counterlath
One of many laths used in preparing one side of a partition or framed wall, when the other side has been covered in and finished.
counterlath: the hidden backup layer that balances wall construction
A counterlath is a thin wooden strip, typically 1/2 inch by 1 1/2 inches, nailed perpendicular to the primary framing laths on one side of a partition or cavity wall. It serves as a mechanical base for plaster or other finish materials on the second side, after the first side has already been plastered and set. The counterlaths are spaced roughly 16 inches on center and run at right angles to the original laths, creating a cross-hatched substrate that grips plaster or other finishes mechanically.
In traditional wet-wall construction, once one face of a partition was fully plastered and hardened, workers would install these counterlaths on the opposite side of the studs or framing members. This perpendicular orientation prevents the finish on the second side from cracking along the grain of the first-side laths and ensures that both faces achieve equal structural grip and finish quality. Without this offset arrangement, the plaster on the back side would tend to key (mechanically bond) along the same line grain as the front, creating a plane of weakness.
Practical role in partition work
Counterlaths become essential in three-coat plaster systems on both faces of a wall. The scratch coat and brown coat on side one compress and shrink slightly; the counterlaths absorb and distribute the mechanical forces from the plaster applied to side two. This cross-grain structure also reduces the risk of one-sided drying causing warping or cupping of the entire assembly. In modern construction, counterlaths have largely been replaced by self-furring metal lath, which achieves similar cross-bracing without the labor of installation, but the principle remains identical in cavity wall detailing.
The name reflects the construction logic: the counterlaths run counter to (at right angles to) the primary laths. This terminology assumes the reader understands that lath-and-plaster walls always require a mechanical key, and that symmetrical two-sided finishes demand structural balance. Failure to install counterlaths, or installing them at too wide a spacing, typically results in plaster checking and delamination on the second face within one to three years of completion.