Retaining Wall Block Tolerance

By Joseph Kowalski
Jan 18, 2014 - 5:19:59 PM

This short article discusses retaining wall block tolerance: what it is, what are the limits, and what happens when blocks are manufactured "out of tolerance".

Segmental retaining wall blocks do not use mortar.  They are simply stacked one upon another.  Sometimes there are grooves, or lips, or pins or clips that align the blocks.  However, the blocks always rest one upon another.

Our past videos (like this one) and articles discuss the critical importance of getting your first row of blocks perfectly level.  The blocks must be level front-to-rear and side-to-side.  A perfectly level first-row of blocks will help make wall construction fast!   Unless .... the blocks are out of tolerance!

When blocks are "out of tolerance" it means that one side of the block is higher or lower than the other side.  You can see that this can cause big problems for these reasons:
  1. Adjacent blocks are not at the same elevation.  The next block you place will rock back and forth;
  2. The next block you place will have its weight pushing on one point.  It's weight is not distributed evenly, so that block is likely to crack; and,
  3. If there is a separation, then the geogrid layers have no friction holding them to the face!
The National Concrete Masonry Association (NCMA) provides guidance for "block tolerance".  They recommend that "SRW unit dimensions shall not differ by more than 1/8 in., as measured in accordance with ASTM C140. This tolerance does not apply to architectural surfaces, such as split faces."

Take a look at these blocks and you can see what problems arise when tolerances are not met.