Sealant Joint Design, What is sealant joint design

Sealant joint design speaks to what is sealant joint design, why use sealant joint design and types of sealant joint design.

These are the major principles that need to be kept in mind while designing a gunnable sealant joint.

For a joint between materials with high coefficients of expansion, the time of year and outside conditions when the sealant is installed must be considered and taken into account when specifying the size of the joint and type of sealant.

Sealant installed in cold weather will have to stretch less during its lifetime but will have to compress more in summer than the same sealant installed in hot weather, which will have to stretch more and compress less.

Installation procedures are also critical to the success of gunnable sealant joints in an exterior wall system. Each joint must be carefully cleaned of oil, dirt, oxide, moisture or concrete or form release compound. It it is necessary to improve adhesion between the sealant and the substrate, the edges of the joint must be primed with a suitable coating.

Then the backer rod or backup rod is inserted. This being a cylindrical strip of highly compressible, very flexible plastic foam material that is just a bit larger in diameter than the width of the joint. It is pushed into the joint where it holds its place by friction, to limit the depth to which the sealant will penetrate in order to maintain the optimum bead and avoid waste of sealant material. Backer rod material is available in a large range of diameters to fit every joint.

The sealant is extruded into the joint from the nozzle of a sealant gun, filling completely the portion of the joint outside the backer rod. Finally the sealant is mechanically tooled, much the same as a masonry mortar joint is tooled, to compress the sealant material firmly against the sides of the joint and backer rod. The tooling also gives the desired surface profile to the sealant. The backer rod’s role is now finished but, being inaccessible, the rod stays in the joint.

Gasket sealants have generally proved to be less sensitive to installation problems than gunnable sealants, For this reason, they are widely used in proprietary cladding systems.


A properly proportioned joint may be both untooled and tooled.

A narrow joint my cause the sealant to elongate beyond its capacity when the panels on either side contract.

If the sealant bead is too deep, sealant is wasted, and the four edges of the sealant bead are stressed excessively when the joint enlarges.

 In a correctly proportioned sealant bead, the backer rod, made of a spongy material that does not stick to the sealant, is inserted into the joint to maintain the desired depth. The width is calculated so that the expected elongation will not exceed the safe range of the sealant and the depth is between 1/8 and 3/8 inch (3 and 9.5 mm).

In a correctly proportioned lap joint, the width of the joint, which is the distance between panels, should be twice the depth of the sealant bead and twice the expected movement in the joint.

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