Can a fire-rated door be undercut?

Author: Lacey Mueller  |  Last update: Wednesday, October 1, 2025

Chapter 4 mandates that the maximum clearance under a fire door be not more than ¾ in. (19 mm). Undercutting is the process required to achieve this ¾ in. (19 mm) door clearance.

Can fire-rated doors have an undercut?

Since fire doors should be certified, the level of undercut will depend on what has been tested and approved by the certifier/certificate. Check with the manufacturer/supplier.

What is the code for door undercut?

306.4. 1.5 Door undercut. Door assemblies in the storm shelter envelope shall be limited to a 3/4-inch (19 mm) maximum undercut.

Can you shave the bottom of a fire door?

Most fire door manufacturers allow some trimming allowance at the bottom of a modern fire door. Before you start to cut the door, speak to the manufacturer for accurate trimming information. The amount that is safe to trim can only be decided on a case-by-case basis.

What are the requirements for a fire rated door?

Standard fire ratings vary, typically ranging from 20 to 180 minutes depending on code criteria. Fire-rated doors are required to be self-closing and positive latching. They must remain closed during a fire to protect the means of egress.

A Conversation Regarding a fire rated door with a 1 inch undercut that has failed inspection and how

Can fire-rated doors be cut down to size?

Whether fire doors can be trimmed always depends on the door manufacturer's guidelines and test evidence. If a door is trimmed outside of the scope of the manufacturer the door's certification is then void and may not work as needed in the event of a fire.

What is the code for a fire-rated door?

b) NFPA 80 – Fire door assemblies (doors, frames and hardware) are designed and tested to withstand smoke, flames and hot gases during a fire. The standard that addresses the requirements for fire door assemblies in depth is NFPA 80 – Standard for Fire Doors and Other Opening Protectives.

What do you put on the bottom of a fire door?

Unfortunately, this also means smoke can easily travel under doors during a fire and flood multiple rooms. A drop-down seal is fitted along the bottom of a fire door and works in unison with a latch on the frame.

Can you modify a fire-rated door?

Where possible, it's best not to modify a fire-rated door as any changes or modifications can affect the integrity of the door if not done correctly or to specification. Any modifications should be in line with the manufacturer's guidelines and should be undertaken by a competent person.

What should never be done with a fire door?

Don't alter your fire door
  • Don't drill into your fire door or cut it in any way.
  • Don't paint over the seals on a fire door.
  • Replace the handles, hinges or any hardware. Always have repairs carried out by a qualified individual.

What is the minimum undercut for a door?

As a guide the average undercut of your door should be between 8mm and 10mm, that's around ⅝ inch to ¾ inch from the bottom of the frame. if you are in doubt, shave or cut off a little and if it's not enough, shave off some more. Remember less is better or you may damage the door's structural integrity.

What is the purpose of door undercut?

By allowing air to flow freely under the door, you create a pathway for the moist air to escape and fresh air to enter. This circulation helps maintain a balanced humidity level and reduces the chances of condensation forming on surfaces.

What is the maximum allowable clearance on a fire-rated door?

Clearance for under the bottom of the door maximum is 3/4” Clearance between door and frame is 1/8”, +/- 1/16” Clearance for meeting edges for a pair of doors is 1/8”, +/- 1/16”

What is the difference between a fire door and a fire rated door?

Fire doors are designed to stop the spread of both: flames and smoke. Fire-rated doors help slow or prevent the spread of both fire and smoke, but they are not designed to be completely fireproof. These doors will eventually burn through in a fire.

Can a fire rated door have a deadbolt?

No. Deadlocks are not approved for use on fire doors. The two main requirements of a fire door is being self-closing and self-latching, so a deadbolt does not satisfy the self-latching requirement for fire doors.

What are the standards for fire rated doors?

In most cases, the BCA requires a fire door to have an FRL of –/60/30. Where a fire door is located in a firewall, a higher FRL is required. The first criterion of zero minutes is a reference to the structural adequacy of the door.

Can you shave a fire door?

Do not trim the top of the door. Most fire door manufacturers allow little trimming allowance at the bottom of a modern fire door, so when purchasing a new fire door, both the height and the width of the door should be specified precisely.

Can I add a clipping to a fire door?

You can make some size alterations by trimming the door. These must be within the limits shown in the instructions or CF for the door. There is a trim allowance that can be made to the two sides and bottom edges of the door. You MUST not remove the label on the top edge of the door.

Can you put a door stop on a fire rated door?

The use of door stops, wedges and other unapproved hold-open devices shall be prohibited. Where it is desired to keep the doors open, the head of the fire department shall require the installation of approved automatic closing devices in accordance with 780 CMR.

What gap is allowed at the bottom of a fire door?

The maximum gap between the bottom of a fire door and the floor should not exceed 10mm or be less than 8mm [as set out in Building Regulation Approved Document F], except in instances where smoke protection is required by Building Regulations in which case the gap underneath the fire door should not exceed 3mm.

Do fire doors need seal at bottom?

Do I need to fit seals on my fire door? ALL fire doors MUST be fitted with the appropriate seals. They are designed to expand under heat, and fill the gaps between the door leaf and frame, thereby preventing the passage of smoke and fire to other parts or compartments of the building.

Does a fire-rated door need a threshold?

Thresholds are not typically required by the codes and standards for fire door assemblies, although there may be a need for them when combustible floor coverings are used, depending on the critical radiant flux and the rating of the fire door assembly.

Can a 20 minute fire rated door have glass?

20 & 45 minute doors still require fire-protection-rated glass up to the “Maximum size tested”. To increase the amount of glass in 60 and 90 minute doors requires fire-resistance -rated glass which is tested to ASTM E119 or UL 263 which also limits the transfer of heat.

What are the requirements for NFPa fire rated doors?

NFPA 80 requires that fire doors be equipped with specific types of hardware and accessories, such as self-closing devices, smoke seals, and fire-rated hinges that are able to withstand the temperatures and pressures of a fire.

What three items must be labeled in a fire rated door opening?

Fire Door Tags

Technicians will analyze and review the material and construction of the three major components of the fire door assembly – the door, the frame, and the kick plate. In the final analysis, they will apply the appropriate time-rated label as required by code.

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