Tn 18A web-nov06. indd

EXPANSION JOINT

CONSTRUCTION

 

Although the primary purpose of expansion joints is to 

accommodate expansive movement, the joint also must resist 

water penetration and air infiltration. A premolded foam or 

neoprene pad that extends through the full wythe thickness 

aids in keeping mortar or other debris from clogging the joint 

and increases water penetration resistance. Fiberboard and 

similar materials are not suitable for this purpose because 

they are not as compressible.

Mortar, ties or wire reinforcement should not extend into 

or bridge the expansion joint. If this occurs, movement will 

be restricted and the expansion joint will not perform as 

intended. Expansion joints should be formed as the wall is 

built, as shown in 

Photo 1

. However, vertical expansion joints 

may be cut into existing brickwork as a remedial action.

Sealants

Sealants are used on the exterior side of expansion joints to prevent water and air penetration. Many different 

types of sealants are available, although those that exhibit the highest expansion and compression capabilities 

are best. Sealants should conform to ASTM C 920, Standard Specification for Elastomeric Joint Sealants [Ref. 1], 

Grade NS, Use M, and be sufficiently compressible, resistant to weathering (ultraviolet light) and bond well to 

adjacent materials. Sealant manufacturers should be consulted for the applicability of their sealants for expansion 

joint applications. Compatibility of sealants with adjacent materials such as brick, flashings, metals, etc., also 

must be taken into consideration. Manufacturers recommend three generic types of elastomeric sealants for use 

on brickwork: polyurethanes, silicones and polysulfides. Most sealants suitable for use in brickwork expansion 

joints meet an ASTM C 920 Class 25 or Class 50 rating that requires them to expand and contract by at least 

25 percent or 50 percent of the initial joint width, respectively. Sealants meeting Class 50 are recommended to 

minimize the number of joints. Many sealants require a primer to be applied to the masonry surface to ensure 

adequate bond. 

Use a circular foam backer rod behind sealants to keep the sealant at a constant depth and provide a surface 

to tool the sealant against. The sealant must not adhere to the backer rod. The depth of the sealant should be 

approximately one-half the width of the expansion joint, with a minimum sealant depth of