When installing a flue liner is there a minimum high from the roof that it must protrude using a macflash or similar.
The existing chimneyhead was removed below roof hight and would need to have steel upright inserted out through the tiles/slates.
As Welsh writes it must rise at least one metre above the highest part of the roof. In any event the chimney must extend more than 16 feet above the highest opening in the fire.
Less than 16 feet and you will have problems with smoke.