Structural vs weatherproofing silicone

Silicone sealants split into two jobs on a facade, and they are not interchangeable.

Weatherproofing (weatherseal) silicone

Seals the joint between components against water and air — the perimeter of a window, the joint between two cladding panels, glazing pockets. What matters:

• Movement capability — rated as a class (e.g. 25 or 50), meaning it tolerates ±25% or ±50% of the joint width in movement without failing. Use a low-modulus, high-movement sealant on wide or moving joints.
• Cure type — neutral cure for almost all construction work; it does not corrode metal or stain stone. Acetoxy (vinegar smell) cures fast and cheap but attacks some metals and cementitious substrates — keep it for glass-to-glass and sanitary work.
• UV and adhesion — must be rated for permanent exposure and for the substrates it touches.

Structural silicone

Bonds glass to the frame so the silicone itself carries the wind load and holds the glass on the building — there is no mechanical clip on the exposed face. This is an engineered application:

• Only specific two-part or approved one-part structural silicones qualify.
• The bite (bond width) and glue-line thickness are calculated from the wind load and the glass size by the sealant maker or the facade engineer.
• Surfaces must be cleaned and primed to a method statement, and adhesion tested.
• It is factory-applied for most systems; site structural glazing needs the manufacturer’s sign-off.

The rule

Never use a weatherseal sealant to structurally bond glass, and do not assume a structural silicone is the right choice for a moving weather joint. Tell us the joint, the width, the substrates and the exposure, and we will match the Wacker grade.