No, this is a printing error. The word “over” should be retained as for the 2000 edition of Approved Document Part B, and so paragraph 9.12 of Volume 2 of Approved Document Part B 2006 should read: “Where the concealed space is over an undivided area which exceeds 40m (this may be in both directions on plan) there is no limit to the size of the cavity if…….”.

Cavity barriers are provided to reduce the risk from unseen fire spread within concealed spaces in accordance with requirement B3(4) of the Building Regulations. The principle of paragraph 9.12 is that it relates to a concealed space over a single undivided room (such as an open-plan office). Because the room below the space is undivided, the occupants will be able to see a fire develop and react to the changing hazard, thus cavity barriers within the space above are less important than with a cellular layout. The conditions in paragraph 9.12 are intended to reduce the risk of a fire starting/spreading in the undivided void and to prevent fire entering the void from outside the room which is not visible to its occupants.

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