Fire door gaps

[RRR]

What is the recommended gap for a notional FD30S timber doorset at the door to floor, there are fire and smoke seal on the leading edge,top and hinge side but nothing at the bottom. The gap at the bottom is 12mm, what is the regulation on this type of door.

[Neil ashdown]

BS 8214: 2016 Timber fire door assemblies: Code of practice states that, for doors that must restrict the spread of cold smoke, the threshold gap should be no greater than 3mm at any point. 

A 12mm gap is likely to be too large even for a fire door without the restricted spread of cold smoke requirement.  You will likely need to install a threshold plate to the floor in remediation. 

[James Gormley]

Hi,I'm supervising on a 20 story apartment block built 10 years ago.  Reducing the gaps head and sides 3mm + or - 1mm so 4mm max.

There are 450 communal doors to have remedial work done to them,  we are reducing the gaps, changing all hinges, changing the intumisent smoke seals, batt and mastic between frame and structure.  The threshold gaps are very bad, we have been told that the maximum permitted threshold gap is to be 10mm. The average gaps is 15mm with the worst 27mm.

I have been told that the manufacturer s certificate will not allow hardwood edging strips to be glued and screwed to the bottom of the door.  I suggest an 8mm metal threshold strip screwed to the carpets.  Was told this would cause a trip hazard. 

What would be the correct solution

[Neil ashdown]

For this type of door, the maximum threshold gap should be 3mm because building regulations require the fire resisting doors in common areas of blocks of flats to provide restricted spread of cold smoke.  Refer to BS 8214:2016 Timber-based fire door assemblies - Code of practice, section 12.3.

The use of a suitable threshold plate will certainly make it much easier to achieve this gap and Building Regulations Part M compliant ones are available.  However, if a hazard assessment has ruled them out it will be necessary to fix a hardwood lipping to the door leaf bottom edge and/or fit a drop-down or static threshold seal. Survey each door to see which remedy is most suitable for the particular door.

Whether or not a timber-based fire door leaf may be fitted with a hardwood lipping will depend on the construction type of that particular fire door and possibly whether the original installer over-trimmed the door leaf when they fitted it. 

When resolving edge and threshold gap issues, it is sometimes necessary to remove the complete door and frame assembly and re-install to the correct tolerances. The gap between the door frame and the surrounding wall substrate should be within the scope of the requirements set out in the document BS 8214:2016 Timber-based fire door assemblies - Code of practice, section 9.

[Mike North]

the door should resist passages smoke to a given performance standard when pressurised from one side at “ambient” temperature to limit the amount of cold smoke passing from one side to the other. The requirements specifically exclude any performs for the threshold, the key point is that ADB only requires the head and jambs to have a performance form standard in respect to cold smoke leakage. When pressurisation is being used in a building as part of the active fire safety provisions then the requirement for smokes seals on the threshold of doors is increase and it is these conditions word threshold sealing would need to be considered

So long as there is no pressurisation present, the only statutory requirement is for head and jambs to resist the passage of cold smoke  

[Neil ashdown]

Fire fire doors, with regard to restricted smoke spread the excerpt above is from building regulations in England & Wales Approved Document B, Table C1.  Looking at 2a it can be seen that smoke leakage is allowed but not exceeding the stated rate, from the head and jambs only. There is no mention of the threshold gap.

Section 12.3 at BS 8214:2016 provide specific guidance about the gap at the threshold and the following information from www.ifsa.org.uk may be useful: 

"Because it is perceived to be difficult to achieve a seal at the threshold, the gap at the bottom of the door is exempted from the leakage criterion in Approved Document B. As the gap at the bottom of the door is straight through and is invariably wider than elsewhere in the assembly, then if the smoke is truly mixed and at an even pressure and temperature, a greater proportion of the smoke is likely to be egressing from this gap than any of the others. The pragmatic decision to ignore the leakage from the bottom assumes that there will be some stratification of the gases and, therefore, less polluted air will be present nearer to the floor level. This cannot be guaranteed and so leakage through the threshold gap would lead to rapid smoke logging of the protected space. There are, therefore, real benefits to be achieved if the threshold is sealed to satisfy similar levels of performance. This is something that has been recognised in BS 9999 and BS 9991, where it is recommended that for an adequately smoke sealed door the threshold is sealed with a drop down seal, or one that makes continuous contact with the floor, or where this not practicable that the gap at the bottom of the door is controlled to 3mm".

 

 

[Mike North]

So long as there is no pressurisation present, the only statutory requirement is for head and jambs to resist the passage of cold smoke