Intumescent strips are fitted to the edges of fire doors. In in extreme heat, these strips expand to seal the gaps between the door leaf and its frame. This allows closed fire doors to act as a barrier to the spread of smoke and flames throughout a building. Therefore, it is essential that fire doors are installed, maintained with their seals intact. They must also be kept shut when not in use.
Intumescent seal on a fire door
Smoke seal vs intumescent strip
‘Intumescent strips’ are embedded in the door, and are dormant under normal conditions. These strips respond to heat, causing them to expand greatly in the event of a fire. This closes the gap between the door and its frame. These seals activate at temperatures that are above human survival levels. Therefore, there is no danger of them expanding and trapping people trying to escape.
A ‘brush’ seal or smoke seal will prevent the escape of cold smoke around the edges of the fire door. While these seals are also intumescent, smoke inhalation can be more dangerous than the fire itself. It is important, when fitting smoke seals, that they do not hinder the full and effective closure of the door.
Do all fire doors need intumescent strips and smoke seals?
In order to be effective, all fire doors must be fitted with intumescent strips. Without these, the door will not ‘seal’ to the frame in the event of a fire. These seals are essential to slow the spread of the fire throughout the building, providing occupants time to evacuate. If intumescent strips are found not to have been fitted in the event of a fire, the Responsible Person could be prosecuted.
Smoke seals, however, are required as specified in the fire risk assessment. Most commonly this is included on doors approaching escape routes and doors which open on to a common space.
All fire doors must have intumescent strips fitted
Why fit intumescent strips and smoke seals?
Intumescent seals only react to extreme heat, so they don’t seal up until the fire is very close. Before this, smoke from the fire which has accumulated will be able to pass through the gaps around the door. This puts building occupants in danger of smoke inhalation, which can be fatal. Smoke seals prevent this, and are typically either a soft brush or a plastic / rubber flap.
When should smoke seals not be fitted?
There are some applications where a gap should not have smoke seals: e.g. if the fire door has been installed on the exit of a room which has no smoke detectors on its own. In this case, the fire alarm system can only be triggered if smoke can leak out around the fire door and set off the fire alarm system in the circulation spaces, but these cases are quite rare.
Fire door ratings and smoke
Fire doors are rated in accordance with the length of time they will resist a fire. Therefore, a door rated ‘FD30’ will resist the passage of fire for 30 minutes, an ‘FD60’ for 60 minutes. If a fire door is rated FD30s, (‘s’ meaning ‘smoke’), it should have been fitted with the appropriate seal to resist the passage of cold smoke for 30 minutes as well. For more information about how fire doors are rated, visit our blog.
Fire door gaps and smoke seals
British Standards set out details on the permitted gaps around a fire door. The gap along the sides, top, and between the leaves of a double door, must be between 2mm and 4mm. Responsible Persons can use a gap gauge to ensure that their fire door gaps are compliant.
Under-door (threshold) gaps should be in accordance with the manufacturer’s installation instructions for the particular doorset design. This blog provides more information about threshold gaps.
Intumescent seals can only be effective with the correct gap size between the door and frame
Can smoke seals be painted?
Fire doors can be painted with ordinary paint. However, fire accessories, including intumescent seals should not be painted, as it can prevent them from being effective in the event of a fire.
Are intumescent strips required in letter boxes?
If a fire door has a letter box installed, this must have been tested and rated to the same standards as the door itself.
If an intumescent letter box needs to be added to a fire door retrospectively, this can only be done if specified in the scope of the fire door’s Certifire Approval documents, and in line with the manufacturer’s instructions. Further to this, alterations can only be made by a trained professional, and with certified hardware. This ensures that the performance of the door, and its associated certification, is not compromised.
What does the threshold drop seal do?
