What is Silicone Sealing?

Silicone sealing is the installation of a silicone-based sealant into a joint, so that the joint can move without letting water through. The material is elastic. It bonds to most building substrates, and it holds up to ultraviolet light far better than most alternatives. The failures are almost never the silicone's fault. They come from preparation, from the wrong chemistry for the substrate, or from a bead sized wrongly for the joint.

At Shepherd and Sons we are members of the Extruded Sealant Association and have been sole contractor for the British Airport Authority since 2008. From working with clients on airfield and highway joints, we've found the discipline transfers directly. A silicone bead is a sealed joint like any other, and the same three questions decide whether it lasts.

Silicones are grouped by what they release as they cure. Acetoxy cure silicones give off acetic acid, which is the vinegar smell. They cure fast and bond well to glass and glazed tile.

They also corrode some metals. They can stain or fail on cementitious and alkaline substrates. That single incompatibility is behind a large share of the silicone that fails early.

Neutral cure silicones, which includes oxime and alkoxy types, release something far less aggressive. They are the choice on concrete, masonry, natural stone and most metals. They cure more slowly, and that is the trade.

In our experience, choosing the chemistry from the substrate works better than choosing it from the application. The same joint detail can appear on tile in one room and on rendered blockwork in the next. Only one of those tolerates an acetoxy.

A sealant only moves if it is allowed to. Get the geometry wrong and even a first-class silicone tears.

A movement joint wants a bead roughly twice as wide as it is deep, and it wants to be bonded on two faces only. The joint moves by stretching the bead between those two faces.

If the sealant also bonds to the back of the joint it is held on three sides. It cannot stretch. It tears instead, usually at the weakest face, and usually within a year.

That is what a backer rod or a bond breaker tape is for. It sets the depth of the bead and stops the third bond forming. Leaving it out is not a saving.

This is where specifications commonly cite the wrong document, so it is worth being precise about which covers what.

• BS EN ISO 11600 classifies construction sealants and is the general classification standard. Its two end-use types are G for glazing and F for facade, and it applies to vertical, non-slump joints.
• BS EN 15651-3 is the standard for sanitary joints: bathrooms, showers, kitchens and similar wet areas.
• BS EN 15651-4 covers cold applied sealants for movement joints in floors and pedestrian walkways, including joints between concrete slabs.
• BS 6213 is the guide to selecting a construction sealant, and BS 6093 the guide to designing the joint in the first place.
• BS 8000-16 is the workmanship code for sealing joints on site.

So a sanitary silicone specified to BS EN ISO 11600 is specified to the wrong standard. So is a horizontal floor joint, because that classification is written for vertical work. The British Adhesives and Sealants Association guide sets out the classification plainly and is free to read. So is their guide to applying construction sealants on site.

A silicone bead is only as good as what it is stuck to. The substrate has to be clean, dry, sound and free of anything that will release.

Old sealant is the usual culprit. Silicone does not bond reliably to cured silicone, so a refresh means cutting the old bead out completely rather than running a new one over it.

Dust, release agents, curing compounds and cleaning residues all defeat adhesion. So does damp, which is why a bathroom sealed straight after tiling often fails at the bottom edge first.

Priming is substrate-specific. Some porous and cementitious surfaces need it, many do not, and the manufacturer decides rather than the applicator.

Tooling is the last chance to get it right. The bead is pressed into the joint to wet both faces properly, then struck to a concave profile so the sealant is thickest in the middle.

A bead tooled flat or left proud is thinner at the edges, and the edges are where the stress goes. That is the difference between a joint that lasts ten years and one that opens in a season.

It is worth being clear about scope. We are a pavement and surfacing contractor. Silicone sealing sits alongside that as internal and commercial sealant work, movement joints, and the wet-area and fit-out packages that go with a building programme.

Structural glazing and curtain walling are a specialist facade discipline with their own certification regime, and a pavement contractor should not present itself as that. What we bring is joint discipline. Preparation, the right chemistry, correct bead geometry and a proper backer. It is the same sequence that governs a movement joint on any surface.

A sanitary bead and a floor movement joint look similar and behave nothing alike.

A bathroom joint accommodates small movement and sees water but no traffic. A floor movement joint accommodates the slab moving, and it takes wheels directly.

That changes what the sealant has to be. A soft silicone in a trafficked floor joint gets picked out by pallet truck wheels within months.

It also changes the geometry. A trafficked joint usually wants the sealant supported and closer to flush, rather than tooled to a deep concave profile.

And it changes the standard. Floor and walkway movement joints have their own classification, separate from both the sanitary and the facade ones.

Where the joint is in a pavement rather than a building, the material usually stops being silicone altogether. That is where a polysulphide or a polyurethane takes over, which is the ground the rest of our joint sealing work sits on.

• The substrate was not clean, dry and sound before the gun touched it.
• The chemistry was wrong for the surface, most often an acetoxy on masonry or concrete.
• The bead was sized to fill a gap rather than to accommodate movement.
• No backer rod, so the sealant bonded on three sides and tore.
• It was tooled badly, leaving the bead thin at the edges where the stress concentrates.

None of those is exotic. All of them are decided in the first ten minutes of the job rather than in the choice of tube.

That is the argument for treating a silicone package as sealing work rather than as finishing work. The same discipline that keeps water out of a runway joint keeps it out of a shower tray, and the failures look remarkably similar when they come.

For silicone sealing and joint sealing across Kent and the South East, get in touch with our team.