QUESTION

Is there an allowable tolerance for head joint alignment in a 50% Running Bond pattern? For instance, we are currently on a project installing 12" x 24" rectified porcelain tile on a floor with 1/8" grout joints. The Homeowner laid a straight edge on the head joints and found a single course where the head joints did not line up perfectly (they were off by 3/16" to 1/4". He is requesting we remove the entire course that is approximately 30' long. In my professional opinion, there is no possible way you'll ever be able to detect that with the naked eye and am trying to discourage him from us removing that course because it will most likely destroy the Wonderboard substrate we used.

ANSWER

ANSWER - I am assuming that when you say head joint that you are referring to the vertical grout joint on the 12" short side of the 12 x 24 inch tile you are installing that lines up with the center of the tile above and below it in the adjacent courses.

Grout joints within a tile installation are always going to vary to some degree as the installer makes adjustments for the sizing variation of the tile.  Rectified tiles will be more consistent in sizing.  Depending on the particular tile, in order to maintain a consistent staggered pattern it may require a minimum grout joint width.  In those cases the overall grout joint width should be adjusted to the minimum required head grout joint width.

Rectified tiles of that size should be able to have a 1/8" wide grout joint, although ANSI A108.02-4.3.8.1 says the grout joint width should be increased from that by the amount of edge warpage it has on the longest edged of the actual tiles being installed.   So in many cases rectified tiles will have about 1/32" edge warpage and that means the tile grout joint should be at least 5/32" wide.

Note that ANSI A108.02-4.3.8.2 states that when using tiles that are greater than 18 inch on one side in a running bond off-set pattern that the maximum offset should be 33% unless the owner or architect approves otherwise.  50% offsets are problematic because the center of the tile and the end of a tile, which have the most warpage, are aligned which can lead to excessive lippage.

ANSI A108.02-4.3.8 says "Nominal centerline of all joints shall be straight with due allowances for hand-molded or rustic tiles."  It doesn't give any quantitative tolerance for grout joint widths.

The MIA says “Joint size: The grater of 25% of the joint width being used or +/- 1/32.  Joints should always read true and straight.”

Currently there is a suggestion in the ANSI A108 committee to add to standards "In no circumstances shall the grout joint width vary more than +/- 25%."

In your case if the 1/8" grout joint width is varying +/- 1/32" then a 3/32" to 5/32" wide grout joint width is acceptable.   If the tile sizing causes the head grout joint to be larger in order to maintain the offset pattern, then the horizontal joints should have been increased.