Joint Dimensions in Caulking: Yes, They Are Important!

The Ins and Outs of Caulking by the late Charles Carll reminds the homeowner that joint dimensions matter.

In narrow joints, a given amount of differential movement between substrates translates into relatively large strain rates in the sealant.

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A perimeter sealant joint around a contemporary flanged window. The joint is of appropriate width at 0.25 inch. The joint had been in service for roughly 3 years when the photo was taken. The joint is mostly intact, but some adhesion failure is evident at lower right corner. As is common in residential construction, neither bondbreaker tape nor sealant backer was used. Joint failure is likely the result of three-sided adhesion, an unprimed siding edge, and other-than-ideal sealant width–depth ratio (sealant depth exceeding width).

The photo shows a 0.25-inch-wide perimeter butt sealant joint around a residential window, which was in accord with the window manufacturer’s installation instructions.

Industry standards call for sealant joint depth that vary with joint width and sealant type. A generic rule for joints up to 0.5 inches wide is that joint depth should not exceed joint width. Minimum acceptable joint depth varies with the sealant type, and sealant manufacturers rarely if ever provide minimum depth recommendations to retail customers.

butt-sealant

Cross-sectional sketches of butt sealant joints.

With butt joints, some minimum depth dimension at the substrate surfaces is necessary for adequate adhesion. The hourglass shape of the sealant cross section that can be seen in the above graphic is considered desirable, as it provides the greatest possible adhesive-bond area at substrate surfaces and provides a region of relatively low stiffness at mid-width of the joint. Tooling of sealant (to be discussed in an upcoming post) results in surface concavity that provides in part for the hour-glass shape of the sealant cross section. With sealants that shrink during cure, concavity of the cured sealant joint surface is likely to be accentuated, and as a result, sealant depth at joint mid-width may be less than anticipated.

When using sealants that shrink, making some trial joints to identify cured sealant depth at joint mid-width can be instructive.