N.Y. ADC. LAW § 27-375 : NY Code - Section 27-375: Interior stairs

Interior  stairs  shall  comply with the
  following requirements:
    (a) Capacity. The capacity of interior stairs shall be  as  listed  in
  table 6-1.
    (b)  Width.  The  width  of  interior  stairs shall be the clear width
  between walls, grilles, guards, or  newel  posts.  Stair  stringers  may
  project into the required width not more than two inches on each side of
  the stair.  No interior stair shall be reduced in width in the direction
  of exit travel. Interior stairs shall be at least forty-four inches wide
  except as follows:
    (1)  Interior  stairs may be not less than thirty-six inches wide when
  serving not more than  thirty  occupants  per  stair  on  any  floor  in
  buildings  classified  in  occupancy  groups J-1 and J-2 or when serving
  buildings classified in occupancy group J-3 and exceeding  four  stories
  in  height,  or  when serving not more than sixty occupants per stair on
  any floor in buildings classified in occupancy groups E, B, and D.
    (2) Interior stairs may be not  less  than  thirty  inches  wide  when
  serving  mezzanines  having  an  occupant load not exceeding twenty-five
  persons or when located in buildings classified in occupancy  group  J-3
  not  more  than  three  stories in height. Interior stairs in four story
  buildings classified in occupancy  group  J-3  shall  be  a  minimum  of
  thirty-three inches in width.
    (c)  Headroom. The clear headroom shall be at least seven feet, except
  that in buildings classified  in  occupancy  groups  J-2  and  J-3,  the
  minimum  clear  headroom  may  be  six  feet eight inches. Headroom in a
  flight of stairs shall  be  measured  vertically  between  two  parallel
  inclined  planes,  one of which contains the line of the nosing or upper
  front edge of each tread and extends to its intersection with a  landing
  and  the  other  of  which is through any point directly above the first
  plane that limits the headroom of the stair.
    (d) Landings and platforms. Landings and platforms shall  be  provided
  at  the  head  and  foot of each flight of stairs, except at the head of
  basement stairs in one- and two-family dwellings, and shall comply  with
  the following:
    (1)  The  minimum width of landings and platforms perpendicular to the
  direction of travel shall be equal to at least the width of  the  stairs
  except  that  on  a  straight-run  stair, the distance between risers of
  upper and lower flights at intermediate landings or platforms  need  not
  be more than forty-four inches.
    (2)  The  maximum  vertical  rise of a single flight of stairs between
  floors, between landings or platforms, or between a floor and a  landing
  or  platform  shall  not  exceed  eight  feet in buildings classified in
  occupancy groups F and H, and twelve feet in all other occupancy groups.
  No flight of stairs shall have less than two risers.
    (3) Landings and platforms  shall  be  enclosed  on  sides  by  walls,
  grilles or guards at least three feet high.
    (e)  Risers  and treads. Risers and treads shall comply with table 6-4
  and with the following:
    (1) The sum of two risers plus one tread exclusive of nosing shall  be
  not less than twenty-four nor more than twenty-five and one-half inches.
    (2)  Riser  height  and tread width shall be constant in any flight of
  stairs from story to story.
    (3) Winders shall not be permitted in required exit stairs  except  in
  one-  and  two-family dwellings and except as permitted in subdivision l
  of this section. The width  of  winder  treads  when  measured  eighteen
  inches  from  the  narrower  end shall be at least equal to the width of
  treads above or below the winding section.

