If the nature of the timber is such that uprights are necessary in order to comply with regulation 12, uprights shall be fitted which shall be-
(a) of sufficient strength for the purpose;
(b) secured in position by angles or metal sockets of sufficient strength for the purpose or by equivalent means; and
(c) so spaced as to provide efficient support taking into account the nature and length of the timber, but subject to the space between any 2 uprights fore and aft not exceeding 3 m.