Ask the Experts

QUESTION

We had an issue recently with a White Thassos mosaic where the individual pieces on the sheet had inconsistencies in spacing on the mesh, with a few pieces exceeding a 2mm joint. The factory specification was made to a 1.5mm joint, but I’m told from our vendor that various factors (handling,

Thassos White Marble Mosaic

shipping, etc) can cause shifting to occur over time.

Can you confirm if there’s an industry standard tolerance for deviation on these types of mosaics? I’m sure the Marble Institute of America has something to this effect, but I don’t have access to their documentation to verify.

ANSWER

I am able to provide the specification for ceramic mosaic tile from ANSI A137.1.  Table 6 of ANSI A137.1 states the specifications for mosaic ceramic tile, including mounting/crooked tiles and for mounting/wide & narrow joints.  Those specifications are:

  • Mounting/Crooked Tiles:  Individual joint range <30% of the average joint width of the sheet.
  • Mounting/Wide & Narrow Joints:  Average joint widths for each tile must be within +/- 25% of the average joint width of the sheet.

ANSI A137.1 section 6.2.2.1.11 describes this a bit more:

The tile shall be uniformly mounted and in patterns specified.  Joints between tile shall be in accordance with manufacturer’s specifications.  For all mounted mosaics, the range of an individual grout joint shall be no more than 30% of the average joint width for the sheet.  For sheets containing mosaics of the same color, the individual average joint widths shall also be within +/- 25% of the average joint width for the sheet.  Sheets shall be accepted or rejected (for grout joint variations) based on the number of grout joints in the sheet.

As you stated, the manufacturer specification for the tile in this installation was for a nominal joint width of 1.5mm.  Some of the joints exceed 2mm.

When we apply the tolerance of <30% of the average joint width of the sheet for the mounting of Crooked Tiles, the acceptable narrowest part of the joint adjacent to a crooked tile would be 1.05mm.

When we apply the tolerance of +/- 25% for Wide & Narrow Joints the acceptable range becomes 1.125mm to 1.875mm.  2mm would be outside the range of tolerance.

Section 9.5 of ANSI A137.1 describes the laboratory testing method for variations in mounted mosaic ceramic tiles.  That is where we learn how many tiles and grout joints are to be examined and how many variations are acceptable based on the size of the individual tiles on the sheet and overall size of the sheet.  I am not certain if the specifications for stone mosaics are similar, but I am able to refer to the following.

In the Marble Institute of America’s “Q&A Manual (Expert Answers to Technical Questions About Working with Natural Stone)” the following is stated with regard for a reasonable tolerance for joint width:

“Tolerance is normally correct at a variation from true (specification) of 1/4 of the specified joint width. With a joint width of 3/32”, a tolerance of ± 1/32” is reasonable. A 3/32” joint is correct at 1/16” wide and at 1/8” wide. The joints should “eye up” straight and true. Be careful though, if the joint is specified at between X and Y.” *

To paraphrase the first sentence of the MIA’s statement, “a variation tolerance of 1/4 of the specified joint width” may be equivalent to +/- 25% for ceramic mosaics.

I have seen many instances where inadequate adhesive has been applied to the mesh allowing individual tiles to become loose or shift.  I have seen instances where the adhesive has dried out and allowed the individual tiles to become loose and shift on the sheet.  Many of these adhesives are water based; when they become damp they re-emulsify and lose their bond.  I personally have received a shipment of tile that had become damp at some point during the fabricating, mounting, packaging, shipping, storage and delivery process to such a degree that the mesh adhesive had lost most of its bond to the back of the tiles and the tiles themselves were mildewed.  I rejected that entire shipment.

When installing any type of mesh- mount mosaic, an installer should inspect each sheet for broken, loose or very inconsistently spaced tiles.  These should be cut off the mesh and individually placed or replaced.  Depending on the size of the installation, I always expect to repair at least several throughout the installation.

Some types of mesh used for mounting are less rigid than others.  Some mesh material used has thinner strands or larger spaces in the mesh fabric that make it flimsier.  This makes it more difficult for the installer to achieve consistent spacing when setting individual sheets and throughout the installation.

I hope this helps.  For more specific guidance, perhaps the Marble Institute of America would be willing to comment on your question. — Mark Heinlein, NTCA Technical Trainer / Presenter

* Quoted from Marble Institute of America “Q&A Manual (Expert Answers to Technical Questions About Working with Natural Stone)”  www.stonesofnorthamerica.com/technical/Expert_Answers_to_Technical_Questions_about_Working_with_Natural_Stone.pdf