There is no requirment in the IRC stairway requirements
for a tread depth of 11 inches. The IRC calls for a
minimum tread depth of 10 inches measured horizontally
between the vertical planes of the foremost projection of
adjacent treads and at a right angle to the tread�s
leading edge. For winder treads the IRC permits a minumum
tread depth of 10 inches at a point 12 inches from the
side where the treads are narrower with a minimum winder
tread depth of 6 inches. (R3111.5.3.2) The FBCR adds a
classification of stairs not found in the IRC, Circular,
and seems to combine the FFPC provisions for curved stairs
and winders found in the Florida Fire Prevention Code. The
provisions presumably are to allow for a stair with a more
gradual turn than a spiral stair. (i.e. monumental stairs
of yore.) The provisions contained in the FBCR, except for
a reduction of minumum tread depth from 10 inches to 9
inches with a 1 inch nosing, are identical to those for
existing curved stairs in the Florida Fire Prevention
Code. (s.7.2.2.2.2.2) New curved stairs in the FFPC are
required to provide a minimum tread depth of 11 inches.
This is the only occurence of the 11 inch tread that is
found. In addition, the FFPC contains a section addressing
winders that differs from curved stairs by limiting the
narrow side to not less than 6 inches while curved stairs
provide the smallest radius is not less than twice the
stair width. (s.7.2.2.2.4 and 7.2.2.2.2) Having said all
that, no, the intent is not to provide a Florida Specific
Amendment to reduce the base code tread depth from 11 to 9
inches as there is no 11 inch requirement.