    (4) Curving or skewed stairs may be used as exits when the  tread  and
  riser  relationship  is  in accordance with table 6-4 when measured at a
  point eighteen inches in from the narrow end of the tread; and no  tread
  shall  be  more  than three inches narrower or three inches wider at any
  point than the width established eighteen inches in from the narrow end.
    (f)  Guards and handrails. Stairs shall have walls, grilles, or guards
  at the sides and shall have handrails on both sides, except that  stairs
  less  than  forty-four inches wide may have a handrail on one side only.
  Handrails shall provide a finger clearance of one and  one-half  inches,
  and  shall  project  not  more  than  three and one-half inches into the
  required stair width.
    (1) Stairs more than eighty-eight inches wide shall have  intermediate
  handrails  dividing  the  stairway into widths that maintain the nominal
  multiples of twenty-two inches, but the widths shall not be greater than
  eighty-eight inches nor less than forty-four inches.
    (2) The height of handrails above the nosing of treads  shall  be  not
  more than thirty-four inches nor less than thirty inches.
    (3)  Handrails  shall  be returned to walls and posts when terminated,
  except in one- and two-family dwellings.
    (4) Handrails shall be designed to support loads  in  compliance  with
  the requirements of subchapter nine of this chapter.
    (5)   Handrails   in  all  stairs  shall  be  of  materials  having  a
  flame-spread rating not exceeding one hundred fifty.
    (g) Stair doors. Doors providing access to stairs  shall  comply  with
  the requirements of subdivision (a) of section 27-342 of article five of
  subchapter five of this chapter and subdivision (e) of section 27-371 of
  this  article.  The swing of stair doors shall not block stairs or stair
  landings, nor shall any door at  any  point  of  its  swing  reduce  the
  effective  width  of  the  landing  or  stair  to less than seventy-five
  percent of the required width of the landing or stair, or to  less  than
  the  width  of the door opening on them. The width of doors from a stair
  shall not be less than the number of units of exit  width  required  for
  the  capacity  of the stair, but in no case shall the door width be less
  than required by subdivision (e) of section 27-371 of this article.
    (h)  Stair  construction.   Risers,   treads,   stringers,   landings,
  platforms,  and  guards,  exclusive  of  handrails,  shall  be  built of
  noncombustible materials except that interior  stairs  in  buildings  of
  construction group II may be built of combustible materials in buildings
  classified  in occupancy group B-2, C, D or E when the buildings are two
  stories in height or less, and  in  buildings  classified  in  occupancy
  group  J-2  or J-3 when the buildings are not more than three stories in
  height, and in the case of J-2 occupancy group,  when  occupied  by  not
  more  than  three families. Interior stairs shall have solid treads. All
  risers shall be closed except as otherwise provided in  subdivision  (i)
  of  this  section.    When  of  combustible  construction, the soffit of
  interior stairs shall be fire protected by  material  having  a  minimum
  fire  resistive  rating  of  one hour or five-eighths inches gypsum wall
  board or equivalent, or the space  beneath  shall  be  enclosed  without
  openings  by  material  having  a one hour fire resistance rating unless
  permitted to have open risers by subdivision (i) of this section.  Where
  two separate interior stairs are contained within the same enclosure (so
  called  "scissor  stairs"), each stair shall be separated from the other
  by noncombustible construction having a fire resistance rating equal  to
  that required for the stair enclosure.
    Stairs, platforms, and landings shall be designed to support all loads
  in  compliance with the requirements of subchapter nine of this chapter.
  Treads  and  landings  shall  be  built  of  or  surfaced  with  nonskid
  materials.

