Smoke shafts are a crucial part of smoke ventilation systems, and a question we frequently receive is: what should the fire rating of a smoke shaft be? Documentation is the first place to look.
Starting with the fire rating of the materials that smoke shafts are made from, Approved Document B section 3.51 part 3 states:
“The smoke shaft should be constructed from a class A1 material.” With Class A1 materials defined as “Materials that cannot contribute to a fire at any stage, including a fully developed fire, can be awarded Class A1”.
The SCA Guide aligns with Approved Document B’s sentiments, stating in section 6.4.1:
Smoke shafts should be constructed of non-combustible material.
BS 9991, Section H, part 184.108.40.206, elaborates on these definitions with:
The smoke shaft should be constructed either of non-combustible materials conforming to BS 476-4 or of any material which, when tested in accordance with BS 476-11, does not flame or cause any rise in the temperature on either the centre of the specimen or the furnace thermocouples.
These documents offer conclusive evidence that smoke shafts must not add to the effects of a fire in any way and must be constructed out of materials which meet the fire rating of Class A1. Class A1 materials are materials which have been given a ‘non-combustible’ seal of approval and can vary from concrete to A1 rated plasterboard among many other fire resistant materials.
With the combustibility of the material a smoke shaft should be made from now confirmed, required fire resistance needs to be understood. Approved Document B defines fire resistance by saying it is:
Measured in minutes. This relates to time elapsed in a standard test and should not be confused with real time.
Revisiting section 6.4.1 of the SCA Guide, it states:
The fire resistance of the shaft should be equivalent to the requirements of the elements of the structure through which it passes.
The guide then goes on to say:
The fire rating of the vent should be equivalent to that of the smoke shaft or at least 60 minutes. This is to ensure that the fire resistance of the smoke shaft is maintained when the vent on the floor of fire origin is in the open position.
Table 4 from BS 9991 offers guidance on the fire resistance of elements of structure, e.g. floors. For buildings of 5m, it requires 30 minutes, for buildings between 5 and 18 metres, 60 minutes, for those over 18 metres, 60 minutes and for those over 30 metres, 120 minutes. This concludes that the smoke shaft should either be 60 or 120 minute fire rated, depending on the height of the building.
Hopefully this answers the question of the fire rating of a smoke shaft, if you have any further knowledge on this topic please don’t hesitate to contact us and speak to our friendly team who will be happy to talk you through the options available. If you’d like to continue learning about smoke ventilation, you’ll find some helpful resources on our blog and YouTube channel.