If smoke protection is required by Building Regulations or the fire risk assessment, the maximum gap underneath the fire door is reduced to 4mm. If the gap under a fire door is too large, surface mounted drop-down smoke seals or rebated drop-down smoke seals can be fitted to existing fire doors. When the door closes, a plunger makes contact with the door frame and lowers the seal to the floor, closing the gap under the door. This is usually a suitable solution for gaps of up to 14mm.
Can fire door seals be replaced?
Where a fire door rebate already exists, or the existing rebated fire door seal has been damaged, rebated intumescent fire door seals can be fitted.
Can fire door seals be fitted retrospectively?
Some older fire doors do not comply with the latest specifications. In some cases, this means that they do not have the appropriate intumescent or smoke seals. Fire door seals can be retrospectively added to these fire doors.
To avoid having to cut a rebate in either the door or the frame, surface mounted fire door seals can be fitted. These are stuck to the frame or door with their self-adhesive backing and sometimes nailed as well to give them increased longevity.
Replace intumescent seals if they are damaged
Who can fit fire door seals?
Fire doors should always be professionally installed, as should any work which structurally alters the door or its hardware. Therefore, rebated fire door seals can only be fitted by a qualified professional, whether as a replacement or retrofit.
Surface mounted fire door seals, on the other hand, can be fitted by a ‘Competent Person’. If new fire door seals are fitted for the first time, make sure that fire door hinges, fire door closers and, where necessary, intumescent door lock protection are fitted as well.
Visit our website to see Safelincs’ full range of fire door seals. This includes ‘fire only’ (intumescent) and ‘fire and smoke’ (intumescent and brush) fire door seals. Safelincs supply both rebated and surface-mounted application, in both FD30 and FD60 ratings. If you are still unsure what type of seal your door requires, or whether a retrofit is appropriate for your door, contact our friendly fire door team on 0800 433 4289 or by emailing firedoors@safelincs.co.uk.
Frequently Asked Questions
What is the difference between intumescent strips and smoke seals?
Intumescent strips remain dormant under normal conditions but expand greatly when exposed to extreme heat during a fire. This expansion seals gaps between the door and frame to block flames and hot gases. Cold smoke seals, often appearing as brush or rubber flaps, work differently by blocking smoke at normal temperatures. While intumescent strips activate only at high temperatures beyond human survival levels, cold smoke seals work immediately to prevent smoke from passing around door edges, protecting against smoke inhalation, which can be fatal before the fire itself reaches you. Many modern products combine both functions as intumescent smoke seals, providing complete protection against fire and smoke.
Do fire doors need intumescent seals?
Yes, all fire door sets must have intumescent seals to function correctly. Without these seals, a fire door cannot perform its essential purpose of containing fire within compartments. The lack of proper seals will allow fire to spread quickly through gaps around the door. The Responsible Person could face prosecution if fire doors are found without properly fitted intumescent seals during an incident. Building regulations require these seals to ensure the door achieves its fire rating (FD30, FD60, etc.). Cold smoke seals, while not always required on every fire door, are typically specified in fire risk assessments for doors near escape routes or those opening into common areas.
How do I fit intumescent fire seals?
The fitting method depends on whether you are using rebated or surface-mounted seals. For rebated seals: 1) Ensure the rebate in the door or frame is clean and free from obstructions, 2) Measure and cut the seal to the correct length, 3) Remove the backing from the adhesive strip, 4) Press firmly into the rebate, ensuring it sits flush, 5) Secure with pins if recommended by the manufacturer. For surface-mounted seals: 1) Clean the surface where the seal will be applied, 2) Measure and cut the seal to size, 3) Remove the backing from the adhesive, 4) Apply to the door or frame, pressing firmly, 5) Some seals require additional fixing with pins for long-term security. Professional installation is required for rebated seals, while Competent Persons can fit surface-mounted types. Always check the fire door after fitting the intumescent seals to ensure it self-closes. Adjustments may need to be made to the door if the newly fitted fire door seals prevent the door from closing.
Can fire door seals be fitted to the door instead of the frame?
