(1) Except as permitted by Sentences (2) and 3.1.10.3. (2), a firewall shall extend above the roof surface to form a parapet not less than,
(a) 150 mm high for a firewall required to have a fire-resistance rating not less than 2 h, and
(b) 900 mm high for a firewall required to have a fire-resistance rating not less than 4 h.
(2) A firewall that separates two buildings with roofs at different elevations need not extend above the upper roof surface to form a parapet, provided the difference in elevation between the roofs is more than 3 m.