§ 132. Stairs. Every interior and exterior stair and fire-stair shall be provided with proper balustrades and handrails. Every such stair and fire-stair three feet eight inches or more in width shall be provided with a handrail on each side. Each tread, exclusive of nosing, shall be not less than nine and one-half inches wide; each riser shall not exceed seven and three-quarter inches in height; and the product of the number of inches in the width of the tread and the number of inches in the height of the riser shall be at least seventy and at the most seventy-five. Winding stairs shall be unlawful in any public part of the dwelling.