Section 27-337. Roof coverings

Roof coverings shall be classified as A, B,
  or C on the basis of their  resistance  to  exterior  fire  exposure  as
  listed  in  reference standard RS 5-9, or as determined by tests made in
  conformance with reference standard RS 5-10 for those not listed.
    (a) Limitations of use. Every roof  placed  on  a  building  shall  be
  covered  with  class A or B roof covering, except class C roof coverings
  may be placed on buildings classified in occupancy group J when not more
  than three stories or forty feet in height, and on  buildings  permitted
  by  this  code  to  be  of  class  II-D or II-E construction. The use of
  roofing having no rating is prohibited, except for  replacement  to  the
  extent  of  twenty-five  percent  of  the  roof area in any twelve month
  period.
    (b)  Combustible   roof   decking.   Unless   attached   directly   to
  noncombustible  framework,  all  roof  coverings  shall  be applied to a
  closely fitted deck; except that wood shingles, to the extent  permitted
  in subdivision (a) of this section, may be applied to wood slats.
    (c) Roof insulation. Combustible roof insulation may be applied on top
  of  roof  decking  or  slab  provided that it is protected with the roof
  covering applied directly thereto.