Expansion and Construction Joints: Sealing Moving Gaps

• Ahmet Toka
• 7 August 2026
• Waterproofing
• 4 min read

A joint is not a defect in a structure; it is a movement zone deliberately left in place. Concrete dries and shrinks; it expands with heat; the structure settles and sways during an earthquake. If these movements are not accommodated somewhere, the structure opens its own crack. Joints determine in advance where that movement will occur.

The problem is this: making a moving gap watertight is far harder than making a fixed surface watertight. In this article we cover joint types, why a membrane stretched over them tears, how to choose the right filler and tape, and the detail solutions that work on site.

A membrane stretched over an expansion joint tears at the first movement cycle. The correct solution is not to prevent the movement but to leave the slack that will accommodate it.

Ahmet Toka, General Manager

Joint Types and Expected Movement

Not every joint is the same, and each calls for a different solution. On site we encounter four main types.

• Expansion joint: The joint that divides the structure into blocks and accommodates the largest movement. It can experience both opening and closing and out-of-plane shear. It is the most demanding joint type.
• Shrinkage (control) joint: A joint cut to determine where the crack will form as the concrete dries. The movement is one-directional and decreases over time.
• Construction (cold) joint: The junction between concrete poured at two different times. No movement is expected, but it is a natural path for water.
• Structural separation joint: A joint separating parts of the structure that settle differently. The likelihood of vertical shear is high.

The first step in detail design is to determine how much that joint will move and in which direction. Without that, no material can be chosen.

Why Is a Membrane Not Enough?

A membrane laid flat over an expansion joint has to accommodate all the movement with its own elongation capacity. When the joint opens the membrane is stretched; if there is not enough elongation allowance it tears. When it closes it wrinkles, and fatigue cracks form in those folds.

The logic of the correct solution is different: leaving slack in advance to accommodate the movement. In practice this is done in two ways. The first is to lay the membrane over the joint with a loose loop (an omega detail); when the joint opens, that loop unfolds and the material is not stretched. The second is to use special joint tapes designed to accommodate movement.

There is also a hidden mistake: if the area of the membrane over the joint that must not be bonded is in fact bonded, the loop allowance loses its function. The detail works precisely because that free zone stays free.

Layers of the Solution

A sound joint detail usually consists not of a single material but of layers working together.

Waterstops Within the Concrete

Waterstops are placed inside the joint during concrete pouring and cut off the passage of water through the concrete section. PVC waterstops, swellable (bentonite or hydrophilic rubber) profiles and injection hoses belong to this group. An injection hose offers a further advantage: if the joint later lets water through, intervention can be made through the hose by injection .

Joint Filler and Sealant

The joint gap is brought to a set depth with a backer rod and an elastic sealant is applied over it. The backer rod does more than fill space: by ensuring the sealant bonds to only two faces (not three) it allows the material to stretch freely. When this detail is skipped, the sealant suffers cohesive failure at the first movement cycle. For material and application details see our joint sealing applications page.

Surface Joint Tapes

Where the waterproofing layer crosses a joint, special tapes with a loop allowance to accommodate movement are used. The tape is bonded to the surface on both sides; the free zone in the middle accommodates the movement. In liquid systems such as polyurethane and polyurea, continuity is provided by applying an additional reinforced layer over the tape.

Decisive Criteria in Sealant Selection

• Movement capacity: The total movement the product can accommodate must exceed the expected movement of the joint. This also determines the calculation of the application width.
• Exposure: A UV-resistant type is chosen for surfaces in sunlight and a water-resistant type for places permanently under water.
• Traffic: Joints crossed by vehicles require products of high Shore hardness that resist abrasion.
• Chemical exposure: On industrial floors the resistance table is checked against the chemical that will be in contact.
• Bonding surface: Concrete, metal or a coating; each may require a different primer.
• Depth-to-width ratio: In most elastic sealants the depth should be about half the width; a deep application reduces the flexibility capacity.

Common Mistakes Made on Site

1. Not using a backer rod. The sealant bonding to the base of the joint as well largely destroys its movement capacity.
2. Cutting the joint too narrow. A joint left too narrow for the expected movement will not work even with the best sealant.
3. Trying to "close" the joint. An expansion joint filled with mortar or rigid material tries to prevent the structure from moving, and new cracks open in the neighbouring areas.
4. Skipping the primer. Most sealants do not achieve sufficient adhesion directly to concrete; a primer is mandatory.
5. Breaking the continuity of the waterproofing. If the waterproofing layer is terminated at the edge of the joint, the joint zone is left unprotected.
6. Forgetting maintenance. Sealant is a consumable; it must be inspected at intervals and renewed when necessary.

For the place of joints within waterproofing as a whole see our waterproofing page.