HOW TO SEAL A MOVEMENT JOINT

Not every gap between two elements is a movement joint. Movement and construction joints can occur between:

• concrete elements
• façade panels
• masonry and concrete
• concrete and metal
• floor elements
• prefabricated structures
• sandwich panels
• dissimilar building materials moving independently of each other

These joints are designed to open and close as the structure moves.

This is the most important choice. Do not look only at whether the product is PU, silicone or hybrid — look at its declared intended use and movement capability.

Construction sealants carry classes such as 25LM, 25HM, 20LM and 20HM. The number is the movement class; LM / HM refers to the mechanical behaviour of the material — low or high modulus.

That does not mean every real joint can be sized by a single movement × 4 rule. You have to account for:

• the maximum expected movement
• the initial joint width
• the temperature at the time of application
• the temperature range of the structure
• production and installation tolerances
• the permissible movement of the specific sealant
• the minimum and maximum width allowed by the manufacturer

For significant structural joints the dimensions must be set by the designer or the technical specification.

If two façade elements have a wide elastic joint between them, a few millimetres of movement is a relatively small percentage of its width. In a very narrow joint the same movement demands enormous deformation of the sealant.

The greater the expected movement, the more the joint needs adequate width. Do not try to compensate for a too-narrow joint with a better sealant.

The joint must not be a deep channel filled to the top with sealant.

Too thick a sealant section becomes harder to deform. More material does not mean a better working movement joint — the aim is a sealant that can extend and compress freely.

In a standard deep movement joint a PE backer rod goes under the sealant. It is not just a filler and has three important functions:

• it sets the depth — placed at the required level, it controls the sealant thickness
• it reduces consumption — you do not fill the whole deep opening with expensive sealant
• it prevents bonding to the back — its most important function

A correctly executed elastic sealant must bond well to the left and right faces, but not to the back of the joint. This is called two-sided adhesion.

When the sealant also bonds to the back it is fixed in three directions and cannot deform freely. Stress concentrates and the result can be:

• tearing within the sealant itself
• debonding from one edge
• premature joint failure

Sometimes a joint is wide enough but too shallow for a backer rod. A suitable bond-breaker tape can then be used at the back. Its job is the same — to prevent three-sided adhesion.

The backer rod is normally chosen slightly larger than the joint width so it stays firmly compressed between the faces. Follow the size recommended by the rod and system manufacturer.

A movement sealant only works if it is firmly bonded to the substrate. The side faces must be sound, clean, dry, dust-free and grease-free. Remove:

• laitance
• friable concrete
• dust
• oils
• residues of old sealant
• poorly bonded paints and coatings

On concrete and other mineral substrates the mechanical preparation is often just as important as the dust removal that follows.

A primer is not universally required and should not be used just in case. It is selected for the sealant, the substrate and the service conditions. Certain concretes, porous mineral substrates, metals, painted surfaces and old or problematic substrates may need a suitable primer; on others the sealant may adhere well without one.

Do not assume a construction sealant bonds to everything. Particular care is needed with:

• PE
• PP
• PTFE
• bituminous materials
• some plastics
• heavily plasticised materials
• powder-coated surfaces
• unknown old coatings

These can bring poor adhesion, plasticiser migration, staining or chemical incompatibility. Check the data sheet and test before doing the whole joint if needed.

Position the nozzle so the sealant fills the joint from the back towards the surface. Do not just run a thin bead over the top.

The material must have continuous contact with both side faces, with no air between sealant and substrate. Work at an even speed with steady pressure on the gun.

Tooling is not cosmetic. Correct shaping presses the fresh material against the joint faces and improves contact with the substrate. The profile must follow the requirements of the specific system.

In many façade joints the combination of backer rod and correct tooling creates a profile that lets the sealant deform more effectively. Tool before a surface skin starts to form.

If the structure has a movement joint, it has to keep doing its job. Do not bridge it with:

• rigid filler
• render
• tile adhesive
• screed
• rigid repair mortar
• any other material that blocks movement

The movement will not disappear — it will find the weakest point and crack there. The movement joint must continue through the system according to the design detail.

On a cold winter day an external joint between two concrete or metal elements can be relatively wide. On a hot summer day the same joint can be significantly narrower.

The width you see when you apply is therefore not necessarily the mean working width. The design has to consider the whole expected temperature and structural range. On large façades, long elements and critical joints this is not decided by eye.