Resistance belongs to the assembly
The common error in discussing fire resistant doors is to attribute performance to the leaf. Fire does not attack a door at one point: heat passes through the leaf face, smoke passes through the gap between leaf and frame, heat radiates through any glazing, and flame travels through the packing between frame and wall. The weakest of those four paths sets the performance of the whole.
That is why the trade specifies a door assembly rather than a door: leaf, frame, hinges, seals, lock, closer, glazing where present, and the installation detail. Change one component and the behaviour of the set changes.
The leaf and its core
Both faces of the leaf are steel, and steel conducts heat quickly, so the leaf's real work sits in the core that slows transfer between the two faces. Mineral wool is widely used because it holds its form at high temperature.
The core must leave no voids. A core that slips or settles creates a cavity, and fire finds the cavity. Bonding the core to the internal frame is therefore a performance requirement, not a tidiness measure.
Frame and installation: the link most often skipped
The frame carries the leaf and resists thermal movement during a fire, but its performance is inseparable from how it meets the wall. The gap between frame and wall must be filled with non-combustible material. Standard polyurethane installation foam is an insulant and it burns.
What we find on site, repeatedly, is a correctly specified leaf hung in a frame bedded with ordinary foam — and flame then travels around the frame. Anchoring at several points per side follows the same logic.
Intumescent seals
In normal conditions the seal is an unremarkable strip. Under heat it swells and closes the gap between leaf and frame. It is the main control on smoke passage, and the component most easily compromised.
- It must run unbroken; a seal torn at a corner leaves a gap exactly there.
- A seal channel filled with site paint cannot swell. This is the most common refurbishment damage we see.
- Hardened, worn or displaced seals are replaced. The door still shuts, but smoke now passes.
Glazing
Adding glass helps supervision and daylight but complicates fire behaviour. Ordinary glass fails in the first thermal shock. Fire-rated glass retains integrity for a defined period and is assessed with its frame and gasket as one detail, and glazed area matters as much as glass type. Glazed models in our fire doors range have pane size and framing fixed at order stage, because cutting an aperture later disrupts the leaf.
A door standing open has no resistance
The sentence sounds blunt and it is accurate. A fire resistant door held open is just a frame. The closer is therefore part of the performance calculation, and its job is to drive the leaf into the latch — not near it. A setting that releases in the last centimetre leaves the door almost closed, and smoke uses exactly that gap.
Propping doors with wedges, buckets or wire is the most common site violation. Where a door must stand open, the answer is an alarm-linked electromagnetic hold-open that releases on activation.
Compared with an ordinary steel door
The two aim at different things: one at forced entry, insulation and appearance, the other at core material, seal behaviour, closer performance and installation detail. Neither substitutes for the other, so specify them separately — lever, closer and lock decisions on building entrance doors should align with the escape route.
Five minutes a year
- Open the door by hand and release it — does the closer drive it into the latch?
- Is the perimeter gap narrow and even?
- Is the seal unbroken, with no torn or painted sections?
- Is the door propped open with anything?
- Has anything been drilled through the leaf for signage or cabling?
Which resistance period and hardware configuration you need comes from the building design. Send quantities, opening sizes in centimetres, escape direction and closer requirements per opening, and we will quote the assemblies with shipping from Istanbul priced separately.
