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Floor zone fire protection in houses

Steve A

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I am buying a new small two bedroom mid terrace house from a mass house builder. The floor construction is timber I beams. These span the whole width of the property and are at about 600 centres.

I am concerned about the spread of flame through the floor void, as although it is compartmented into 600 widths, there are numerous holes for ducts and services. I understand the integrity of the floor needs to be 30minutes for escape purposes, but a fire once in the void assisted by the battening out of the plaster board on both the ground and first floor could fan the flames and lead to collapse in a short period of time. Surely there should be fire stops at the floor level and preferably a fire blanket or two to reduce the spread of flames? Steve A

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This is the normal construction of any private dwelling house, the upper floor should achieve 30 mins from the underside according to ADB and 12 mm plaster board slimmed should achieve this. If the premises has been built to all the necessary standards then a fire starting in the floor void would be consider low risk. 

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 Timber I beams. My concern with this type of construction is that all the reliance is in the integrity of the plasterboard and effective fire stopping.

With traditional solid joist construction the voids in the floor are small and impede the spread of flames. This type of construction allows the spread of flames to envelop the whole floor area. There are numerous openings in the sterling board webs, which would burn very quickly. The plastic central heating pipes pass through the webs as do all the electrics. The services run in the voids from the ground floor to the floor zone, and then on to the first floor and then to the roof space.  I notice that the building inspector has requested a fire collar round the vents from the extract fans and at the floor levels.

The open vents for the extract fans in the ceiling and plastic ducting within the floor zones offer no resistance to a fire and the gaps between the walls and the rear of the plasterboard which link the lower reaches of the ground floor with the floor zone and the complete height of the first floor to the roof space, offer and excellent draw to feed flames and accelerate the fire causing collapse. The 30 minute plasterboard protection is therefore meaningless. In fact if the fire was predominantly in voids or floor zone, the protection offered by the plasterboard may conceal the fire and its exact location way past a time it would still be safe to be in the building.

I suppose my main concern with this type of construction is that more emphasis should be placed on the fire stopping than the plasterboard. The plasterboard can be seen, I doubt whether the majority of owners would know if the fire stopping had been added or where to look for it.

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