Joint design decides whether the sealant works

A 615% elongation figure means nothing if the bead is 2 mm deep and bonded on three sides. Joint geometry sets the demand; the sealant only has to meet it.

The width-to-depth ratio

For a moving joint, depth is usually kept between 0.5 and 0.7 of width. A 20 mm wide joint gets a 10–14 mm deep bead. Deeper than that and the sealant is being stretched from a thin section; shallower and there is not enough material to absorb movement.

For a static joint the ratio can be squarer, but the bead still needs enough depth to bond.

Backer rod is not optional

The backer rod sets the depth, supports the bead during tooling and — critically — prevents three-sided adhesion. A bead bonded to two faces can stretch; a bead bonded to three cannot, and it tears at the weakest point.

Closed-cell rod for most joints; open-cell only where the rod will not absorb the sealant’s plasticiser.

Movement class

Movement is expressed as a percentage of the joint width: a 20 mm joint that opens 5 mm is a 25% joint. Match the sealant’s movement capability to the calculated movement, then add margin for temperature and settlement.

Elastic recovery matters as much as elongation. KA340A is specified at 615% elongation with 75% recovery; KA340B at 550% with 80%. A sealant that stretches but does not return accumulates strain.

Surface preparation, again

Concrete must be cured, dry and dust-free. Abrade, blow out, and prime porous or dusty substrates. Do not seal green concrete — the moisture that cures the concrete will interfere with the bond.

The four failures, in order of frequency

1. Three-sided adhesion — no backer rod, or rod compressed too far.
2. Poor preparation — dust, release agent, damp substrate.
3. Wrong movement class — a 25% joint sealed with a 12% product.
4. Bead too thin — wiped to a feather edge during tooling.

A more expensive sealant does not correct any of them.

Recess geometry, dimensioned

A joint drawing that says “seal joint” is not a design. Four dimensions and one material callout make it one:

Dimension	Typical value	Consequence if wrong
Joint width	10–30 mm for façade work	Sets the movement class
Recess depth	0.5–0.7 × width	Too deep: strain concentrates; too shallow: no material
Rod diameter	1.2–1.3 × width	Too small: slides; too large: buckles
Recess setback	2–5 mm below the surface	Flush beads take traffic and UV; deep beads collect dirt
Primer	By substrate, specified	Omitted on porous or damp concrete

The setback is the detail most often left to the installer. A bead finished flush with a trafficable surface wears through in months; a bead set 5 mm deep in a façade collects dirt and looks failed even when it is sound. Neither is a product problem, and both are decided on the drawing.

The drawing pack a contractor should receive

1. Joint location plan, with widths and movement class for each type.
2. A section detail for each joint type, dimensioned as above.
3. Substrate schedule and primer specification.
4. Sealant class, not just a brand.
5. Inspection points: rod placement, bead depth at a stop end, bond line after the first cycle.

A pack missing items 1 and 2 means the sealant choice was made before the joint was designed, which is the wrong order and the source of most failures on site.

What this means for buyers

1. Specify joint width, depth and movement before choosing a product.
2. Budget backer rod and primer as part of the system, not as extras.
3. Check recovery, not only elongation.
4. Inspect the bond line after the first wet cycle, not on installation day.