§ 52. Stairs. 1. In every multiple dwelling erected after April eighteenth, nineteen hundred twenty-nine, every interior stair, fire-stair and fire-tower and every exterior stair in connection with any dwelling altered or erected after January first, nineteen hundred fifty-one, shall be provided with proper balustrades or railings and all such interior and exterior stairs shall be kept in good repair and free from any encumbrance. Every such stair, fire-stair and fire-tower more than three feet eight inches wide shall be provided with a handrail on each side.

2. The upper surface of every balustrade or railing placed in any stair after April eighteenth, nineteen hundred twenty-nine, shall be at least two feet six inches and at most two feet eight inches above the front edge of the stair treads, and at any stair landing shall be at least two feet eight inches and at most three feet above the level of such landing.

3. The treads and risers of every stair, fire-stair and fire-tower constructed after April eighteenth, nineteen hundred twenty-nine, in any multiple dwelling shall be of uniform height and width in any one flight. Each tread, exclusive of nosing, shall be not less than nine and one-half inches wide; each riser shall not exceed seven and three-quarters 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 most seventy-five.

4. No winding stairs shall be constructed in any multiple dwelling.

5. a. Except as otherwise provided in paragraph b of this subdivision, every stair constructed after April eighteenth, nineteen hundred twenty-nine, leading to a cellar or basement from the first story above shall be entirely enclosed with fireproof walls and be provided with fireproof doors and assemblies at both top and bottom, with the doors self-closing; except that, in a non-fireproof multiple dwelling erected before such date, where such a stair is permitted such enclosing walls may be fire-retarded.

b. When the first floor or a part thereof, in a fireproof multiple dwelling, is used for business purposes, a stair leading to a cellar or basement from such business space shall be enclosed in fireproof walls having a fire-resistive rating of at least three hours and be provided with a fireproof door and assembly at the bottom, with the door self-closing. No opening shall be permitted between such business space and the remainder of the dwelling.

6. The department shall have the power to make supplementary regulations relating to fire-towers.

7. In every multiple dwelling erected under plans filed with the department after January first, nineteen hundred sixty, on every story above the entrance story every door opening into such stair shall be so hung and arranged that in opening and when opened it shall at no point reduce the clear and unobstructed required width of the stair or stair landing.

8. The provisions of this section shall not apply to a stair within an apartment provided that each level of the apartment is provided with required means of egress complying with the provisions of this article.