An intumescent strip sits in the gap between a fire door and its frame. In a fire it swells and closes that gap, so smoke and flame cannot get round the edge of the leaf. Choosing the wrong size is one of the most common reasons a fire door fails an inspection.
The short answer
| Door | Typical strip | Notes |
|---|---|---|
| FD30, single leaf | One 15 x 4mm strip | Usually fitted in the frame head and both jambs |
| FD60, single leaf | One 20 x 4mm strip, or two 10 x 4mm strips | Follow the door maker's test evidence |
| Double doors | As specified for the door set | Meeting edges often need their own strip |
These are typical sizes, not rules. The door or door set manufacturer's test evidence decides what goes on that door. If the paperwork says something different, the paperwork wins.
Fire only, or fire and smoke
A fire only strip expands with heat but does nothing about cold smoke in the early stages of a fire. A fire and smoke strip adds a brush or blade that seals the gap at room temperature. Doors marked with an S, such as FD30S, need smoke control, so they need a fire and smoke strip or a separate smoke seal.
Gaps matter as much as strips
Strips are tested with a particular gap between the leaf and the frame, typically around 3mm. A door that has dropped or been planed down so the gap is far wider than that will not seal reliably, whatever strip is fitted. Check the gap along the full height before replacing strips.
Fitting
- Fit the strip into the groove in the frame or door edge, centred, in one continuous length per side.
- Do not paint over strips or seals. Paint can stop a brush seal working.
- Replace strips that are damaged, missing or painted, rather than patching them.
- Remember the hardware. Locks, latches and hinges cut into the leaf often need their own intumescent protection, such as Interdens kits.
Lines on account
Astroflame strips, fire and smoke seals, Interdens hardware kits and fire rated letter plates and viewers are all stocked. See the fire and door safety department.