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Flue pipe clearances - heat shielding - UK stove building regs J

In Document J, pages 32 and 33

Single skin flue pipe - distance to combustibles

Single skin, uninsulated flue pipe must be at least three times its diameter (OD) from combustible materials. Combustible materials must therefore not be within the grey shaded area shown in the diagram below.

diagram of single skin flue pipe clearances to combustible materials

For example a 125 mm single skin flue pipe must be at least 3 x 125 mm = 375 mm from combustible materials (being pedantic the actual figure is a little higher as 125 mm refers to the internal diameter not outside diameter).

Heat shielding combustible materials

The combustible material can be heat shielded. The flue must be at least 1.5 times its external diameter away from the heat shield, the heat shield must extend at least 1.5 times the flue's external diameter to each side of the flue, and there then has to be an air gap of at least 12 mm between the shielding material and the combustible material.
Practically this can be achieved using 12 mm fireboard. Strips of cut fireboard are used as spacers behind to achieve the 12 mm air gap.

flue pipe distance combustibles heat shielding diagram