Staircase stringer is a structural member installed on either side or in the center of a flight of stairs into which the treads and risers are fixed. The primary function of the stringer is to provide a framework or support to the tread and risers and hide the edge of the stairs exposing the tread profile.
In this article, we discuss the types, procedure to cut stringer and code requirements of staircase stringer.
Types of Staircase Stringers
The staircase stringers are classified into 3 types :
1) Routed or Housed Stringers
2) Sawtooth or Open Stringers
3) Mono Stringers
1. Routed or Housed Stringers
Routed stringers consist of notches (hollowed out) in which the treads, risers, and wedges can be inserted. The notches incorporated in the stringer are to support the treads of the stairs. This type of stringers is also known as Box Stringers.
Fig 1: Routed or Housed Stringers.
2. Sawtooth or Open Stringers
Open stringer is cut open on the rise and run when viewed from the side. The sides of the tread are exposed which need additional treatment for a better look.
Fig 2: Sawtooth or Open Stringers
3. Mono Stringers
A mono stringer stair uses a single ‘beam-like’ stringer that supports the center of the treads from the bottom. There are no risers are in mono stringer stairs.
Fig 3: Mono Stringers
Procedure for Cutting Open Stringers
1. Calculating Tread and Riser
The first step in finding out the stringer dimensions is to calculate the size of tread and riser. The height between the flooring and deck is measured using a tape (say 56 inches).
The height is divided by the size of risers to find out the number of steps required. The standard size of the riser is taken as 7 inches.
56″/7″= 8 Steps
To calculate the run of the stairs, the number of steps is multiplied with the desired width of the tread. The building codes laid out in the IRC recommend that each step should have a minimum width of 10 inches in order to provide secure footing.
8 Steps X 10 inches = 80 Inches
Fig 4: Dimensions of Staircase.
2. Marking of Tread and Riser on the Stringer
The dimension of tread and riser is marked on framing square and sketched out on the wooden plank of standard size 2×12 inches stock board. Make sure you leave at least 7 inches at the head of the board for the final riser, which you can trim to fit later.
Fig 5: Marking of tread and riser on square frame.
· Fig 6: Marking of tread and riser on stringer.
3. Cutting of Stringer
With all the cut lines drawn, carefully cut along them with a circular saw. In the notches at all the tread-riser intersections, stop cutting when the blade reaches the line. The strength of the stringer weakens if the tread and riser lines are overcut. Use the first stringer as a template to cut the subsequent stringers.
Fig 7: Cutting of Stringer.
The live load to be considered while designing the stairs is specified as 40 pounds per square foot for residential applications and 100 pounds per square foot for other applications.
The majority of the code provisions address dimensional restraints, such as width, rise/run, and vertical clearance. The image below provides a summary of code requirements and building construction handbook recommendations for the dimensional restraints of stair rise and run.
Fig 8: Code provision for Stringers.