Intumescent strips for fire doors: What you need to know

Out of all the measures in place to protect a building from fire, fire doors are among the most visible and frequently used. However, many people do not know the full extent of their role within a fire protection strategy. As well as providing a safe evacuation route, fire doors help prevent a fire from spreading throughout a building. This is partially due to the use of intumescent strips.

Intumescent paint expands when exposed to extreme heat, closing gaps between the door and frame in the process. This helps stop fire and smoke from passing into other rooms. Depending on the type of intumescent strip, you can expect to get 30 to 60 minutes of fire resistance with this installation. However, intumescent strips can be rendered ineffective when fire doors are frequently misused – a common occurrence in buildings with multiple occupants. For example, in 2019 it was estimated that 64% of premises visited by fire services had fire doors wedged open.

This article will first provide an overview of industry best practice surrounding intumescent strips, helping both building owners and health and safety professionals understand their importance within a passive fire protection system. It will then touch upon some installation guidelines, so those responsible for fire safety maintenance can be confident in their compliance with fire door regulations.

What are intumescent strips?

Intumescent strips are fitted to fire doors, and when exposed to extreme heat they expand to close any gaps or edges that might facilitate the spread of smoke and fire.

How much protection is offered by an intumescent strip?

Intumescent strips for fire doors are generally designed to provide around 30 – 60 minutes of fire resistance, helping contain the fire to one area and prevent it from spreading. This in turn provides occupants with enough time to safely evacuate the building whilst emergency services work to extinguish the fire.

Do all fire doors need intumescent strips?

All fire doors are rated based on their fire resistance features. This is known as an FD rating and is measured based on several different criteria. FD 30 and FD 60 are the main categories of fire doors. FD 30 means a fire door has a minimum of 30 minutes of fire resistance. A door with a rating of FD60S offers a minimum of 60 minutes of fire resistance and is a smoke sealed door. It is generally accepted that to provide the required level of fire resistance, all fire doors should be reinforced with either intumescent strips, cold smoke seals or a combination of both.

According to the British Standard 476 fire safety standards, most doors need to be S-type (smoke sealed) doors. Having a fireproof door installed with intumescent strips can help the door qualify for these standards. Seals need to limit leakage to a rate of 3 cubic metres of smoke through 1 metre of door edge per hour. If you require additional clarification on fire door requirements, we advise you to contact the manufacturer of your fire door’s make and model.

If a door does not meet the regulatory definition of a fire door, adding intumescent strips alone won’t mean it will provide the same level of protection as a fire door. While nothing is stopping you from installing intumescent strips on a regular door, the door will still fall short of regulatory requirements.

Fire door intumescent strip regulations

The main point of reference for compliant fire door installations is their fire rating, which would be provided by the manufacturer in the form of testing data. These ratings depict the amount of time that a door can withstand fire damage. For example, a fire door with the rating of FD30 means it has a fire resistance rating of 30 minutes (these can range between FD30 to FD240). To learn more, consult our guide to fire door regulations.

A building’s regulations will also stipulate where its fire doors are, as well as where intumescent strips need to be installed. In a standard fire door, the door leaf has to be able to move within the frame, and for this to be possible, there has to be a small gap around the perimeter.

As a general rule, the gap of a fire door should not be larger than 3mm – 4mm, and this should be the case for the door’s two long edges, as well as across the top edge of the door leaf. The gap at the bottom of the door must be approximately 10mm for non-smoke conditions, but when smoke seals are in place this gap should be reduced to 3mm – 4mm.

Fitting intumescent strips to fire doors

Fire door seals are fitted to the stiles and head of a door set, either in the grooves of the doorframe or mounted on the surface. They are designed to swell once the ambient temperature exceeds 200°C, which will usually occur within the first 10-20 minutes of a fire. Properly installed intumescent strips will fill gaps between the door and frame to achieve fire compartmentation.

Regulations surrounding intumescent strips also require specific installation and fitting based on your building and fire door type. For example, some doors require a cold smoke seal to protect people and property from smoke damage. In cases where smoke leakage is important to detect a fire, cold smoke seals may not be required.

How to fit intumescent strips

Intumescent strips are fairly easy to install, provided the fire door in question has the right grooves in place. Otherwise, you will need to create grooves using a router that matches the width of the intumescent strip.

Once you have your groove in place, you can measure and cut your strip to ensure it’s the right width for the door. You should then check to see that the groove is clean and free of any dust or debris. Otherwise, the strip’s self-adhesive back may not stick into the groove properly. Upon fitting the strip, you should test the door so you can be certain that the door still closes properly. You may need to use a door closer so it shuts properly. If so, fire safety regulations stipulate that it must be a Grade 3 door closer or above.

Can you paint intumescent strips?

While it’s not advisable to paint over a smoke seal, painting intumescent strips is slightly more ambiguous. If you wish to paint your intumescent strip, for instance, you’d want to avoid heavy application or getting any paint on the ‘brush’ strip section. While painting an intumescent strip is unlikely to compromise its effectiveness, from a fire protection compliance standpoint, it is most likely best to err on the side of caution.

Maintaining smoke seals and intumescent strips

Like all other fireproofing materials, intumescent strips should never be “set it and forget it.” Intumescent strips can crack, warp, or partially detach from a door with age. They can also sustain damage from door use or building repairs. Therefore, they should be checked at least every six months for imperfections or other issues that might inhibit them from working properly. Any damaged or missing strips should ideally be replaced with a new one of the same size, type and brand as the original.

It’s important to carry out this inspection alongside other fire door maintenance tasks, such as checking hinges, locks, and other door elements. The door frame must be free of damage or gaps and it must close soundly. It must also have a minimum of three hinges that are in good working order. This helps ensure the fire strips maintain the same level of effectiveness as the original installation.

CLM Fireproofing are the UK’s leading experts in passive fire protection, with over 35 years of experience in the industry. Our FIRAS-accredited specialists offer fully compliant fire door inspection, installation and maintenance services (including intumescent strip fitting). Contact the CLM Fireproofing team today to find out more.