Tooling sealant against a round backer rod with a concave tool produces a bead shaped like an hourglass on its side. The wide contact at each face gives good adhesion area, while the thin middle stretches easily, keeping stress at the bond lines low. The center depth is typically about half the joint width, with a minimum of roughly 1/4 inch.
Tool it concave so you get the hourglass profile, a flat-faced bead that thick won't stretch.
A convex, bulging bead is the opposite of an hourglass and stretches far worse; tool it concave.
Also heard as
- hourglass shape
- hourglass joint
Related terms
Width-to-depth ratio
The proportion between a sealant joint's width and the sealant's depth, usually targeted at about 2 to 1.
Tooling
Pressing and shaping freshly applied sealant with a tool to force it against the joint faces and give it a neat profile.
Backer rod
A round foam cord pushed into a joint to set sealant depth and keep the sealant from bonding to the back.
Movement capability
How far a sealant can stretch and compress, as a percentage of joint width, while keeping its seal.