Expansion Joint in Masonry — Backer Rod and Sealant

An expansion joint is a planned, continuous gap in a masonry wall that allows the brick or block to expand and contract with temperature and moisture changes without cracking or crushing adjacent sections.

What It Is

Brick expands slightly and permanently after it is fired — a phenomenon called moisture expansion. Combined with thermal cycling, long runs of brick wall will accumulate enough movement to crack or buckle if there is no relief. An expansion joint provides that relief by inserting a soft, compressible gap through the full thickness of the wall at regular intervals, typically every 25–50 feet in standard brick veneer construction.

The joint consists of three components: the gap itself (typically 3/8–1 inch wide), a backer rod that partially fills the opening to control sealant depth, and a flexible sealant that closes the joint to water while remaining able to compress and extend as the wall moves. The backer rod is a closed-cell foam cylinder pushed into the joint to about half depth, giving the sealant a back surface to bond to and keeping the sealant at the correct thickness ratio for maximum movement capacity.

Expansion joints in brick veneer walls are placed at corners, at changes in wall height or thickness, at intersections with dissimilar materials, and at regular intervals in long straight runs. The Brick Industry Association recommends a maximum spacing of 25 feet in exterior walls exposed to full sun, and up to 50 feet for less exposed conditions.

Expansion joints differ from control-joints: control joints are used in concrete and CMU to manage shrinkage cracking and are typically filled with a non-compressible backup; expansion joints are compressible and absorb true expansion.

Where It Is Used

Expansion joints appear in exterior brick veneer walls, brick cavity walls, brick retaining walls, brick pavement, and long runs of CMU construction. They are required by most masonry standards in exterior walls and are placed at structural discontinuities and at the intervals specified by the design or by manufacturer recommendations.

How to Identify One

Look for a vertical joint in a brick wall that runs from top to bottom and does not contain mortar. Instead, the joint has a flexible caulk or sealant material — often a dark gray or tan color — that is slightly recessed or flush with the wall face. In older walls, the sealant may be cracked, missing, or replaced with rigid mortar (which defeats the purpose).

Replacement

Failed sealant in an expansion joint — cracked, pulled away from the brick, or hardened — should be replaced before water infiltrates. The old sealant is cut out, the backer rod is inspected and replaced if compressed or deteriorated, and new polyurethane or silicone sealant compatible with masonry is tooled in. Sealant replacement is a maintenance task that does not require a permit and should be done every 10–20 years depending on exposure.