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Are Silicone Sealants Any Good?

Silicone sealants certainly have their place in the construction industry. They are terrifically versatile products that fill many crevices and gaps.

Silicone sealants are a type of adhesive that cure (dry) to form a flexible, durable, and water-resistant seal. They are commonly used in a wide range of applications, including construction, automotive repair, home improvement, and many other industries. They are known for their excellent adhesion, weather resistance, and flexibility over a broad range of temperatures and conditions.

Silicone sealants are composed primarily of silicone polymers, which are large, chain-like molecules made up of silicon and oxygen atoms. They also contain fillers, crosslinkers, and catalysts, which help to determine the sealant’s properties and curing behaviour.

Silicone sealants come in a variety of formulations, each designed for specific applications. Some are designed to withstand high temperatures, making them suitable for use in engines or ovens. Others are designed to resist mould and mildew, making them ideal for use in bathrooms and other damp environments. In addition, there are food-grade silicone sealants used in the food and beverage industry, which meet strict safety standards for contact with food and drink.


In the construction industry, where harsh conditions, exposure to the elements, and fluctuating temperatures are the norm, a material’s ability to resist these challenging conditions is critical.

In the realm of bathroom renovations and installations, silicone sealants are indispensable. Given their waterproof nature and mould-resistant properties, they provide excellent protection in areas with regular water exposure. Whether it’s sealing around bathtubs, shower trays, sinks, or tiles, silicone sealants ensure that moisture does not seep behind fixtures, thereby preventing water damage and the subsequent growth of mould and mildew.

The versatility of silicone sealants stretches far beyond these applications. They can also be used in sealing window and door frames, filling cracks and gaps in walls and floors, and even in some DIY projects around the house.

One key advantage of silicone sealants over other types is their flexibility. Buildings naturally move due to a variety of factors, including temperature changes and settling. Silicone sealants have the unique ability to stretch and compress, allowing them to maintain their seal even as the structure around them moves. This flexibility makes them perfect for use in areas subject to movement or vibration, ensuring a lasting seal.

Do silicone sealants degrade?

Indeed, while silicone sealants are remarkably durable and long-lasting, they are not impervious to degradation over time. Several factors can contribute to the eventual deterioration of silicone sealants, which we’ll explore further here.

1. Environmental Exposure – Long-term exposure to harsh environmental conditions such as UV rays from the sun, temperature extremes, moisture, and pollutants can contribute to the degradation of silicone sealants. UV radiation, in particular, can break down the sealant’s structure over time, leading to cracking, chalking, and loss of adhesion.

2. Mechanical Stress – Regular or severe mechanical stress, including heavy vibrations, movement of the structure, or physical impact, can cause silicone sealants to degrade or fail.

3. Chemical Exposure – Silicone sealants can also be degraded by exposure to certain chemicals. For example, exposure to solvents, acids, or alkalis can cause the sealant to soften, lose adhesion, or even disintegrate. In the case of bathrooms or kitchens, certain cleaning agents could potentially cause degradation.

4. Incorrect Application – Silicone sealants may degrade prematurely if they are not applied properly. This could include failure to clean and prepare the surfaces adequately, applying the sealant too thinly or too thickly, or not allowing it to cure properly.

5. Biological Factors – In some conditions, particularly damp environments, silicone sealants can develop mould or mildew growth, which can contribute to degradation over time.

Mould silicone sealant
Mouldy silicone sealant

Silicone sealants in EWI

Whilst they do have their uses, degradation over time is the major reason for us not recommending the use of silicone sealants, especially when installing verge trims. Continuous exposure to harsh weather conditions and UV radiation leads to the degradation of silicone sealants over time.

Verge trims, being an integral part of the roofing system, need to withstand extreme environmental conditions and mechanical stress. The prolonged exposure to sunlight, rain, wind, and temperature fluctuations can accelerate the deterioration of silicone sealants, leading to cracking, hardening, and loss of adhesion. This also results in water ingress, potentially causing damage to the underlying roofing structure and the interior of the building.

The industry constantly evolves. As such, new verge trim regulations dictate that there must be a secondary waterproof membrane behind the verge trims.

New verge trim specification

Given their popularity, verge trims are often installed in bulk. Cold bridging norms were not established until PAS 2017 and were only mandated under PAS 2019. As a result, dependence on mastic and single joint installations led to significant water ingress behind the EWI system. Our partners at EWI Pro have also addressed these issues, especially the solutions to the new regulations.

The revised guidelines address multiple pressure points associated with the use of verge trims:

  • Multiple lines of defence – Sealants won’t be relied upon as the primary barrier against water infiltration. Instead, a secondary trim (cover trim) or a suitable membrane or flashing will ensure additional protection. All joints and render abutments should comply with PAS requirements by having a double seal.
  • In the case of eaves with an insufficient roof overhang, the top of the EWI system should include a secondary waterproof membrane and/or flashing that tucks under the existing sarking felt where present.
  • The overhang should align with the exposure zone and profile type. Specific overhang measurements are recommended based on the BRE wind-driven rain map.
  • Overlapping, pre-fabricated units/connectors must be used at gable-to-eaves junctions. On-site fabrication is not permitted, although minor on-site trimming/bending is allowed as per the manufacturer’s recommendations.
  • To mitigate the effects of thermal bridging, thermal insulation at gable-to-eaves junctions should reach the level of the loft insulation top or higher, depending on the Retrofit Designer’s requirements.
  • Gable apexes should be created using pre-fabricated elements. Although minor on-site trimming/bending of pre-fabricated apex profiles is permitted, as per the manufacturer’s recommendations.

If you have used the new verge trims, let us know in the comments!


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