The Mortice Lock Fire Gap: Why Your UNION StrongBOLT Locks Need Intumescent Protection
When installing or upgrading fire doors in commercial, residential, or public buildings, attention is naturally focused on the door leaf itself, the frame, and the overhead door closers. However, one of the most critical - and frequently overlooked - aspects of fire door safety is what happens inside the door.
Specifically, when you carve out a cavity to fit a mortice lock, you are removing a substantial amount of protective timber. This creates a vulnerability where heat can build up and fire can breach the door.
This is where intumescent kits come in. In this guide, we will explore why these thin, expanding wraps are essential for maintaining fire door integrity, and how to choose the right UNION StrongBOLT Intumescent Kit for your project.
The Mortice Lock Fire Gap: Why Intumescent Protection is Vital
A timber fire door is a highly engineered barrier designed to hold back smoke and flames for a specified period - typically 30 minutes (FD30) or 60 minutes (FD60).
To install a mortice lock, such as the widely specified UNION StrongBOLT range, a locksmith or carpenter must cut a deep pocket (a mortice) into the edge of the door. This process:
- Reduces the timber thickness: The solid wood that acts as a natural barrier to fire is replaced by a metal lockcase.
- Creates a heat conductor: Metal conducts heat far more rapidly than wood. During a fire, the metal lockcase can transfer extreme temperatures straight through the core of the door, causing the surrounding timber to char, degrade, and eventually fail.
- Allows smoke and flame passage: If the timber around the lock burns away, it creates a direct path for toxic smoke and fire to bypass the door.
An intumescent lock kit solves this problem. Made from an ultra-thin (typically 0.8mm), flexible graphite-based material, these pre-cut wraps are applied directly to the lockcase and strike plate. When exposed to temperatures exceeding 120°C, the material undergoes a rapid chemical reaction, expanding to many times its original volume. This expansion seals any voids around the lock, creating a robust, insulating barrier that prevents heat transfer and blocks the passage of fire.
Choosing Your UNION StrongBOLT Intumescent Kit
UNION, one of the most respected heritage names in British door hardware, has designed dedicated intumescent kits specifically tailored to fit their popular non-BS StrongBOLT series.
Using a pre-cut kit is highly recommended over trying to wrap a lockcase with generic intumescent paper. Pre-cut kits ensure a perfect fit without interfering with the lock’s mechanical operation, saving valuable installation time and ensuring compliance.
Depending on the type of lock you are installing, you will need to choose between two main configurations:
1. The UNION StrongBOLT Intumescent Deadlock Kit
If you are installing a deadlock - a lock operated solely by a key, with a single deadbolt and no latch mechanism - this is the kit you need. It is pre-cut to match the profile of the StrongBOLT deadlock case and its corresponding strike plate. It is fully fire tested for both FD30 and FD60 timber doorsets.
2. The UNION StrongBOLT Intumescent Sashlock Kit
For doors that require a handle to operate a latch alongside a key-operated deadbolt, you will be installing a sashlock. Because sashlock cases are larger and feature openings for both the latch bolt and the deadbolt, they require a different shape of protective wrap. The StrongBOLT Sashlock Kit is precisely cut to accommodate these dual mechanisms while maintaining complete fire integrity.
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Key Features of UNION StrongBOLT Intumescent Kits
- FD30 and FD60 Fire Tested: These kits are fully tested and proven to maintain the fire rating of timber doorsets for up to 60 minutes, providing peace of mind for building managers, landlords, and installers.
- Ultra-Thin Profile: At just 0.8mm thick, the high-performance flexible graphite sheet fits snugly around the lockcase without requiring you to mortice a wider pocket than standard, preserving as much of the door's original timber structure as possible.
- Self-Adhesive Backing: Installation is quick and clean. Simply peel off the backing paper and wrap the self-adhesive material directly onto the lockcase and behind the strike plate before final installation.
- Tailored Fit: Because they are designed specifically for the UNION StrongBOLT range, there is no need for awkward cutting or trimming on-site.
Best Practice Tips for Installation
To ensure your fire doors perform exactly as intended in an emergency, keep these best practices in mind during installation:
- Clean the Surfaces: Before applying the self-adhesive intumescent sheets, ensure the metal lockcase and the back of the strike plate are free from dust, grease, or wood shavings. This ensures a strong, permanent bond.
- Do Not Block Moving Parts: Ensure the intumescent material is wrapped tightly around the lockcase and does not obstruct the movement of the latch, deadbolt, or keyway.
- Always Match the Kit to the Lock: Never attempt to stretch a deadlock kit to fit a sashlock, or vice versa. The gaps left by an ill-fitting wrap can compromise the fire barrier.
- Combine with Edge Seals: Remember that lock kits protect the mortice cavity, but you still need standard intumescent strip seals (with or without brush piles for smoke) around the perimeter of the door frame to seal the gaps between the door leaf and the frame.
Explore our full range of intumescent strips, seals, and fire safety hardware to ensure your premises remain fully protected and compliant.
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