Backer rod and the right joint depth-to-width ratio

A movement joint that is just filled solid with sealant will tear itself apart. Backer rod fixes that by controlling the sealant’s shape and stopping it bonding on three sides.

What backer rod does

• Sets the depth. Push the rod in to leave the sealant the right thickness — for joints 6–12 mm wide, a depth about half the width; for wider joints, roughly 10–12 mm deep. The sealant should be thinnest in the middle (an hourglass section), so it can stretch and compress.
• Acts as a bond-breaker. The sealant must stick only to the two joint faces, not the back. If it grips all three sides it cannot move and it tears. Closed-cell or bi-cellular rod, or a bond-breaker tape where the joint is too shallow for rod.
• Tools the joint. You press the sealant against the rod when you tool it, forcing full contact with the sides.

Sizing the rod

Use rod about 25% larger than the joint width so it stays put under slight compression. Do not stretch it or twist it in. Do not puncture closed-cell rod — it outgasses and bubbles the sealant; use bi-cellular or open-cell rod for hot conditions.

Depth without rod

Where the joint is too shallow for rod, lay bond-breaker tape along the back before sealing. Never seal a moving joint three-sided.

Rule of thumb

• Width × 2 = maximum depth for narrow joints.
• Aim for a 2:1 width-to-depth once you are past ~12 mm wide.
• Always a bond-breaker at the back.

We stock closed-cell and bi-cellular backer rod in the common diameters and bond-breaker tape. Tell us the joint width and movement and we will match the rod and the sealant.