TGD J 2014 – 3.9.3 – Wall thickness:

3.9.3 Wall thickness: The wall thickness of a brick or blockwork chimney or flue block chimney should be at least 25 mm. Any chimney wall which: (a) is part of the wall of a compartment of the same building, another building or another dwelling, should give at least the fire resistance needed for the compartment or separating wall (see Technical Guidance Document B – Fire Safety), or (b) passes through a compartment wall or a compartment floor, should have at least half the fire resistance needed for the compartment wall or floor (see Technical Guidance Document B – Fire Safety). If the compartment wall or floor is masonry material, it may also form the chimney wall.

C:BCARTGDtgd jTGD%20J%202014TGD%20J%202014%20-%20SECTION%203.9.3.pdf