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. MEANS OF EGRESS <br />1009.4.2 is set aside for spiral stairways conforming to <br />Section 1009.9 (see Exception 3). However, Excep- <br />tion 5 allows a different maximum riser and minimum <br />tread under limited condilions, but retains the, mini- <br />mum riser height and measurement method of Section <br />1009.4.2. <br />7iie requirements for dimensional uniformity are <br />found i� Section 1009.4.4. <br />Exceptions 1 and 2 are for unique elements of verti- <br />cal egress. Section 1009.4.2 is nol applicable to these <br />elements because of their construction issues and lim- <br />ited application. �or a discussion on alternating tread <br />devices, see Section 1009.10. For ships ladders, see <br />Section 1009.11. <br />Except?on 3 is for spiral staircases, a unique lype of <br />stairway. •'�ection 1009.4.2 is not applicabte to this stair <br />type, again because of construction issues and limited <br />applications. For a discussion on spiral staircases; see <br />Section 1009.9. <br />Exception 4 provides a practical exception where <br />assembly faCilities are designed for viewing. See Sec- <br />tions 1028.11 lhrough 1028.11.3 for assembly aisle <br />stair-limiting dimensions. <br />Exceptien 5 allows revisions to the 7 incheslll <br />inches (178 mm/279 mm) riser/tread requiremenls for <br />Group R-3 and any associated utility (such as b2rns, <br />connected garages or detached garages) and within <br />individual units of Group R-2 and their associated util- <br />ity areas (such as attached garages). This change is <br />allowed because of lhe low occupant load and the high <br />degree of occupant familiarily with the slairways. <br />When this exception is taken for slainvays that have <br />solid risers, each lread is required to have a nosing <br />projec;ion with a minimum dimension of'/, inch (19.1 <br />mm) and maximum dimension of 1'/, inches (32 mm) <br />where the lread deplh is less than 11 inches (279 mm). <br />Nosing projeclions are crealed where the nosing of the <br />tread above extends beyond the trailing edge oF the . <br />tread below or when a solid riser is angled under the <br />k�d above and connected ;o the trailing edge of the <br />T4EAO DEFRI (IG�IIiG EDG@ i0 LUOING EOGE) <br />1P MIN. <br />� <br />I <br />/ <br />STNRWAY WITIIOUf NOSING J <br />For SI: 1 inch = 25.4 mm. <br />tread below. Nosir�g projections are not required <br />residenlial stairs with open risers and 10-inch (; <br />mm) treads. A nosing projection provides a gre; <br />stepping surface for those ascending the stainvay. <br />users descending the stairway, the nosing projeci <br />aliows the toe of the (oot to be placed further a� <br />irom the riser above, providing the necessary cIE <br />ance for the heel of the foot as it swings down in an <br />to its posilion on the tre�d [see Figure 1009.4.3]. <br />Exception 6 allows for the replacement of an e� <br />ing stair. Where a change of occupancy would reqi <br />compliance with current standards, this exception <br />lows a stairway that may be steeper than that pen <br />ted, provided it does not constilute a hazard (see S <br />iion 3404.1). This language is consistent with t <br />found for existing stairways in the International Er <br />ing Building Code� (IEBC°'). <br />Exception 7 allows steeper stairs in spaces o( <br />more than 250 square feet (23 mz) in correctional fa <br />ilies (Group I-3) with a maximum riser height o <br />inches (203 mm) and a minimum tread deplh a <br />inches (229 mm) because of lhe minimal occup <br />load and the familiarity oF the users with the slairti� <br />a <br />1009.4.3 �Vinder trcads. Windcr treads are not permitte� <br />menna of egress srainvnys exccpt within a 8welling umt. ; <br />Exccptions: <br />1. Curvcdstainvny.rinaccordanccwithScctionl <br />2. Spiral sminvays in accordance with Section I <br />�oThe intent of this section is to coordinate ihe ge <br />provisions for stairway tread and riser dimensio <br />Section 1009.4.2 with the provisions for winder tr <br />permitted in curved and spiral slairways (see Sec <br />1009.8 and 1C09.9). Winders are permitled in m <br />of egress stairways wilhin dwelling unils where c <br />pant loads are smaller and do not block visual � <br />that prompt lhe autonomic adjuslmenis in ihr, g <br />the user as well as increased familiarity. This is <br />�ally fan-shaped treads at the turning of an L-sh <br />CORRELT MEOIOD OF <br />MEfS11RING TIIE HEIGM7 <br />OF flI5ER5 N1D OEPTMS <br />OFTREADS <br />Figure 1009.4.2 <br />TREADIRISER DIMENSIONS <br />7• MA%.1�' MIN.) <br />RISER HEIG11( <br />�,.TEPPING ^uURGALL TO <br />- STEPPINGSIIRFPL@) <br />Coo�'° <br />