Sealant Joint Design: Movement, Depth & Backing

Short answer

Make the joint wide enough that the expected movement stays within the sealant's movement capability. Keep the bead shallower than it is wide, bonded to the two sides only, with backer rod or bond-breaker tape behind it. Most joint failures we see come from getting one of these wrong, not from the sealant.

A good sealant in a badly shaped joint will still fail. The width, depth and backing decide how much the sealant has to stretch, and whether it can. This covers ordinary building joints. Façade and structural glazing joints need a designer.

Size the joint for movement

Buildings move. Panels expand in the sun and shrink at night; frames flex; concrete shrinks as it dries. The joint has to absorb that movement.

A sealant's movement capability is a percentage of the joint width. To find the minimum width, divide the expected movement by that percentage. For 3 mm of movement:

Pereseal N or Pereseal AF (±30%): at least 10 mm wide.
Pereseal 802 or Pereseal MP (±25%): at least 12 mm wide.
Pereseal 702 (±15%): at least 20 mm wide.

On façades and large panels, the movement figure should come from the designer. Don't size a joint from the gap you see on the day.

Three views of the same façade joint at cool, installation and hot conditions with changing width — The same joint is narrower on a hot afternoon than on a cool night.

Get the depth right

A bead that is too thin tears. One that is too deep cures slowly, gets stiff and pulls at the edges. For our silicones, typical depth × width combinations are 6 × 6 mm, 8 × 12 mm and 10 × 20 mm. The minimum bead is 3 × 3 mm and the maximum joint width is 30 mm. 702 and FR take joints 5–50 mm wide, at least 5 mm deep.

Tooled properly, the bead should be slightly concave: thicker where it grips the sides, thinner in the middle. That hourglass shape lets it stretch without tearing at the edge.

Bond to two sides, not three

A movement joint should stick to the two faces it bridges and nothing else. If the sealant also bonds to the back of the joint, it can't stretch freely. The strain concentrates at the corners and the bead tears or peels.

Backer rod or bond-breaker tape stops that third bond. Rod also sets the depth; tape is for joints too shallow to take a rod.

Backer rod basics

Use closed-cell foam rod, slightly wider than the joint, so it stays in place by compression.
Push it in to the right depth with a blunt tool. Don't puncture it: gas from a punctured closed-cell rod can blow bubbles in fresh sealant.
Don't use rope, expanding-foam offcuts or paper. They soak up water and give the sealant nothing sound to sit on.

Give the sealant sound edges

Sealant sticks to whatever surface is there. If that surface is dusty concrete, flaky paint or crumbling render, the joint fails when that layer lets go, even though the sealant bond held. Cut back to sound material, and clean and dry both faces before sealing.

Keep water moving past the joint rather than sitting on it, and leave access so the joint can be inspected and resealed. Every sealant joint needs replacing eventually. See inspecting exterior joints.