Learn the maximum gap you can have underneath a fire door to ensure it is still efficient.
Written by Esme
6th October 2021
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The maximum gap underneath a fire door is typically 8-10mm, depending on the door manufacturer's specification. If a door has cold smoke protection, the gap between the bottom edge of the door and floor must be no greater than 3mm - roughly the thickness of a pound coin.
Always speak to the fire door manufacturer to confirm their recommended threshold gap if unsure.
Why is there a limit to the size of the gap under a fire door?
If the gap under a fire door is too large, it will not prevent cold smoke from passing through. Cold smoke is toxic when inhaled and is a major contributor to fire-related fatalities. It would also mean that air can flow into the room, adding fuel to the fire, helping it spread more rapidly.
The limit is in place to protect occupants, providing valuable time to evacuate safely in the event of a fire.
Can you cut off the edges of a fire door?
While it is permissible to trim the sides of a fire door to achieve the correct fit, trimming the top edge is strictly prohibited. The top edge usually contains the certification label, which confirms the door’s fire-rated status. Removing this can invalidate the certification and compromise the door’s performance in a fire.
You must never trim more than the allowance specified in the manufacturer’s instructions, as exceeding this limit can affect the door’s ability to provide adequate fire resistance. Always refer to the manufacturer’s guidance before carrying out any modifications.
Threshold seals for fire doors
For enhanced protection at the base of a fire door or if the gap is larger than what is permitted, a drop-down seal can be installed. These seals automatically lower when the door is closed, creating an effective barrier against smoke while maintaining a smooth opening and closing action.
We stock a wide selection of high-peformance drop-down seals, including rebated or surface-mounted options from Lorient, CCE and Norseal - designed to suit various door types and performance requirements.
How we can help
At FireSealsDirect, we stock a comprehensive range of fire door hardware and sealing solutions from trusted suppliers, including Lorient, Pyroplex, and Rutland.
For the latest updates on fire door regulations, specifications, and fire-stopping guidance, visit our Advice Centre or contact our Technical Helpline for free, impartial advice.
