What Size Intumescent Strip Do I Need for a Fire Door?

What Size Intumescent Strip Do I Need for a Fire Door?

If you are replacing or specifying an intumescent strip for a fire door, it is tempting to start with the fire rating. FD30 door? Buy a 30-minute seal. FD60? Fit a bigger one.

Unfortunately, it is not quite that simple. No standard intumescent strip size applies to every FD30 or FD60 fire door. The correct seal depends on the door construction and the evidence supporting it, including the type of intumescent material, its dimensions, position, and configuration.

So while sizes such as 10mm, 15mm and 20mm are commonly encountered, the width of the strip is only one part of the specification.

Is there a standard intumescent strip size for an FD30 fire door?

No. An FD30 rating tells you the fire-resistance period the door assembly must provide; it does not prescribe a universal intumescent seal size. A relatively straightforward single-leaf, latched timber door may need less intumescent material than a larger leaf, a pair of doors or a different door construction. The seal arrangement used in the fire test is therefore far more useful than a rule of thumb based on the FD rating alone.

This is why two doors described as FD30 can legitimately have different intumescent seal specifications.

What size intumescent strip is used on an FD60 fire door?

FD60 doors will often require more intumescent material around the leaf perimeter than comparable FD30 doors, but again there is no single strip size that applies to every 60-minute door.

The additional material might be provided by a wider seal, more than one seal, or a different configuration. Door size, construction, whether it is single or double leaf, and the way the door is restrained can all affect the tested arrangement.

If the door manufacturer's evidence specifies two seals, for example, replacing them with one wider strip because the total width looks similar is not an equivalent substitution.

What do 10mm, 15mm and 20mm intumescent strip sizes mean?

The quoted size normally refers to the seal profile dimensions, but seeing a 10mm, 15mm, or 20mm groove in a door or frame doesn't tell you everything you need to know about the seal that belongs there.

Intumescent materials are not all the same. Different materials can activate at different temperatures and produce different amounts of expansion and pressure. That behaviour matters during a fire, which is why matching the physical dimensions of an existing strip is not enough on its own.

If you have the door manufacturer's test or certification information, use it to identify the seal that was tested or is permitted within the scope of the evidence.

Does the intumescent strip go in the door or the frame?

Intumescent seals can be fitted in the edge of the door leaf or in the frame, depending on the doorset design and supporting evidence. On some doors the position may appear interchangeable, but it should not be moved simply because another location is easier to route or fit.

Pairs of doors need particular care because seals may also be required at the meeting stiles. The expansion pressure generated in this area can affect the way the two leaves behave during a fire, so the tested arrangement matters.

Can I replace an intumescent strip with another brand or material?

Do not assume that you can. Two intumescent strips can have the same external dimensions and still behave differently when heated.

The safest starting point is the evidence for the door or doorset. If a particular material, product type, or configuration is specified, follow it unless an alternative is covered by appropriate evidence.

This is also why choosing a replacement solely because it fits the existing groove can be misleading. The groove shows the available space; it does not show how the seal needs to perform in a fire.

What if I do not know which intumescent strip was originally fitted?

Start by identifying the door and looking for the information that came with it. Labels, plugs, certification marks, installation records, fire door schedules, and the door manufacturer may all help establish the original specification.

If the evidence cannot be found, avoid guessing from the apparent fire rating or groove size. An existing fire door can have a more complicated history than it first appears, particularly if seals, ironmongery, or other components have already been changed.
If you can't establish the correct specification, get competent technical advice before replacing the seals.

Do smoke seals affect the size of the intumescent strip?

Fire resistance and cold-smoke control are different requirements. Where a door needs both, the smoke-control element may be incorporated into the intumescent seal or provided separately.

So if an FD30S or FD60S door has a combined fire and smoke seal, replacing it with a plain intumescent strip of the same width would remove the smoke-control function. You need to check the full requirement, not just the dimensions of the intumescent core.

So how do I know which intumescent strip I need?

If the door is new, the answer should come from the fire test evidence, certification or information supplied by the door manufacturer. Look for the specified intumescent material, size, number of seals and their position around the doorset.

For an existing door, establish as much as possible about the door and its original specification before removing or replacing anything. If you are still unsure, Norseal's technical team can help you work through the available information and identify an appropriate fire door sealing solution.

FAQs about intumescent strip sizes

What size intumescent strip do I need for an FD30 door?

There is no single strip size for every FD30 door. Check the fire test evidence, certification, or the door manufacturer's specification for the correct material, size, position, and number of seals.

What size intumescent strip do I need for an FD60 door?

FD60 doors often use more intumescent material than comparable FD30 doors, but there is no universal FD60 seal size. Follow the tested or certified doorset specification.

Can I use a 15mm intumescent strip on an FD30 door?

Only if the evidence supports that size and seal type for the door. A 15mm strip may suit some FD30 constructions, but the fire rating alone does not determine the seal size.

Can I fit a 20mm intumescent strip instead of a 15mm strip?

Not simply because it is larger. More intumescent material isn't automatically better, and changing the size or expansion characteristics can change how the doorset behaves in a fire.

Are all intumescent strips the same?

No. Different intumescent materials expand differently when heated. Strips that look similar or have the same dimensions are not necessarily interchangeable.

Can I measure the groove to find the right intumescent strip?

The groove size is useful, but it is not enough to identify the correct seal. You also need to know what material and seal arrangement the door was tested or approved with.

Can intumescent strips be fitted in the frame instead of the door?

Sometimes, but the position should follow the evidence for the doorset. Do not move a seal from the door edge to the frame simply because it is easier to fit there.

Does an FD30S door need a smoke seal as well as an intumescent strip?

Yes, the 'S' denotes a smoke-control requirement. This may be achieved with a combined intumescent and smoke seal or with separate seals, depending on the doorset specification.