In the case of wooden ladders the employer shall ensure that -
(a) the ladders are constructed of straight grained wood, free from defects, and with the grain running in the length of the stiles and rungs; and
(b) the ladders are not painted or covered in any manner, unless it has been established that there are no cracks or other inherent weaknesses: Provided that ladders may be treated with oil or covered with clear varnish or wood preservative.