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Building stair stringers: codes & strength requirements for stair support systems; how to build safe stair stringers that don't sag, squeak, collapse, or lose their treads.
This article describes the construction and building code specifications for the support of residential stairways: how strong should the stringers be, and what are the load bearing requirements of stairs and their supporting stringer beams.
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Reader Question: sagging stairway: is there a code I can check on stair stringer construction?
Rich O said:
So, used improper terminology below, believe I should have used "stringer" versus "riser", the entire 12' stringer (wood construction) is now deflecting as if a support has failed. Thanks again, Rich
My house in Westminster CO is just out of warranty and one of the staircase risers has started to visibly deflect (and squeak) when traversing the stairs. I've requested the builder take a look, in the meanwhile, is there a specific code I can refer to in order to ascertain if the stair was properly constructed? Thanks, Rich
Reply: Model building code specifications for stair support stringers
Yes a riser is the vertical board enclosing the space between horizontal walking surfaces, i.e. treads. The stringer is the support for the stairs - it functions as an angled beam and must support both the dead loads and live loads of the building stairway.
Let's be clear first that a significant concern with the strength of supporting stair stringers (effectively angle beams) arise because a typical 2x12" (or rarely 2x14") stringer is in some stair designed notched to carry the stair treads on the horizontal notch face and stair risers on the vertical notch face.
My deck stair photo at left shows very deep notching in a stair stringer (as well as multiple other safety hazards and code violations). Less than 2" of lumber was left in this stair stringer 2x after the builder cut his tread notches.
Notched stair stringers are widely used and are acceptable if sufficient supporting strength is provided. I prefer to avoid deflection or even collapse problems by using a solid stringer, connecting stair treads to cleats.
It's also worth emphasizing that in all cases the connections are as important as the beam strength. The connections of stair stringer to the risen-to platform, of treads to stringer, and of railings and posts are critical for safe stair construction.
Those worries out of the way for a moment and assuming your question pertains to a notched stair stringer system that is sagging or deflecting, if a stair stringer is deflecting, depending on the amount, it may be damaged or may have been inadequate to begin with.
For safety, investigate and determine what repair or additional support is needed.
Because stringer notch depth for designs that actually notch the 2x12 or 2x14 framing lumber typically used can vary depending on stair design, codes do not specify explicit lumber dimensions. Instead codes typically specify the strength required of the ending design. One may need to increase the number of stringers accordingly.
For example the IRC specifies that stairs shall be designed to withstand a live load of 40 pounds - as with a building floor.
Before an inspection has been performed we don't know if the deflection of the stair stringer to which you refer is due to damage (rot for example) or a design inadequacy. But the initial focus must be on the detection of and response to any *immediate safety hazard* such as conditions that could permit a stair collapse, fall, or injury.
Here is a model building code citation for stair stringer support requirements for residential stairs
R301.5 Live Load [Specifications for Stair Construction]
Minimum required live load for Stairs 40 psf. c.
c. Individual stair treads shall be designed for the uniformly distributed live load or a 300-pound concentrated load acting over an area of 4 square inches, whichever produces the greater stresses.
301.1.1 Alternative provisions [for supporting stair stringers]
As an alternative to the requirements in Section R301.1 the following standards are permitted subject to the limitations of this code and the limitations therein. Where engineered design is used in conjunction with these standards the design shall comply with the International Building Code.
1. American Forest and Paper Association (AF&PA) Wood Frame Construction Manual (WFCM).
2. American Iron and Steel Institute (AISI), Standard for Cold-Formed Steel Framing—Prescriptive Method for One- and Two-family Dwellings (COFS/PM).
Stair Support Stringer Analysis & Strength Requirements Code & Study Citations
Rich: I wanted to add that in addition to the important Frank Lam (et als) study of stair support stringers, one of the most thoughtful articles I've found on stair stringer design addresses the lack of specificity of stair stringer design in the model codes and provides some helpful engineering analysis and assumptions. See the first stair stringer code and strength requirement citation just below.
The preceding citations focus on the supporting stringer used in stair construction and on angled beams and strength requirements. For a complete list of stair building codes see the two live links immediately below.
Continue reading at DECK STAIR BUILDING DETAILS for exact procedures for laying out and cutting a stair stringer. or select a topic from the More Reading links shown below.
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