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.