    (i) Stair enclosures.
    (1) Interior stairs shall be enclosed with construction complying with
  the requirements of table 3-4 except that:
    (a)  In  buildings  three  stories  or  less in height excluding those
  classified in occupancy group J-1 or J-2 combustible construction  group
  II,  the  enclosing  construction  may  have  a  one hour fire resistant
  rating.
    (b) Stairs in buildings or spaces classified in  occupancy  group  J-3
  and  not  more than three stories in height, need not be enclosed except
  as otherwise required in subdivision (a) of section  27-341  of  article
  five  of subchapter five of this chapter. Stairs may have open risers in
  one family dwellings and group homes.
    (c) Unenclosed stairs in buildings classified  in  assembly  occupancy
  group  F  may  be  permitted  as  provided  in  subchapter eight of this
  chapter.
    (d) Stairs from floors or mezzanines may be unenclosed, with  open  or
  closed risers.
    (e)   In   buildings   classified  in  occupancy  group  J-2  occupied
  exclusively by not more than one family on each story without  boarders,
  roomers  or  lodgers  and  not  more  than  three stories in height, the
  enclosing construction may have a one hour fire-resistance rating  which
  may  be  constructed  of  combustible  material  provided that the stair
  enclosure is protected with an automatic sprinkler system complying with
  the construction provisions of subchapter seventeen of this chapter.
    (f) In buildings classified in occupancy group J-1  or  J-2  not  more
  than  two  stories  in  height of combustible construction group II, the
  enclosing construction may have a one hour fire-resistance rating  which
  may  be  constructed  of  combustible  material; however, where only one
  vertical exit  is  provided  the  stair  enclosure  shall  be  protected
  throughout  with an automatic sprinkler system constructed in accordance
  with the provisions of subchapter seventeen of this chapter.
    (g) Except as provided in subparagraphs  (a),  (e)  and  (f)  of  this
  paragraph,  in all buildings or spaces classified in occupancy group J-1
  or J-2, the enclosing construction shall be of masonry  or  an  approved
  equivalent material having at least a two hour fire-resistant rating.
    (2)  Access  stairs  connecting not more than two stories which do not
  serve as a required exit may be  constructed  without  an  enclosure  in
  buildings  classified  in  other  than  occupancy group H-2. Such stairs
  shall be additional to and shall not obstruct or interfere with required
  exit facilities. When the first  story  below  grade  is  served  by  an
  interior, unenclosed access stair, it shall be sprinklered in accordance
  with  the  construction  provisions  of  subchapter  seventeen  of  this
  chapter.
    (3) The interior finish of  interior  stair  enclosures  shall  be  in
  accordance with the requirements of table 5-4.
    (4)   Stair   enclosures  shall  be  vented  in  accordance  with  the
  requirements for shafts in subdivision (d) of section 27-344 of  article
  five of subchapter five of this chapter except that stair enclosures for
  buildings  or  spaces  classified in occupancy group J-1 or J-2 shall be
  vented as follows:
    a. In occupancy group J-2 buildings three stories in height  and  with
  not  more than one dwelling unit per story or two stories in height with
  not more than two dwelling units per story, shall  be  provided  with  a
  skylight at least nine square feet in area, glazed with plain glass with
  a  wire  screen  over  and  under  and  provided  with  fixed or movable
  ventilators having a minimum open area of forty square inches.
    b. In occupancy group J-1 or J-2 buildings two stories in height  with
  more than two dwelling units per story shall be provided with a skylight

