As  against  external  fire,  a  tee-iron  and  book-tile  roof  will
give  sufficient  protection,  but  it  will  not  offer  the  resistance
to  shock  or  load  possessed  by  arch  construction.  This  is
important  where  walls  or  portions  of  walls  of  adjacent  build-
ings are  liable  to  fall  upon  the  roof-surface  during  a  con-
flagration.
Considering  internal  fire,  this  construction  provides  no
adequate  protection  for  the  under  sides  of  the  tees.  Where
book  tiles  are  used  the  tees  project  below  the  bottom  surfaces
of  the  terra-cotta  blocks,  and  a  very  thick  coat  of  plaster  on
the  terra-cotta  is  necessary  to  cover  the  tees  even  with  a  very
thin  coating.  Where  rabbeted  tiles  are  employed,  the  plaster
coating  over  the  tee  is  thicker,  but  as  it  has  little  or  no  bond
to  the  iron  surface,  the  protection  is  only  nominal.  This
difficulty  of  satisfactorily  fireproofing  the  tee-irons  renders
this  type  of  roof  unsuitable  for  structures  intended  to  be  of
the  best  fire-resisting  construction.
Another  objection  lies  in  its  comparative  thinness,  as  this
method  does  not  provide  a  good  insulation  against  changes
in  temperature.  The  temperature  of  the  spaces  under  the
roof  will  be  easily  affected  by  outside  changes,  and  condensa-
tion will  occur  under  even  slight  differences  in  temperature.
Roofs  with  Suspended  Ceilings. — Where  appearances
must  be  considered,  as  in  mercantile  or  office  buildings,  a
level  ceiling  must  be  had  under  the  sloping  roof-surface.
This  is  generally  accomplished  by  suspending  a  light  ceiling
construction  from  the  roof-beams.
This  does  not  change  the  roof  construction  itself  in  any
way  from  the  preceding  form.  The  same  general  details  are
used,  with  the  addition  of  the  suspended  ceiling  for  the  sake
of  appearances.  Fig.  131  shows  the  roof  and  ceiling  con-


278


THE   FIREPROOF1NG   OF  STEEL   BUILDINGS.
