Backer rod controls sealant depth, gives a firm surface to tool against and acts as a bond breaker so the sealant only sticks to the two joint faces. It is sized about 25 percent larger than the joint so it stays put by compression. Closed-cell polyethylene sheds water but releases gas if punctured; open-cell polyurethane cannot outgas but soaks up water; bi-cellular types sit between.
Joint's 5/8 so grab the 3/4 backer rod, and don't stab it with the screwdriver or it'll gas through the bead.
Use a blunt roller or wheel tool to set rod at depth; a putty knife or screwdriver punctures closed-cell rod and causes bubbles.
Also heard as
- backer
- rod
- backup material
- sealant backing
Related terms
Bond breaker
A tape or material at the back of a joint that sealant will not stick to, preventing three-sided adhesion.
Hourglass profile
The ideal cross-section of a sealant joint: thick at both bond faces and thinner in the middle.
Rodding
Installing backer rod into joints ahead of the sealant crew.
Outgassing
Gas escaping into curing sealant or membrane from punctured rod or damp concrete, leaving bubbles and pinholes.
Three-sided adhesion
Sealant bonded to both joint faces and the back of the joint, which locks it in place so it tears when the joint moves.