Brick expands, CMU shrinks
Brick grows over time, so brickwork gets expansion joints (BIA TN 18A, 2019). Concrete masonry shrinks, so CMU gets control joints. A CMU backup with brick veneer gets expansion joints in the brick and control joints in the CMU. Swapping them is the reversal TN 18A lists as a common error.
Brick joint spacing and shelf angles
BIA TN 18A recommendations for vertical expansion joints:
- 25 ft (7.6 m) o.c. maximum in brickwork without openings.
- 20 ft (6.1 m) o.c. maximum with multiple openings.
Horizontal expansion joints go immediately below shelf angles, with at least 1/4 in. (6.4 mm) of open space or compressible material under the angle. Packing that joint solid is the error: the expanding brick then bears on the angle below.
The spacing formula
BIA TN 18A Equation 1: Se = (wj × ej) / 0.09
- Se = maximum expansion joint spacing, in inches
- wj = joint width, in inches
- ej = percent compressibility of the least compressible of sealant, backer and filler, entered as a percent
For a 3/8 in. joint at 50%: (0.375 × 50) / 0.09 = 208.3 in., about 17 ft 4 in. TN 18A's worked example, 278 in. (23 ft 2 in.), is for a 1/2 in. joint.
The trap is entering 50% as 0.50, which gives an answer 100 times too small (about 2.1 in.). Multiplying by 0.09 instead of dividing is the other slip.
Total movement
TN 18A estimates movement of unrestrained brickwork as length × 0.0009. For 100 ft: 100 × 12 = 1200 in., and 1200 × 0.0009 = 1.08 in. Labelling the 0.09 ft result as 0.09 in. is the unit trap.
CMU control joint spacing
The IBC-based DoD code §2104.1.6 (2021 and 2024) prints no spacing number. It requires control joint spacing and placement per NCMA TEK 10-2C (empirical method) or TEK 10-3. The 25 ft and 20 ft figures are BIA brick expansion joint figures, and Equation 1 is for brick, not CMU.