Flats

[col10]

How do people feel about the OV in the stairway for small buildings recommended in diag 9 in ADB being omitted,   bearing in mind that B5, access for fire service does not apply and people are supposed to escape before the fire service arrive.  Many of these are conversions and the 1m2 OV cant always be fitted in.

[Jim Scott]

Are talking existing buildings?

[col10]

Conversions and new build, but sometimes the conversions are more difficult, due to the layout having to be planned into an existing building.

[Jim Scott]

If they are new build, why would we even consider not providing an OV?

Why do you think that B5 doesn't apply also?

[col10]

Because the aim is to provide a building which, before the fire service arrive,  people can escape from, and the OV is  meant  for fire service use and not meant for the use of people escaping from the building.

The fire fighting shafts in B5 are for buildings with a floor over 18m.

[kurnal]

Functional requirement B5 applies to all buildings, the extent of the provision will depend on the use, size and height of the building. eg firefighting shafts are only a requirement in high blocks of flats. For small buildings with a stay put evacuation strategy the OV is intended to enable firefighters to maintain safe conditions in the staircase fore firefighting operations and, should it be necessary, to enable the safe evacuation of residents if the fire develops to an unexpected extent.

[wee brian]

Its a B1 requirement not B5.

Who says they are all out before the brigade get there?

[Jim Scott]

Compartmentation and ventilation are the most important factors within flats, particularly with a stay put strategy in place, commensurate with ADB.

I really struggle to understand how any justification could be sought to reduce this requirement in a new build or newly converted building.  I appreciate that there are many exsisting that do not comply, although that is another matter.

As Wee Brian indicates, the ventilation requirement is a B1 concern, although providing additional benefits to B5.

[col10]

If it's meant for fire service use then it should be in B5, not B1.

[kurnal]

Not necessarily- the assumption is that persons will stay put in their flats, the fire service will attend and make a judgement as to any need for further actions, which may include evacuation of the other flats. To do so safely the staircase has to be clear of smoke. Therefore it falls under B1 and B5. B1 comes first cos that's the order of things.

[wee brian]

ADB uses OVs for larger buildings too, again the expectation is that the fire service will operate them but their purpose is to assist with the evacuation of those residents that have "stayed put" if necessary. Hence its in B1.

[col10]

WB.  Where, in ADB, are the OVs for larger buildings?

[kurnal]

The provisions for the very small buildings in the diagrams you refer to are a relaxation of the far more onerous requirements for larger buildings which incorporate ventilated lobbies and AOVs in both lobbies and staircase. This is solely to protect the staircase and is detailed with diagram 7 and 8 and  the text  para 2.26 of the ADB. The measures are therefore on a sliding scale. In any case an AOV is a much more onerous requirement than an OV. The diagrams you refer to simply illustrate the special provisions for very small blocks. In all cases the whole principle is to keep the staircase free of smoke for the reasons referred to earlier.

[wee brian]

ADB uses OVs for multiple stair buildings.

[LeeM]

The OV is obviously only of use when the fire service open it, until then you are relying on the S rated doorset to protect the stair. It will do so for a reasonable period of time. However (and as found by the BRE, BD2410 report) a 1sqm vent will not protect the stair when the flat door is open for anything length of time. Therefore, and appreciate fire fighters don't read these reports, small single stair buildings should be evacuated by the FRS before fire-fighting, or they should hope the wind is blowing in the right direction and the door at the base creates a stack flow effect increasing the extract potential of the roof vent, in which case it is possible to have a clear stair below the top storey.
If you cant get 1sqm OV then at least get one AOV top and bottom which will achieve same extract potential, as its B1 not B5.