Stairways and Stairwell – Design and Layout
If architectural building plans are being used for a project, the rise and the run of the stairways in the building would have usually been designed by the architect, and the unit rise (rise height), unit run (tread width), and the number of risers and treads for each stairway would also have been already determined along with the stairwell opening.
In these cases, the stairwell openings are framed first in accordance with the plan. The carpenters, then, layout and construct the rough stairways using the architect's plan. Carpenters may be required to design and construct any stairways and stairwell.
An important element in stair design is the mathematical relationship between the riser (unit rise) and the tread (unit run) dimensions.
The ratio of these two elements (or similarly, the ratio of the total rise and total run of the stairways) determines the slope of stairways.
Riser to Tread Ratio
One of three simple generally-accepted rules is normally used for determining the riser to tread (unit rise-to-run) ratio:
Unit run (tread) + (2 × unit rise) = 24” to 25”
Unit run + unit rise = 17” to 18”
Unit run × unit rise = 70” to 75”
Determining Riser Height and Tread Run
Normally, the preferred slope of stairways is between 30 to 35 degrees for maximum ease in climbing and safe descent.
Whenever possible, a unit rise between 7" and 7 ½" and a unit run between 10 ½" and 12" is recommended; this will result in a slope of 30 to 35 degrees. The maximum unit rise and minimum unit run permitted by code are often used in building plans or by contractors to minimize stairway space.
The tread run (unit run) or depth is measured from the face of one riser to the face of the next and does not include any nosing.
Determining Stairwell Openings
Stairwell – an opening in the floor for a stairway.
If a stairwell has been framed but the unit rise and run of the stairway are not specified, they must be determined. If the stairwell hasn’t been framed, the opening for the desired headroom clearance should be determined as follows. Click on each button to find out more:
The stair opening width for a straight stairway is determined by the desired width of the stairway plus the thickness of any skirt board(s).
The stairwell width for an open U-shaped stairway could be as wide as both stairways, plus the skirt board, plus the amount of space required to turn the U-shaped handrail.
In instances when the stairway has already been framed, the unit rise, run, and headroom clearance dimensions that are used may not be available.
In these rare cases you must determine the stairway unit rise and run, along with the total run based on the length of the stairwell opening, the total stairwell rise and the minimum headroom clearance allowed.
Laying Out and Cutting a Stringer
Marking a cutout stringer is a simple task. It can be done with either a framing square or a pitch board.
When using a framing square, the blade will represent the unit run and the tongue will represent the unit rise.
To make the stair layout go faster, a set of stair gauges can be used to set the unit rise and run measurements on the framing square.
Reinforcing Cutout Stringers
Some carpenters will add a 2 × 4 or 2 × 6 reinforcement known as a strongback to all longer cutout stair stringers that are supported only at the header and the floor, and not fastened to an adjacent wall.
This strongback is secured to one side of the cut string below the cut notch to add strength and rigidity.
It is also common practice to secure a 2 × 4 ledger to the floor and/ or header to add strength to the stair assembly. The stair stringers are notched to fit over the 2 × 4 ledger.
The size and position of the riser boards establish the height, depth, and spacing of the stair. When the stairs are wide, a center brace is needed for additional support.