Roof expansion joints are essential to any roofing system, designed to absorb movement from thermal shifts, seismic activity and structural settling.
Without them, the roof is susceptible to cracks, leaks and even complete roof failure. This not only threatens the integrity of the structure but also the safety of its occupants.
Selecting the right expansion joint is more than a design choice. It’s a crucial investment in the building’s durability, safety and long-term performance.
Roof expansion joints, also known as movement joints, are strategic separations built into a roofing system to accommodate natural building movement caused by temperature fluctuations, seismic activity, wind loads and settling. They are placed at key stress points like transitions, elevation changes and areas where there are different materials.
By absorbing and isolating movement, expansion joints prevent stress from transferring to the roof membrane, reducing the risk of cracks, membrane tearing and flashing failure.
In addition to managing movement, roof expansion joint covers and seals play an essential role in weatherproofing. A properly designed system includes a watertight cover, and in some instances, a secondary moisture barrier.
This seals out water, air and debris, which is especially important on flat or low-slope roofs where drainage can be limited. Without a reliable expansion joint system, water can infiltrate the roof, potentially compromising the whole structure.
A properly designed and installed expansion joint system enables the roof to flex without damage, preserves structural integrity, prevents water intrusion and extends the overall service life of the roof.
Understanding the role of roof expansion joints is essential for ensuring a durable, long-lasting roofing system. Below, we outline how these joints function and the key benefits they provide.
Roof expansion joints are essential for accommodating thermal movement and building shifts, especially in large-span structures or climates with significant temperature swings.
By absorbing expansion and contraction along designated paths, they preserve roof strength, prevent buckling, tearing or seam separation, and maintain a watertight seal when paired with the proper cover.
This flexibility reduces structural stress on concrete, masonry and other rigid materials, protecting against cracks, premature roof failure and costly repairs while extending the overall service life of the roofing system.
Choosing the right type of expansion joint is important for long-term performance. The two primary types of joint conditions are roof-to-roof expansion joints and roof-to-wall expansion joints.
Roof-to-roof expansion joints are designed to manage movement between independent roof sections, allowing each to expand and contract without transferring stress.
Spanning the gap between roof decks, these joints accommodate both horizontal and vertical displacement while maintaining structural stability.
To ensure watertight protection, they are typically finished with durable metallic or flexible rubber covers that shield the joint from water infiltration.
Roof-to-wall expansion joints manage movement where the roof deck meets a vertical structure such as a parapet, elevator shaft or exterior wall. They stop stress-related damage at the intersection of horizontal and vertical planes — this is where differential movement from thermal shifts, wind loads or settling is common.
By creating a flexible, weatherproof seal, roof-to-wall systems protect against water intrusion while preserving the integrity and durability of the building envelope.
Roof expansion joints use materials engineered for flexibility, durability and waterproofing.
Preformed bellows made from EPDM, PVC or thermoplastic elastomers are frequently used because they accommodate multidirectional movement and resist UV, ozone and weather exposure.
These may be combined with metal cover plates for added protection and to support light foot traffic.
Foam-backed rod systems are also widely used, especially in retrofit applications. These systems typically consist of a compressible foam core with a flexible membrane overlay, providing a watertight seal that can expand and contract as needed.
They are easy to install, conform to irregular surfaces and work well in both roof-to-roof and roof-to-wall applications. However, they are most effective for smaller joints rather than larger openings or seismic conditions.
Choosing the right roof expansion joint system for a project requires a clear understanding of the building’s structural movement, roofing type and environmental conditions.
Considerations should also include adjacent wall conditions, parapets and any glass or glazing if the joint runs near or parallel to a clerestory or glass wall. Additional factors such as roof slope, end conditions, warranty requirements, thermal performance, drainage, and — most importantly — water and moisture management, must also be addressed.
Start by identifying the expected movement range, both horizontally and vertically, based on the building size, layout, materials, local climate and seismic conditions.
You should also consider whether the roof membrane will be single-ply, built-up, modified bitumen or another material — you need to choose a joint system that’s compatible with that material.
Other decision factors include whether the joint will be exposed to foot traffic, equipment loads, chemicals, jet-fuel, animal fats or harsh UV exposure.
It's crucial to minimize mistakes during installation to ensure a proper weatherproof seal from the start. Even small amounts of water can begin leaking in immediately and may lead to costly repairs over time.
These are the most common mistakes to watch out for:
Incorrect sizing of expansion joints is a common installation mistake that can lead to joint failure, membrane stress and water intrusion. Undersized joints won’t accommodate the full range of building movement, while oversized joints can compromise the seal.
To prevent these problems, joint sizes should be based on precise structural calculations that account for expected thermal expansion, seismic activity and building settlement.
Consider both horizontal and vertical displacement. Always follow the manufacturer’s specifications and consult with structural engineers during the design phase to ensure proper joint width, depth and system selection.
Incorrect flashing is a leading cause of roof expansion joint failure. Poorly installed flashing causes leaks at critical transition points, with common mistakes including improper overlaps, incompatible materials or failure to fully integrate the flashing with the roofing membrane.
Delaying the installation of expansion joints until late in the roofing process is a critical mistake that can cause improper tie-ins, rushed detailing and compromised waterproofing. Expansion joints must be installed in coordination with the roofing membrane to ensure seamless integration.
Underestimating building movement is another common misstep that can lead to costly repairs. Accurately assessing the range and type of movement is essential.
Always conduct a thorough structural analysis and consult the manufacturer's specifications to select expansion joints with the proper movement capabilities.
For more than 75 years, Nystrom has helped create buildings that last. Our team understands the challenges architects and contractors face on a day-to-day basis, which is why we provide solutions specifically made with their needs in mind.
With timely responses, quick delivery and on-site assistance, professionals can trust that our team delivers high-performing products and unmatched customer service.
Ready to get started on your next project? Check out our expansion joints product line or contact us for help finding the right solution for your upcoming job.