Technical Booklet E Fire Safety·Page 126·Diagram 4.12
Fire resistance requirements for flues in compartment walls and floors
Flue walls passing through compartment floors or built into compartment walls must have a fire resistance of at least one half that required for the compartment wall or floor, and must be of non-combustible construction.
In all cases flue walls should have a fire resistance of at least one half of that required for the compartment wall or compartment floor and be of non-combustible construction.
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