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How Much Can I Trim Off My Fire Doors?

Safelincs is often asked how much can safely be trimmed from the edges of fire doors. Here we will be specifically addressing wooden FD30 and FD60 fire doors.

The Official Answer

While nothing must be trimmed off the top of a fire door, there is sadly no general guidance to define how much can be trimmed from the sides and bottom of a fire door.

The maximum amount that can be removed from the sides and bottom of a fire door depends from the specific design of each fire door and has to be verified by the manufacturer for each door type in fire testing of complete fire door assemblies (fire door plus fire door frame)

What is a Fire Door?

A fire door is designed to resist fire and stop the spread of fire from one fire compartment of a building to the next. They play a critical role in fire safety, as they allow relatively free movement through a building without sacrificing the passive fire protection offered by the structure’s compartmentation. Fire doors can also protect escape routes and therefore allow occupants to evacuate.

Fire Door Construction

Wooden fire doors are typically around 44mm (FD30) or 54mm (FD60) thick, excluding any protruding design elements. Internally they are constructed from a core – usually particleboard, chipboard, or solid timber – along with a variety of surface options. Some fire doors may use timber framing around a core, others may have a thin hard wood lipping around the sides of the fire door, and some may have a plywood or MDF sheet glued directly to a core. A core can sometimes have a tube-core construction with circular voids within the core to reduce the weight and cost of the fire door.

Reasons for Trimming Fire Doors

There are a variety of reasons why material may need to be trimmed from a fire door. When fitting a new door into an existing fire-resistant frame the measurements may not have been adjusted to allow clearance around the sides of the door. At the top and along the vertical edges there must be a 2 and 4mm gap, while there should be no more than a 10mm gap at the bottom of the door. If the fire door has to be smoke resistant as well as fire resistant (eg FD30s or FD60s, where the ‘s’ stands for ‘smoke resistant’) the threshold gap should only be 3mm.

Fire doors and frames may also deform over time and need trimming.

Problems with Trimming Fire Doors

Trimming too much material from the edges of a fire door can result in a loss of integrity as its construction may become weakened and the more vulnerable core material might become exposed. This means the fire door’s test certificate may be void because the door has been altered outside the scope of its certification and its performance may be affected, rendering the door unsuitable.

What To Do

Always remember: Accurate trimming information can only be provided by the manufacturer!

If you are fitting a new fire door into an existing fire door frame make sure you buy a fire door that has sufficient trimming allowance for the minor trimming needed to suit the fire door frame. Preferably you should buy custom made fire doors in non-standard sizes to fit the frame exactly.

If you need to trim an existing fire door and don’t have information about the manufacturer you must make sure the door is suitable for trimming. You must bear in mind that you have no reference points so there’s no way you can be sure that the trimmed fire door will provide the level of fire separation protection required. Consider carefully whether the fire door should be replaced rather than altered.

If you still want to go ahead only general guidance can be given and there’s no guarantee of fire performance here. Remove the door from its frame and look at the top and bottom edges. If the door is a flush door and has wooden vertical stiles of less than 40mm thickness do not attempt to trim these vertical edges as you risk seriously weakening the door’s construction. Also, only trim a timber based flush fire door if you can ascertain it has a solid timber or chipboard core and that the core material is solid (not tube-core) throughout. Any hardwood lipping fitted must be at least 6mm thick after trimming so if you are trimming more material you must re-fit hardwood lipping after trimming.

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.

If the fire door is a panel door construction and you have no information from the manufacturer you should seek advice from a fire door specialist before attempting to trim the door.

Remember, by trimming a timber based fire door without having the required trimming information from the manufacturer you are at risk to adversely affecting its fire performance. If in doubt seek advice from a fire door specialist.

If you purchased your fire door from Safelincs and require additional trimming information, please call our fire door team on 0800 612 4837 or email

The documentation for your fire door should be kept to ensure it is available should future trimming should be required.


Reviewed: 02/11/2021 (doc:462 V1.1). Our articles are reviewed regularly. However, any changes made to standards or legislation following the review date will not have been considered. Please note that we provide abridged, easy-to-understand guidance. To make detailed decisions about your fire safety provisions, you might require further advice or need to consult the full standards and legislation.


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