Yes, intumescent seals can be fitted to either the door or the frame for single fire doors. Both options comply with regulations as long as the seals meet the required fire rating. For double doors or door-and-a-half configurations, the seals should be fitted to the door leaves themselves to ensure the gaps between door leaves are properly sealed during a fire.
What types of intumescent strips are available?
Several types of intumescent strips are available to suit different fire door requirements: 1) Standard intumescent strips (fire-only seals), 2) Combined intumescent and smoke seals (fire and smoke protection), 3) Rebated seals that fit into grooves in the door or frame, 4) Surface-mounted seals that attach to the face of doors or frames, 5) Threshold drop seals for under door gaps, 6) Specifically rated seals for FD30 (30-minute) or FD60 (60-minute) doors. Each type serves specific applications depending on your building regulations requirements and fire risk assessment findings.
Why are cold smoke seals important?
Cold smoke seals provide protection against smoke spread at normal temperatures before a fire heats up enough to activate intumescent strips. This is crucial because: 1) Smoke inhalation causes most fire-related deaths, 2) Smoke travels much faster than fire through a building, 3) Even small gaps around doors allow significant smoke passage, 4) Clear escape routes free from smoke improve survival chances. Combining intumescent strips for fire containment and cold smoke seals for smoke containment provides complete protection.
When should intumescent smoke seals be used?
Intumescent smoke seals, which combine both fire and smoke protection, should be used: 1) On doors along escape routes, 2) For doors opening onto common areas or shared spaces, 3) When specified by your fire risk assessment, 4) In buildings with vulnerable occupants, 5) Where smoke control is critical for evacuation strategy. Some exceptions exist where smoke seals might not be appropriate, such as rooms without smoke detectors where smoke leakage is needed to trigger external alarms. Always consult your fire risk assessment.
What are the requirements for intumescent seals for fire doors?
Fire doors must have correctly specified intumescent seals that: 1) Match the fire rating of the door (FD30, FD60, etc.), 2) Seal gaps of between 2mm and 4mm around the sides and top, 3) Remain unmodified by paint or other coatings, 4) Are correctly fitted according to manufacturer instructions, 5) Remain intact and undamaged. The British Standards set out specific requirements for gap sizes and seal placement. The gap along the sides, top, and between double door leaves must be between 2mm and 4mm.
Can damaged intumescent fire seals be replaced?
Yes, damaged intumescent fire seals can and should be replaced as soon as damage is noticed. Damaged seals compromise the fire resistance of the entire door assembly. For replacement: 1) Remove the damaged seal completely, 2) Clean the rebate or surface, 3) Install a new seal of the same type and fire rating, 4) Ensure proper adhesion, 5) Check fire door self-closes after the new seal is fitted. Rebated seals require professional replacement, while surface-mounted seals can often be replaced by a Competent Person within your organisation.
How do I maintain the intumescent seals?
Regular inspection of intumescent seals is essential for fire safety. Check for: 1) Physical damage or degradation, 2) Proper adhesion to the door or frame, 3) Complete coverage with no missing sections, 4) Signs of tampering or paint overspray, 5) Correct operation of any moving parts on drop seals. We recommend including seal checks in your regular fire door inspections. Replace any damaged seals promptly to maintain fire safety compliance.
Why should I choose Safelincs for my intumescent seal needs?
Safelincs provides quality intumescent seals backed by our commitment to customer satisfaction. Our team can help you select the right seals for your fire doors, whether you need a single replacement or products for an entire building. As winners of customer service awards and the trusted supplier to major organisations, including the Church of England and the Catholic Church in the UK, we offer: 1) Quality products that meet all relevant standards, 2) Expert advice on selection and installation, 3) Competitive pricing, 4) Next-day delivery on many items, 5) Ongoing support for all your fire safety needs. Contact our friendly fire door team on 0800 433 4289 or email firedoors@safelincs.co.uk for personalised advice on your intumescent seal requirements.
