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Undercut. What’s That?

Simply put, the undercut is the gap between the underside of the door leaf the top of the floor covering.

Does it matter what size it is?

Again, simply put, yes.

Fire resistance testing is in accordance with BS 476: Part 22: 1987. For fire resistance applications, door gaps and alignment tolerances must fall within the permitting range.

Threshold 10mm between bottom of leaf & top of floor covering.

During manufacture the undercut is commonly measured as the bottom of the door leaf to the bottom of the doorframe, with the presumption that the doorframe will sit on the floor slab and not the final floor finish. Therefore, as a default measurement the undercut is 10mm.

However, doorset schedulers may consider floor finished thickness and amend this dimension to suit thicker floor finishes such as ceramic tiles.

What remains important to achieve a fully certified installed doorsets is that the gap between the bottom of the leaf and the top of the floor covering does not exceed 10mm.

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Well, there is always a but!

If the doorset design is required to provide a smoke control function to comply with Building Regulations, the doorset must meet one of the following criteria:

(a) have a leakage rate not exceeding 3m3/m/hour (head and jambs only) when tested

at 25Pa under BS 476 Fire tests on building materials and structures, Section 31.1


(b) meet the additional classification requirement of Sa when tested to BS EN 1634-

3: 2004 – Fire resistance tests for door and shutter assemblies, Part 3 – Smoke

control doors.

Smoke seals or combined intumescent & smoke seals that are fitted to the door to achieve the performance requirements specified above, must have been tested in

accordance with the associated test method. Providing the smoke seal seals, any interruptions, the doorset will comply with current smoke control legislation under

Approved Document B; and a suffix ‘S’ or ‘Sa’, as appropriate, may be added to the designation.

Any other components installed where smoke leakage may occur must also be taken into account.

However, there is other guidance available, including BS EN 9999-2017 – Code of practice for fire safety in the design, management and use of buildings, which may

impose different or additional requirements, such as consideration of the gap between door leaf and threshold. In this instance and now practiced, the gap between the bottom of the door leaf and top of the floor finish must not exceed 3mm.

No threshold sealing image

3mm OK, no problem?

Well for manufacturers we are not interested in the undercut size. It is just a dimension we work to during manufacture. However, for installers it can cause us tremendous installation issues, none of which are within our control.


Well, it’s back to tolerance issues (again).

Doorset manufacturers and installers work to incredibly small, permitted tolerances, on many occasions +/-1mm. However, partition companies and floor finishes have greater leniency in margin.

Imagine, if you will, a doorset fitted perfectly vertical and square within an opening. With a typical door leaf measuring over 900mm wide. The permitted gap between the bottom of the leaf and the top of the floor finish is 3mm. However, the floor finisher is permitted a 3mm tolerance over a 1000mm distance. His floor therefore may legitimately increase or decrease the undercut below the door leaf by nearly 3mm. 4mm undercut? Fire certification ~ void.

But that gets worse.

Assuming the closed position is corrected. The door leaf needs to open. If the floor level increases in level within the arc of the leaf, the door may not open. The leaf cannot be trimmed to open because the gap will be too large when it is closed again. Fire Certification ~ void.

Is there a solution?

Yes and it’s not a costly one.

The simple application of a tested drop down (automatic threshold) seal will allow the manufacturer to increase the undercut to 10mm. The seal, when the doorset is closed, will drop to seal the undercut space. When the door is open the seal will raise.

drop down seal image

Responsibility for the appropriate smoke sealing specification and performance of thevdoors should be agreed between the relevant parties (i.e. specifier, manufacturer, contractor) prior to commencing manufacture or installation. The incorrect specification and fitting of smoke seals may impair the operation of a doorset and therefore compromise the fire resistance performance. Advice should be sought from the seal manufacturers regarding the correct specification and installation of smoke seals or combined smoke and intumescent seals.


RW Joinery are both certified timber performance fire doorset manufacturers and certified installers and has significant experience in the treatment of fire stopping between doorset and structural openings from both the certified manufacture of fire doorsets and the certified installation of the same.

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