Section 27-328. Fire retardant treated wood

(a) Material. Fire-retardant
  treated wood shall be pressure treated with fire retardant chemicals  in
  accordance  with  reference standards RS 5-3 and RS 5-4. Where used as a
  structural element or as furring, the material shall have a flame spread
  rating not greater than  twenty-five  when  tested  in  accordance  with
  reference  standard  RS 5-5 when exposed for a period of at least thirty
  minutes, with no evidence of significant progressive  combustion.  Where
  used  as interior finish or trim, the material shall have a flame spread
  rating that meets the requirements of section 27-348 of this  subchapter
  for  the location in which it is used. Subsequent to treatment, material
  two inches thick or less shall be air dried or kiln dried to an  average
  moisture content of not more than nineteen percent.
    (b)   Label.   All   fire-retardant   treated   wood  shall  bear  the
  identification of a testing laboratory or  producer  certifying  to  the
  performance  thereof,  in accordance with the acceptance requirements of
  section 27-131 of article seven of subchapter one of this chapter.
    (c) Application. Fire-retardant treated wood may  not  be  used  where
  exposed to the weather or in interior spaces where the relative humidity
  is normally eighty percent or more. There shall be no fabrication of the
  material  after  treatment,  such  as  cutting, shaping, or grooving for
  splines or ring connectors so as to expose  untreated  surfaces,  except
  that  the  material  may  be  cut  to  length, shaped, or grooved if the
  exposed surfaces or edges are tightly butted against other material that
  is noncombustible  or  that  is  fire  retardant  treated,  so  that  no
  untreated wood is left exposed to danger of ignition. Holes may be bored
  or  cut  for  plumbing or heating pipes and for electric outlets only if
  the openings are coverd with tightly-fitted  noncombustible  escutcheons
  or cover plates. The allowable working stresses of the material shall be
  ninety  percent  of  the allowable stresses for untreated lumber of like
  classification.
    (d) Where permitted in construction group  I.  Fire-retardant  treated
  wood  may  be used in buildings of construction group I in the following
  cases:
    (1) As permitted by table 3-4.
    (2) For interior non-bearing partitions that are not required to  have
  a fire-resistance rating.
    (3)  For  interior furring and blocking of exterior walls, furring and
  blocking of interior walls and  partitions,  and  framing  of  suspended
  ceilings  provided  the furring, blocking, and framing do not affect the
  integrity, or reduce the fire-resistance  rating,  of  the  construction
  element.
    (4) For interior finish and trim.
    (e)  Area  increase.  Fire-retardant  treated  wood  may  be  used  in
  construction group II buildings in  lieu  of  untreated  wood  for  wall
  studs,   bearing  partition  studs,  columns,  beams,  girders,  joists,
  rafters, trusses, sole and cap plates, subflooring and roof  decks,  and
  when  so used, the area limitations of tables 4-1 and 4-2, for buildings
  of construction group II, may be increased by thirty-three and one-third
  percent.