602
Section 4 Finishing
header permits a slight extension in the fl oor area
above a stairway, Figure 18-7. When a closet is
located directly above the stairway, the closet fl oor
is sometimes raised for additional headroom.
18.4 Stair Design
Most important in stair design is the math-
ematical relationship between the riser and
tread. There are three generally accepted rules
for calculating the rise-run or riser-tread ratio. It
is wise to observe them:
• The sum of two risers and one tread should
equal 24″–25″.
• The sum of one riser and one tread should
equal 17″–18″.
• The height of the riser times the width of the
tread should equal 70″–75″.
According to the fi rst rule, a riser 7 1/2″ high
requires a tread of 10″. A 6 1/2″ riser requires a
12″ tread.
The current edition of the International
Residential Code, developed by the Interna-
tional Code Council, includes a change in the
allowable tread width and height of risers.
The previous standard for residences was an 8
1/4″ rise and a 9″ tread. Under the new code, a
7 3/4″ rise and a 10″ tread (assuming a nosing
of at least 3/4″) are required. The International
Residential Code, which is updated every three
years, has no legal force until it is adopted by
state or local governments.
In residential structures, treads (excluding
nosing) are seldom less than 9″ or more than
12″. Nosing is a small extension of the tread. In a
given run of stairs, it is extremely important that
all of the treads be the same size. The same is true
of the risers. A person tends to subconsciously
measure the fi rst few steps and will probably
trip if subsequent risers are not the same.
When the rise-run combination is wrong,
climbing the stairs will be tiring and cause extra
strain on leg muscles. Further, the toe may kick
the riser if the tread is too narrow. A unit rise
of 7″–7 5/8″ with an appropriate tread width
provides both comfort and safety. Main or prin-
cipal stairs are usually planned to have a rise
in this range. Service stairs are often steeper,
but risers should be no higher than 8″. As stair
rise is increased, the run must be decreased. See
Figure 18-8.
A main stair should be wide enough to allow
two people to pass without contact. Further, it
should provide space so furniture can be moved
up or down, Figure 18-9. A minimum width of
3′ is generally recommended, Figure 18-10. FHA
permits a minimum width of 2′-8″, measured
clear of the handrail. On service stairs, the
requirement is reduced to 2′-6″. Furniture
moving is an important consideration and extra
clearance should be provided in closed stairs of
the L- and U-type, especially those that include
wedge-shaped treads, or winders.
Stairs should have a continuous rail along the
side for safety and convenience. A handrail (also
called a stair rail) is used on open stairways that
are constructed with a low partition or banister.
In closed stairs, the support rail is called a wall
rail. It is attached to the wall with special metal
brackets. Except for very wide stairs, a rail on
Winders: Wedge-shaped treads installed where stairs
turn.
Handrail: A pole installed above and parallel to stair
steps to act as a support for persons using the stairs.
Also called a stair rail.
Wall rail: In closed stairs, the support rail that is
attached to the wall.
Finish flooring line
Headroom
line
Auxiliary
header
Main
header
Non-load-bearing partition
Joist
Figure 18-7. Extending the upper fl oor area with a shallow
auxiliary header to provide headroom. The partition over
the auxiliary header must not be load bearing.