(1) Roof-slates shall be of impermeable fine-grained rock, free from knots and knurls, and of such nature and so located that the durability or weatherproofing of the roof is not adversely affected thereby.

(2) Roof-slates shall, in addition, meet the following general requirements-

a) every slate shall be of uniform thickness throughout, which shall be not less than-

(i) 5 millimetres for all sizes not larger than 300 x 250 millimetres;

(ii) 6 millimetres for all sizes larger than 300 x 250 milimetres;

b) slates shall have a water absorption not exceeding 0,8 per centum of their dry mass, when subjected to the test set out in the First Schedule;

c) the width of each slate shall be at least half its length;

d) for slates laid on open battens, the roof slope shall be not less than 30 degrees, except as provided for in paragraph (e), and the sizes of slate used shall comply with those given in Table V for the appropriate roof-slope;

TABLE V

e) where additional weatherproofing to the satisfaction of the local authority, the roof-slope may be less than 30 degrees, and in that case shall be not less than 20 degrees.