If you manage a property with fire doors, you may have heard about intumescent strips and smoke seals without being entirely sure what they do — or whether every fire door needs them. These components are often discussed during inspections, yet they are easy to overlook until a defect is recorded.
In practice, most fire-rated door sets are designed to work with a specific seal arrangement. That arrangement may include intumescent material, smoke seals, or a combined product, depending on the door specification, frame type and intended fire performance. Responsible persons and duty holders should confirm applicable requirements for their building rather than assuming one arrangement applies everywhere.
This guide explains what intumescent strips and smoke seals are, why they matter, where they may be fitted, common defects and how structured inspections can help record visible issues. It is general information only and does not constitute legal advice.
What intumescent strips are
Intumescent strips are materials fitted around a fire door that expand when exposed to heat. As they expand, they can help fill gaps between the door leaf and frame, restricting the passage of fire and hot gases for a period consistent with the door's intended fire rating.
Intumescent material is typically installed within grooves in the door frame or door edge, or supplied as part of a combined seal product. It is usually concealed when the door is closed and may not be obvious during a casual walk-through. That is one reason why seal condition is a common focus during structured fire door checks.
It is worth noting that intumescent strips are not a standalone solution. They work as part of a complete fire door set, including the door leaf, frame, hinges, closer, gaps, ironmongery and any glazing. If other components are defective, seal performance alone may not achieve the intended outcome.
What smoke seals are
Smoke seals are designed to restrict the movement of smoke around a closed door. Smoke can spread quickly during the early stages of a fire, often before temperatures rise enough to activate intumescent material. For that reason, many fire door specifications include smoke control as well as fire resistance.
Smoke seals are often brush-based or fin-based and may be fitted separately or combined with intumescent material in a single product. On site, you may see a continuous seal around the head and jambs of the door, sometimes with a separate threshold arrangement depending on the door specification.
A door may have intumescent material without a visible smoke seal, or it may have a combined product. The correct arrangement depends on the door manufacturer's specification and the intended performance. Replacing seals with an apparently similar product without checking compatibility can be a common remedial mistake.
Why seals matter
Fire doors are intended to help maintain compartmentation — dividing a building into sections that can slow the spread of fire and smoke. Even when a door leaf and frame appear sound, gaps around the door can allow smoke and fire gases to pass through if seals are missing, damaged or incorrectly fitted.
Seals also interact with door gaps. Industry tolerances for gaps around fire doors are typically measured in millimetres, and seal products are designed to work within defined gap parameters. If gaps are excessive, or if the door does not close fully into the frame, seal performance may be compromised regardless of whether intumescent material is present.
For property managers, facilities teams and landlords, seal condition is often one of the most practical indicators that a fire door may need attention. It is visible, can be photographed and can be recorded without invasive testing. That does not replace certification review or specialist advice, but it can support day-to-day monitoring and remedial planning.
Where seals may be fitted
On most timber fire door sets, intumescent and smoke seals are fitted around the head and both jambs of the frame, or into the door edge where the design requires it. The meeting stile of double doors and the rebated edges of door pairs may also have specific seal requirements.
Threshold gaps are a separate consideration. Not every fire door has a threshold seal, and whether one is required depends on the door specification and the floor finish. Some doors are designed to maintain performance with a defined bottom gap, while others may need a threshold seal or drop seal to meet the intended smoke control criteria.
Glazed panels, letter plates, ventilation grilles and other apertures may have their own seal and fire-stopping requirements. A seal around the door perimeter does not automatically mean the full door set is correctly specified. Inspection scope should include the door as a complete assembly where access allows.
- Head of the door frame or top edge of the door leaf
- Both vertical jambs
- Meeting stiles on door pairs where applicable
- Threshold or drop seal arrangements where specified
- Glazing beads and vision panel surrounds where present
Common seal defects
During fire door inspections, several seal-related defects appear repeatedly across residential and commercial buildings. These are visible issues that can be recorded and prioritised without specialist laboratory testing.
Missing seals are a straightforward concern. A door may have had seals at installation, but they can be removed during refurbishment, damaged by heavy use or simply never replaced after repair work. A frame groove with no seal, or a torn remnant of material, is a common finding.
Damaged or incomplete seals are equally common. Brush seals can become matted or detached. Intumescent strips can be painted over, which may affect performance depending on the product. Seals can be cut short at corners, leaving unsealed gaps. In high-traffic areas, seals at the leading edge of the door are often worn down by repeated contact with the frame.
Incorrectly fitted seals are sometimes found where a non-compatible product has been used, where seals have been fitted to only part of the frame, or where a seal has been compressed so heavily by an ill-fitting door that it no longer sits correctly. Gaps that exceed permitted tolerances can also mean that even intact seals may not perform as intended.
- Missing intumescent or smoke seals
- Seals damaged, torn or worn by heavy use
- Seals painted over or partially removed
- Gaps in seal continuity, especially at corners
- Non-compatible replacement seals fitted
- Excessive door gaps affecting seal contact
Why missing or damaged seals may be recorded
When a fire door inspection identifies missing or damaged seals, the finding is typically recorded because seals are a visible, inspectable part of the door set. Inspectors can photograph the defect, note its location and include it in a structured report for the responsible person or managing agent.
Recording a seal defect does not automatically mean a building is non-compliant, and an inspection report does not prove compliance. It means a visible issue has been identified that may warrant further review, remedial action or specialist follow-up depending on the door specification and building context.
Prioritisation may consider factors such as the door's location, its role in the building's compartmentation strategy, the extent of seal damage and whether other defects are present on the same door. A bedroom door in a licensed HMO, a corridor door in a care home and a riser door in a block of flats may all have seal defects, but the remedial urgency and appropriate action can differ.
Clear records can support remedial planning, contractor briefings and follow-up checks after repair work. Where photographs are included, they can help reduce ambiguity about what was found and what was addressed.
Do all fire doors need the same seal arrangement?
No. Fire doors are manufactured and tested as complete door sets with specific components, including the seal type and arrangement. A seal kit that suits one door may not be suitable for another, even if both doors appear similar on site.
Fire rating, door construction, frame type, gap tolerances and whether smoke control is required all influence the correct seal specification. Double doors, doors with glazed panels, doors with narrow stiles and doors in steel frames may each have different requirements from a standard timber single-leaf door.
If seals need replacing, the appropriate approach is usually to identify the door specification — through labels, certification data or the original installation record where available — and source compatible components. Fitting a generic strip without confirming compatibility may not restore the door to its intended performance.
Where the door specification cannot be confirmed from visible evidence alone, further investigation may be needed. That is one reason why professional inspection and survey input can be useful, particularly across larger or mixed portfolios.
How inspections can support remedial planning
Structured fire door inspections can help property teams build a clearer picture of seal condition across a building or portfolio. Rather than relying on ad hoc observations, a consistent inspection method can record which doors have seal defects, where those defects are located and how they relate to other issues on the same door set.
Reports may include photographs, defect descriptions and priority notes where applicable. This can support conversations with contractors, help brief remedial works and provide a baseline for follow-up checks after repairs. Depending on scope, inspections may cover communal doors, flat entrance doors, bedroom doors and service riser doors where access is agreed.
Inspection reports are a record of visible conditions at the time of inspection. They do not prove compliance and should not be treated as a substitute for legal advice, certification review or specialist fire engineering input where that is required.
If you manage HMOs, care homes or other higher-risk residential properties in London, reviewing seal condition alongside closers, gaps and signage can be a practical starting point for understanding which doors may need attention.