Open Roof Voids

[SeekingAdvice]

Hi

During recent investigation in roof voids above flats it has become apparent the roof void compartmentation does not mirror the flats and corridors below. Instead there are shared open roof voids covering 3 - 4 flats. This is a relatively new build and have been advised that this is accepted construction practice, assuming there is managed access to the roof void, detection in place and the ceilings are double boarded. I understand that new build housing schemes are now been constructed with completely open roof voids if the measures mentioned are in place. This does seem a bit conflicting with the guidance documents I have reviewed.

Any advice would be welcomed :)

[Tom Sutton]

Does it meet the required standards for B3 section 8 compartmentation of ADB, I do not think so it looks like the architects have come up with an alternative. 

I had a block of flats and the ceiling to the top eight flats was a concrete slab with access, from the two staircase enclosures. I lost the roof but there was no fire damage to the flats only a small amount of water damage, so I can see their reasoning but would double boarding achieve anything near that standard or what is required by ADB, I doubt it?

What's being said on other forums, are we seeing the standards required by some BCO/AI these days.

[AnthonyB]

Fire resisting ceilings (assuming boarded rather than a concrete slab) cannot usually prevent the downward spread of fire so should not normally be accepted, but despite what is likely to come out of the Grenfell Enquiry AI's and BCO's continue to play fast and loose with the standards despite not automatically having to be being fire experts and having instead to understand every single aspect of the building regulations.

[SeekingAdvice]

Thank you both for your help on this. Would you happen to know if there is any research information on the downward spread of fire in buildings ( I am likely to get challenged on this, once I present this information :)

I have reviewed B3 section 8 compartmentation of ADB and can not see any reference to anything other than full height roof void compartmentation that mirrors flats and communal fire doors. Given that many new developments do not adopt this approach there must be something that they are hinging this design on and if not do you have any views from the developer point of view?

Kind regards

[Tom Sutton]

I think it is sufficient to know that fire will spread upwards, sidewards and downwards, fastest upwards, slower sidewards, even slower downwards but will spread in all directions depending on the construction. If you want a more scientific approach then google "research on downwards spread of fire" I am sure someone will have researched it.

For a new flats development, an architect would use ADB or BS 9991 or an engineered solution, I know ADB or BS 9991 requires 60 mins separation between every dwelling which I think would not be achieved by a double layer of plasterboard, but I would need to research that.

[Tridentfirepl]

It depends a lot in the construction of the ceiling, provision of compartment walls and cavity barrier's. In a roof void cavity barrier's should be installed directly in line with any compartment walls below, but if the ceiling is fire rated the cavity barrier's are not required to be inline with the walls below and are used to reduce the roof space to maximum 20m in length. The issue is that a fire rated ceiling is normally only tested from below for fire resistance and not from above, so a fire in a roof void could compromise the ceiling in less than the required time of fire resistance and if the dividing walls in the roof space do not align with the walls below 2 or more flats may be compromised by the fire. This would render a stay out policy ineffective. 

I would expect to see solid block or brick compartment walls separating each flat in the roof space fire stopped to the head as per ADB with an intermediate cavity barrier installed if the distance exceeds 20m between both compartment walls.