Sealants & Joints

Sealant joints are the weathertight line between every panel, every window frame and every movement joint in a façade, and between the façade and the roof, balconies and podium. A joint is designed to move: panels expand and contract with temperature, frames deflect under wind, and concrete structures shorten. The joint is a system of a sealant bead, a backing rod that sets its depth and shape, and a bond breaker so the sealant sticks only to the two sides of the joint and not to the back.

Joint geometry decides life as much as product. A good movement joint has a bead roughly half as deep as it is wide, an hourglass profile thinner at the centre, adhesion on two faces only, and a width sized for the movement the joint must take. Joints made too narrow, too deep, or bonded to the back (three-sided adhesion) fail early whatever the product.

What shortens life:

• Exposure: north and west elevations in Australia, south and west in the northern hemisphere, and any elevation with heavy wind-driven rain.
• Poor joint design: joints too narrow for the movement, beads too thick, no backing rod or bond breaker.
• Poor preparation: dust, release agents on precast, old sealant residue, wet or cold substrates, missed primer.
• Incompatibility between new sealant and old, or between sealant and coatings, gaskets or stone.
• Horizontal joints that hold water, and joints at the bottom of sloped surfaces.
• Patching new sealant over old, which bonds to the old bead rather than the substrate.

Adhesive failure

The sealant peels away from one face of the joint, leaving a clean substrate. It is caused by poor surface preparation, missing primer, contamination such as precast release agents or old sealant residue, or a substrate that was wet or dusty at installation. Adhesive failure usually starts early in the life of the joint and appears along long runs.

Cohesive failure

The bead splits along its middle while staying bonded to both faces. It means the joint has moved more than the sealant can take, the bead is too thin at its centre, or the sealant has aged and lost elasticity. Cohesive failure late in life is the expected end of a well-installed joint.

Substrate failure

The sealant stays intact but pulls a skin of concrete, render or stone off the joint face. The substrate is weaker than the sealant, often because a high-modulus sealant was used on a weak or carbonated surface.

Hardening and surface breakdown

UV and heat drive out plasticisers and break down the surface. The bead hardens, crazes and cracks, loses the ability to stretch, and eventually splits at the first big movement. Polysulphides and older polyurethanes harden most; silicones least.

Three-sided adhesion

Without a bond breaker or backing rod, sealant bonds to the back of the joint as well as its sides. It cannot stretch, so it tears at the first opening of the joint, often within a few years.

Staining and compatibility

Some silicones bleed fluid into porous stone and concrete, leaving a dark band either side of the joint. Dirt pick-up darkens soft sealants. Incompatible sealants soften, discolour or fail to cure when applied over or next to another product.

Hazardous content

Polysulphide and some other sealants applied from the 1950s to about 1980 may contain polychlorinated biphenyls (PCBs), which also migrate into adjacent concrete and masonry. Removal is then a hazardous materials operation.

From the street and from aerial imagery

• Dark staining or efflorescence below joints; streaking down panels from horizontal joints.
• Sealant in several colours and ages, which means patching rather than a programme.
• Visible gaps at panel corners and window perimeters; daylight through joints from inside.

Close up on site

• Gaps between sealant and substrate along one face (adhesive failure) or splits down the centre of the bead (cohesive failure).
• Crazing, chalking, cracking and hardness: an aged bead does not spring back when pressed.
• Bead dimensions: very thin or very thick beads, sealant smeared over the face rather than in the joint.
• Sealant bonded to old sealant rather than the substrate; patching over failed joints.
• Fluid staining on stone and concrete beside silicone joints.
• Fire-rated joints at slab edges and through walls: missing, gapped or replaced with non-rated material.

Inside the building

• Damp, staining and mould at window heads, sills and reveals, and on the inside of external walls at joint lines.
• Leaks that worsen in wind-driven rain and recur after local patches.

In the records

• Date of the last sealant programme and the product used; no programme on a panel façade older than 15 years is itself a finding.
• Leak log by elevation and location; repeated patching at the same spots.
• Hazardous materials register entries for sealants on buildings of the PCB era.

Where a joint has failed tells you why. Look at which face the sealant has left, and whether the substrate came with it.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A façade engineer or sealant consultant leads, using rope access or building maintenance units for close access.

• Close-up survey of a representative sample of joints on every elevation, recording failure mode, hardness, bead size and the percentage of joint length failed.
• Adhesion testing by the hand pull method in ASTM C1521 at a sample of joints on each elevation and substrate.
• Joint measurement: width, depth and movement, compared with the movement capacity of the proposed product.
• Compatibility and adhesion test patches of the proposed sealant on each substrate, left to cure and pulled before the programme starts.
• PCB sampling of sealants on buildings of the 1950s to 1980 era before any removal.
• Water testing where leaks are being traced.

The report should state the sealant type and age, failure mode and extent by elevation, adhesion results, joint geometry problems, any hazardous content, and a specification and staging for the programme.

• Cut out and replace: remove the old sealant completely, clean and prime the faces, install a new backing rod and bond breaker, and apply and tool new sealant to the right profile. This is the standard programme.
• Bridge or overband seals: a preformed silicone strip bonded over the joint where the old sealant cannot be fully removed, the joint is too narrow, or the substrate is weak.
• Joint redesign: widen joints that are too narrow for their movement, or change to a lower-modulus product.
• Fire-rated joints: replace with a tested system matched to the joint width, movement and required rating.
• Hazardous sealants: removal under a hazardous materials method, including treatment of contaminated adjacent substrate.

Interactions: sealant programmes share access with façade inspections, glazing work and cleaning, and are most economical when combined; adjacent coatings may need to be renewed at the same time.

Survey and testing are a lump sum with rates per test. Sealant replacement is measured by linear metre of joint by joint type, with bridge seals and fire-rated joints priced separately. Access is usually the dominant cost on multi-storey façades. PCB removal adds containment, disposal and substrate treatment.

The programme recurs: a façade resealed today will need it again within one to two decades on exposed elevations. It is coded as lifecycle; patching is repairs and maintenance; fire-rated joint repairs are code compliance.

• Australia. Fire-rated joints follow the National Construction Code with fire resistance tested to AS 1530.4 and installed to AS 4072.1; façade sealants are specified to the ISO 11600 classification of movement capability. On strata buildings, façade joints are common property and a frequent sinking-fund omission.
• United Kingdom. BS 6213 guides the selection of construction sealants, BS 6093 the design of joints, and BS EN ISO 11600 classifies sealants by movement capability; fire-stopping falls under Approved Document B. On leasehold blocks a sealant programme is a service-charge item subject to section 20 consultation.
• United States. ASTM C920 specifies elastomeric joint sealants, ASTM C1193 guides their use and ASTM C1472 the calculation of joint movement; ASTM C1521 sets the adhesion test. The EPA publishes guidance on PCBs in caulk in buildings built or renovated between 1950 and 1979. Façade ordinances record sealant condition as part of each examination.

• Date, extent, product and contractor of the last sealant programme, and its warranty.
• Façade inspection reports noting joint condition by elevation.
• Leak log and maintenance log.
• Façade specification naming sealants, coatings and panel finishes, for compatibility.
• Hazardous materials register, including any PCB testing.
• Fire-stopping records for slab edges and fire-separating joints.