(1) Every external masonry or concrete wall of a habitable portion of a building shall be capable of withstanding the artificial rain test described in the Second Schedule and shall be-

a) constructed of masonry building-units so as to be at least 200 millimetres thick; or

b) constructed of plain normal concrete at least 200 millimetres thick or reinforced concrete at least 100 millimetres thick, and be treated if necessary; or

c) constructed of no-fines concrete at least 200 millimetres thick; or

d) of such lesser thickness as is permitted by this Chapter for the relevant conditions.

(2) Every external wall of a habitable portion of a building which is not of masonry or concrete shall be weatherproof to the satisfaction of the local authority.