  of  at least twenty square feet in area, glazed with plain glass, with a
  wire  screen  over  and  under  and  provided  with  fixed  or   movable
  ventilators having a minimum open area of forty square inches.
    c.  In  occupancy  group J-1 buildings exceeding two stories in height
  and in occupancy group J-2 buildings three stories in height  with  more
  than  one  dwelling  unit per story or exceeding three stories in height
  shall be provided with a skylight at least twenty square feet  in  area,
  glazed  with  plain glass with a wire screen over and under and provided
  with fixed or movable ventilators having a  minimum  open  area  of  one
  hundred   forty-four   square  inches.  In  lieu  of  the  skylight  and
  ventilators a window of equal area may be provided  with  fixed  louvres
  having  a  minimum  open  area  of  one hundred forty-four square inches
  installed in or immediately adjacent to the window.
    (5) When dwelling  units  are  located  over  a  space  classified  in
  occupancy  group C or E on the street floor, they shall be provided with
  a separate enclosed interior stair, or with an exterior stair.
    (6) Impact resistance. Stair  enclosures  serving  occupancy  group  E
  spaces  (office  spaces)  in high rise buildings constructed pursuant to
  applications filed on or after July 1, 2006 shall comply with  rules  to
  be   promulgated   by   the  commissioner  establishing  minimum  impact
  resistance standards. Such rules shall permit compliance with assemblies
  comprising approved reinforced construction  boards  affixed  onto  stud
  framing.  The  commissioner  shall  promulgate  such  rules on or before
  January 1, 2006.
    (j) Openings and obstructions to stair enclosures. No  piping  of  any
  kind,  with  the exception of piping required or permitted in subchapter
  seventeen of this code, shall be permitted within a stair enclosure.  No
  openings of any kind, other than windows, fire department access panels,
  exit doors and openings specifically authorized in reference standard RS
  5-18  shall  be  permitted  within  a stair enclosure. Pipes required or
  permitted by such  subchapter  seventeen  and  protected  in  accordance
  therewith  which  do not reduce the required clearances of the enclosure
  may be permitted. Ducts protected in accordance with the requirements of
  subchapter thirteen of this chapter, which do not  reduce  the  required
  clearances of the enclosure, may be permitted. In addition, in buildings
  in  occupancy  group  J-2, which are three stories or less in height and
  occupied by not more than two families on each story,  a  door  from  an
  apartment  may  open  directly into a stair, and the door may swing into
  the apartment.
    (k) Roof access. (1) Except as otherwise provided  for  in  paragraphs
  two  and three of this subdivision, in buildings or in building sections
  more than three stories or forty feet high with roofs having a slope  of
  less  than  twenty  degrees,  access to the roof shall be provided by at
  least one interior stair, except that access to setback roof  areas  may
  be  through  a  door  or  window  opening  to  the roof. Interior stairs
  extending to roofs shall be  enclosed  in  bulkheads  of  fire-resistant
  construction  meeting  the  requirements  of  subchapter  five  of  this
  chapter.
    (2) In buildings or in building sections classified in occupancy group
  J-1 or J-2 more than two stories in height, except as otherwise provided
  for in paragraph three of this subdivision, with roofs having a slope of
  fifteen degrees or less all interior stairs, except those terminating at
  a level of a setback roof,  shall  extend  to  the  roof  and  shall  be
  enclosed   in  bulkheads  of  fire-resistive  construction  meeting  the
  requirements of subchapter five of this chapter. Stairs  terminating  at
  the  level  of  a setback shall provide access to the setback roof areas
  through a door except where the setback is less than four feet in width,

  measured from the inside of the parapet wall, and less than ten feet  in
  length.
    (3) In buildings or in building sections classified in occupancy group
  J-1  or  J-2  two  stories  in  height  and in occupancy group J-2 three
  stories in height with not more than one dwelling unit  per  story  with
  roofs  having  a  slope  of  fifteen degrees or less, access to the roof
  shall be provided through a scuttle at least twenty-one inches in  width
  and  twenty-eight inches in length and shall comply with subdivision (c)
  of section 27-338 of article four of subchapter five  of  this  chapter.
  Scuttles  shall be located within each stair enclosure with a stationary
  iron ladder leading thereto.
    (l) Spiral stairs. Spiral stairs may serve as  access  stairs  between
  two  floors or levels in accordance with the provisions of paragraph two
  of subdivision (i) of  this  section.  Such  stairs  may  not  serve  as
  required  exits,  except  that  unenclosed  spiral  stairs when built of
  noncombustible materials and having a tread length of  at  least  thirty
  inches  may  serve  as  exits  from  mezzanines  or  balconies having an
  occupant load not exceeding twenty-five persons.
 
           Table 6-4 Maximum Riser Height and Minimum Tread Width
  ========================================================================
    Occupancy Group Classification       Maximum Riser    Minimum Tread{1}
               of Building                Height (in.)      Width (in.)
  ------------------------------------------------------------------------
  Residential J-3 (with closed risers)..      8 1/4    9 plus 1 1/4 nosing
  Residential J-3 (with open risers)....      8 1/4    9 plus 1/2 nosing
  Residential J-2 (with only three......
  dwelling units).......................      8 1/4    9 plus 1 1/4 nosing
  Assembly F............................      7 1/2    9 1/2 plus nosing
  Institutional H-2.....................      7       10 plus nosing
  All others{2}.........................      7 3/4    9 1/2 plus nosing
  ========================================================================
 
  Notes:
    {1} Treads may be undercut a distance equal to the  nosing.  A  nosing
  shall not be required when tread width is eleven inches or wider.
    {2}  The  proportions  and  dimensions  of  treads  and  risers may be
  adjusted in buildings classified in occupancy group G to suit the age of
  occupants, subject to the approval of the commissioner.