than 2 inches in length - some go through the entire length of the tile. I am battling with the installer as to what is an "acceptable" void. They tell me that as long as 80% of the tile is solid, it meets requirements. The problem is that these are large tiles and 80% still leaves almost 10 inches of void - Is this acceptable. Your response and help is greatly appreciated, this was a VERY expensive job, and I just want to know if I have a legitimate issue. Thank you for your time.


ANSWER - First of all not all hollow sounds are voids.  Having hollow sounds is not a deficiency and there are no standards addressing hollow sounds, other to say that hollow sounds don't necessary mean it is a problem.

Hollow sounds can be an indication of a deficiency; particularly if some tiles or parts of tiles sound hollow and other parts do not.  It could be an indication that there are voids or loose tile or something else loose within the tile assembly; or hollow sounds can be an indication of the type of material or the substrate configuration that the tile was installed over.

Based on your description, it is possible the hollow sounds are voids.  The only way to make sure is to remove a few tiles in various sounding conditions for verification.

ANSI A108.5 standard for installing floor tiles says that the average uniform contact area shall be not less than 80% contact except on exterior or shower applications that should have 95% contact.   It says that the 80% or 95% coverage shall be sufficiently distributed to give full support to the tile with particular attention to this support under all corners of the tile.

According to TCNA 2016 Handbook and the MIA, for natural stone the minimum coverage is 95% with no voids exceeding 2 square inches and no voids within 2 inches of the corners.  All edges of the stone are to be fully supported.

Where there are excessive voids the tile is susceptible to damage if something heavy is dropped on it or something is moved over it with a heavy concentrated load.

The ANSI A108.5 standard does say "average uniform contact" and "sufficiently distributed to give full support" regarding the thin-set adhesive between the back of the tile and its substrate.   On that basis, large voids under the tile is not acceptable.

Spot bonding of tile that only gives partial contact leaving large voids is not an acceptable method for tile installed on floors with thin-set mortars.

2 thoughts on “What are the allowable size of voids under porcelain tiles?

  1. Peter dems says:

    Hi Donato

    Had a tile guy who just installed large 2×3′ porcelain wall tiles in shower and the two at bottom are 1/3 hollow it seems. He did not backbutter the entire tile. Could this become a problem being on the bottom of shower wall where hot water and moisture will be constant? Thank yo

  2. Donato Pompo says:

    Technically according to industry standards hollow sounds are not a defect, although they can be a symptom of a defect. Tiles can sound hollow for many different reasons.

    Generally speaking if a portion of a tile sounds hollow and other portions sound solid, that might indicate that there are excessive voids or the tile isn’t bonded in those areas. The only way to determine it for sure is to remove the tile to see what the underlying conditions are where it is hollow sounding versus solid sounding.

    ANSI A108.19 for gauged porcelain tile panels require that the tiles are back buttered by troweling the back of the tile so the trowel ridges are parallel to the short side of the tile. The substrate is to be troweled in the same direction so the parallel thin-set ridges are line up with each other. The standard allows for up to 15% voids on the back of the tile, but with no voids larger than 2 square inches, which is about the size of a golf ball.

    Porcelain tiles are impervious and the large tile panels have fewer grout joints that are smaller in width. You don’t want excessive voids behind the tile in a shower so water doesn’t collect in those areas. You also want to make sure the tile is safely attached to the walls.

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