Maximise Rooflight Durability: Importance of Backing Rods in Rooflight

In overhead glazing systems like rooflights and skylights, sealant joints are critical for maintaining watertight, energy efficient and durable structures. However, temperature fluctuations cause substrates such as glass and metal frames to expand and contract, putting stress on these joints.

A key component in ensuring their longevity is the backing rod. A simple yet essential element that controls joint depth, prevents adhesion issues and enhances performance. This post explores how backing rods contribute to resilient sealant joints in rooflights, addressing thermal movement, insulation, and condensation challenges.

How Thermal Movement Impacts Rooflight Sealant Joints

Rooflight materials which is typically glass and a metal frame ( aluminium or steel ) expand in the heat and contract in the cold. This daily cycle narrows and widens the joint gaps, putting constant stress on the sealant.

Without proper design, the result is:

• Cracks or splitting in the sealant

• Debonding from glass or frame edges

• Water leaks and costly energy loss

Solution: using a backing rod helps maintain the correct sealant profile so the joint can flex and recover through every season.

Key Principles for Durable Sealant Joints in Rooflights

A robust sealant joint depends on several fundamentals to handle the stresses of thermal movement:

Optimal Depth-to-Width Ratio

The sealant, typically silicone in glazing applications, must stretch and compress without overstressing. A 2:1 width-to-depth ratio is ideal for moving joints, allowing up to 25-50% movement capability depending on the sealant type. Too shallow a joint restricts flexibility, while excessive depth can cause curing issues or sagging.

Avoiding Excess Sealant

Bubbled or raised silicone after curing indicates overapplication. Excess sealant creates voids or expands unnecessarily, compromising both aesthetics and performance. Proper application ensures the sealant stays within its designed movement range.

Importance of Tooling

Tooling the sealant after application eliminates air pockets, ensures full contact with substrates and creates a smooth, concave profile that sheds water effectively. This step is critical for long term joint integrity, especially in overhead installations.

Cold weather amplifies these challenges, making materials brittle and joints more prone to cracking. A well designed joint, however remains flexible enough to withstand seasonal movement without losing adhesion.

The Critical Role of Backing Rods in Rooflight Joints

Backing rods, typically made from closed-cell polyethylene, are indispensable for creating durable sealant joints in rooflights. They serve multiple functions that enhance joint performance:

Controlling Joint Depth

Backing rods set the correct depth for the sealant, ensuring the optimal depth-to-width ratio. This prevents the sealant from being applied too deeply, which could lead to curing issues or reduced flexibility.

Preventing Sagging

In wider or overhead joints, uncured sealant can slump. Backing rods provide support, holding the sealant in place during curing to maintain a uniform profile.

Avoiding Three-Sided Adhesion

Sealants perform best with two-sided adhesion, bonding only to the opposing substrates to allow free movement. Without a backing rod, the sealant may adhere to the bottom of the frame(three sided adhesion), restricting flexibility and increasing the risk of failure as the joint moves.

By addressing these factors, backing rods ensure sealant joints remain resilient against the constant expansion and contraction in rooflights.

Enhancing Energy Efficiency in Glass to Glass Joints

In glass to glass joints, common in large rooflight installations, backing rods play additional roles in improving thermal performance and moisture control.

Reducing Thermal Bridging

Backing rods act as spacers, preventing the outer silicone sealant from contacting the inner part of the glass unit (e.g. in insulated glazing units or IGUs). This separation minimises heat loss through the joint, improving the rooflight’s U-value and reducing heating costs.

Preventing Internal Condensation

In cold weather, the outer sealant can become chilled. Without a backing rod, this cold joint could cool the inner glass surface, causing condensation inside the unit. By keeping the outer and inner joints separate, (same way as a spacer does in a glazing unit) backing rods reduce this risk, preserving visibility and protecting the glazing system from moisture-related damage.

Conclusion: Building Resilient Rooflights with Backing Rods

Backing rods are a small but mighty component in overhead glazing, ensuring sealant joints can withstand thermal movement, maintain energy efficiency, and prevent issues like condensation. Whether you’re designing, installing, or maintaining rooflights, prioritizing proper joint design with backing rods is essential for long-term performance.

For your next glazing project, consult with manufacturers to select compatible backing rods and sealants tailored to your system. A well-engineered joint doesn’t just seal—it adapts to the elements. Share your thoughts or experiences with rooflight joint design in the comments below, and stay tuned for more glazing insights!