Fire door gap

[KevinD]

Dear All,

Please could you confirm the following regarding the maximum gap at the bottom of fire doors. 

Fire doors have just been fitted to what should be a protected refuge area.  However, the gap at the bottom of the fire doors is large enough to get my fingers underneath even though the flooring (carpet) has already been fitted.  This makes the gap between the carpet and doors around 10-15mm.

Please could you confirm (or otherwise!) my understanding that the maximim 8-10mm gap usually cited as permissible at the bottom of the fire door should be to allow for the carpet (i.e. it should be 8-10mm max from bottom of the door to the surface onto which the carpet etc is laid).  From the bottom of the door to the concrete I would guess it is more like 15-18mm
Thanks in advance

[Tom Sutton]

You need BS 8214:2008 which says "Under-door (threshold) gaps should be in accordance with the manufacturer’s installation instructions for the particular doorset design". Previous version BS 8214:1990 stated a maximum of 8mm providing cold smoke seals were fitted, 3mm if not.

[KevinD]

Thanks Tom, it sounds like I need to go back to the manufacturer in that case if the BS no longer contains this information.
Regards

[CivvyFSO]

From TRADA:

The recommended leaf edge to frame gap specified in BS 8214: 1990 is 4mm on all edges. Intumescent seals are engineered to react within this size of gap to give optimum sealing and clamping performance.

It is through extensive testing of fire rated doorsets, together with knowledge of the pressure regime within a fire test, that a larger gap is permitted at the threshold of the door. There is negative pressure at the threshold during test conditions and so cool air is drawn in underneath the door. It is for this reason that there is unlikely to be a failure at the threshold and also why there is no need to fit a perimeter intumescent strip at this location (doorsets below FD60 performance). A larger gap at the threshold is also useful from a practical end use application for accommodating floor coverings. We recommend a gap of 10mm from the bottom of the leaf to the structural surround.

[KevinD]

Thanks Civvy, that's very useful indeed.
Regards

[Tom Sutton]

From TRADA:

We recommend a gap of 10mm from the bottom of the leaf to the structural surround.[/i]

I think TRADA is a little out of date the "BS 8214:1990 Incorporating Amendment No. 1" states 8mm if a cold smoke seal is fitted 3mm if not also the 1990 version has been updated again in 2008.

[Phoenix]

Irrespective of what the guidance says, why not think about what is likely to happen under fire conditions.  This is what the guidance is based  on.  If the fire has no ventilation (for example, if the window hasn't broken) then it will be sucking in all the air it can from everywhere it can (i.e. under the door) but the neutral plane will be descending in the room as fire pressures build up.  The smoke layer will also be descending.  If the fire does not ventilate itself (for example, by breaking the window) then combustion will decrease rapidly as oxygen runs out (you can only get so much under a door - even with a 15mm or so gap).  The neutral plane will be right down and some small amounts of smoke will come out the bottom of the door.  Not a great problem.

What about if the fire is ventilated (open window) or ventilates itself (breaks a window)?  Then we have a different situation altogether.  Flashover is likely.  The gap under the door is much more vulnerable if the room is flashing over.  The fire's not concerned in trying to draw miniscule amounts of air under the door, it has plenty of air from the window.  It will pump out hot gases under the door as the whole space will be subject to slightly higher pressures than ambient.  Not good.

Most rooms have potential ventilation (windows) so the best advice is to keep that gap as small as possible.

Stu

[Tom Sutton]

"the best advice is to keep that gap as small as possible."

Stu that's what BS 8214 states it simple puts on limits on the size of the gaps it also appears to be more concerned with cold smoke control, with regards to the threshold gap.

[KevinD]

Apologies all to labour this (and Stu I take on board what you say) but perhaps could I just simplify with a question and scenario as it seems the BS guide no longer applies and TRADA is out of of date (this seems to be what is being implied in this thread?).

If it is accepted that a refuge (in which 2-3 wheelchair users may need to be waiting as they are evacuated by trained staff using evac chair - not ideal I know) should be protected by at least a FD30S door (so protecting from fire and all smoke including cold), in your opinions, is it acceptable to have a fire door with a gap of 15mm and no smoke seals along the underside/threshold?  To me the immediate answer would be no.

Regards

[kurnal]

But on the other hand a refuge is likely to be approached from a corridor or lobby  and is unlikely to have doors to rooms opening directly into it?

A fire is unlikely to develop to flashover conditions in the corridor, though pressure differentials  and variations  are likely to be greater between the rooms, corridors and the staircase due to wind and other effects.

Best advice is as per BS8214 but theres nothing wrong with a qualitative judgement based on the circumstances of the case.

Perhaps if there is concern - eg rooms opening  directly into a stairwell used as a refuge we should also question whether this is an appropriate location for a refuge.

[KevinD]

Thanks Kurnal.  I understand the issue re the refuge and I'm not especially happy about it myself but I just wanted to understand:

If you'd specified a FD30S door as it was needed to protected a route/refuge etc and when fitted it had a 15mm+ gap and no smoke seals along the threshold, would you accept this?

Thanks

[kurnal]

If it was a new fit I most likely would not accept it. If it was an old door in an existing building I may or may not accept it depending on the circumstances.

[Tom Sutton]

If you'd specified a FD30S door as it was needed to protected a route/refuge etc and when fitted it had a 15mm+ gap and no smoke seals along the threshold, would you accept this?

If I had to defend in court a FD30s fire door with 15mm threshold gap then I would be pushed to say in what circumstance I would accept it. However 8 to 10mm I would be be happy using previous version of BS 8214 to justify my decision.

[CivvyFSO]

Just a slightly different take on it:

If you haven't managed to get a person out of the refuge prior to smoke is coming out of the bottom of the door you have more problems than your doors.