23. In any relevant work—
(a)the materials used shall—
(i)be of a suitable nature and quality in relation to the purposes for and the conditions in which they are used;
(ii)be adequately mixed and prepared;
(iii)be applied, used or fixed so as adequately to perform the functions for which they are designed; and
(iv)not continue to emit any harmful substance longer than is reasonable in the circumstances; and
(b)the standards of materials and workmanship need be no more than are necessary to—
(i)secure the health, safety, welfare and convenience of persons in or about the building; and
(ii)further the conservation of fuel and power.