Stair Stringer Specifications
Building stair stringers: strength & construction codes & requirements
STAIR STRINGER SPECIFICATIONS - CONTENTS: about how should stair support stringers be constructed, what alternative materials can be used & what are the stair stringer building code specifications for strength & load bearing requirements.
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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.
Stair Stringer Codes & Specifications: required strength for stair supports
Stair Tread Support Choices & Relation to Stair Stringer Notching
Stair treads can be placed directly on top of the horizontal surface of cutouts in the notched stair stringer.
This approach is very secure provided that the stringer depth is great enough and the cutout depth shallow enough that the remaining stringer material is strong enough to carry the weight of the stairway and its users (1-? arrow in sketch at left).
[Click to enlarge any image]
Of course if the stairway is supported by securing the stringer to walls on both sides, this cutout depth weakness problem is mitigated or eliminated.
Stair treads can be placed between the stringers and supported by cleats or supported by grooves routed inside the un-notched stringer.
If the stair treads are set into grooves routed into the stringer, in a 2x stringer the groove depth should be 1/2 the thickness of the stringer (typically 3/4") and additional support by cleats or by gluing and end-screwing through the stringer sides into the treads is good practice.
Stair treads butted against an un-grooved stringer inner face and then nailed or screwed through the stringer into the stair tread.
We do not recommend this stair tread approach as too often we find the treads break away from the stringer causing catastrophic stair tread collapse and perhaps serious injuries. If you encounter a stair built this way you should add glued & screwed cleats to support the treads. Also see Unsafe Stair Tread-to-Stringer Connections .
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.
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.
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
using the 2003 IRC.
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.
Christopher R. Fournier, P.E., "Wood Framed Stair Stringer Design & Construction", Structure Magazine, March 2013, p. 45-46. Mr. Fournier is a structural engineer in North Conway NH. His article is available online, retrieved 4/19/14 original source: www.structuremag.org/Archives/2013-3/C-StrucDesign-Fournier-March13.pdf
Aghayere, Abi, and Jason Vigil. "Introduction: Wood Properties, Species, and Grades." Structural Wood Design: A Practice-Oriented Approach Using the ASD Method: 1-24.
Aghayere, Abi, and Jason Vigil. "Frontmatter." Structural Wood Design: A Practice-Oriented Approach Using the ASD Method (2008): i-xii. [See Chapter 27, "Combined Dead and Live Loads on Stair Stringers"]
De Tennis, Danielle. "Strength Testing and Analysis of a Stair Header Connection." PhD diss., Milwaukee School of Engineering, 2010.
Higgins, Christopher. "Prefabricated steel stair performance under combined seismic and gravity loads." Journal of structural engineering 135, no. 2 (2009): 122-129.
Kourakis, Ioannis, and Shawn Li. "Design of a Glass Stair." In Structures Congress 2010, pp. 2650-2659. ASCE, 2010.
Lam, Frank, George Lee, Huijun Yan, Jianzhong Gu, and Ata A. Saravi. "Structural performance of wood-based stair stringers." Forest products journal 54, no. 4 (2004).
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.
BUILDING CODES for STAIRS - Model & actual building codes for stairs, railings, guardrails, landings, risers, treads, and steps
CODES for STAIRS & RAILINGS - Specifications for Stairs, railings, guardrails, landings, treads, & steps based on model & actual building codes
Continue reading at DECK STAIR BUILDING DETAILS for exact procedures for laying out and cutting a stair stringer. or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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 "The Elimination of Unsafe Guardrails, a Progress Report," Elliott O. Stephenson, Building Standards, March-April 1993
 "Are Functional Handrails Within Our Grasp" Jake Pauls, Building Standards, January-February 1991
 Access Ramp building codes:
 Access Ramp Standards:
ADA (Americans with Disabilities Act), Public Law 101-336. 7/26/90 is very often cited by other sources for good design of stairs and ramps etc. even where disabled individuals are not the design target.
ANSI A117.4 Accessible and Usable buildings and Facilities (earlier version was incorporated into the ADA)
ASTM F 1637, Standard Practice for Safe Walking Surfaces, (Similar to the above standard
 Falls and Related Injuries: Slips, Trips, Missteps, and Their Consequences, Lawyers & Judges Publishing, (June 2002), ISBN-10: 0913875430 ISBN-13: 978-0913875438 "Falls in the home and public places are the second leading cause of unintentional injury deaths in the United States, but are overlooked in most literature. This book is unique in that it is entirely devoted to falls. Of use to primary care physicians, nurses, insurance adjusters, architects, writers of building codes, attorneys, or anyone who cares for the elderly, this book will tell you how, why, and when people will likely fall, what most likely will be injured, and how such injuries come about. "
 The National Institute of Standards and Technology, NIST (nee National Bureau of Standards NBS) is a US government agency - see www.nist.gov
"A Parametric Study of Wall Moisture Contents Using a Revised Variable Indoor Relative Humidity Version of the "Moist" Transient Heat and Moisture Transfer Model [copy on file as/interiors/MOIST_Model_NIST_b95074.pdf ] - ", George Tsongas, Doug Burch, Carolyn Roos, Malcom Cunningham; this paper describes software and the prediction of wall moisture contents. - PDF Document from NI
 Slips, Trips, Missteps and Their Consequences, Second Edition, Gary M. Bakken, H. Harvey Cohen,A. S. Hyde, Jon R. Abele, ISBN-13: 978-1-933264-01-1 or
ISBN 10: 1-933264-01-2,
available from the publisher, Lawyers ^ Judges Publishing Company,Inc., www.lawyersandjudges.com email@example.com and also from the InspectAPedia Bookstore (Amazon.com)
 The Stairway Manufacturers' Association, (877) 500-5759, provides a pictorial guide to the stair and railing portion of the International Residential Code. [copy on file as http://www.stairways.org/pdf/2006%20Stair%20IRC%20SCREEN.pdf ] -
[14a] "Visual Interpretation Of The International Residential Code (IRC) 2006 Stair Building Code", The Stairway Manufacturers Association, [Portions of this document reproduce sections from the 2006 International Residential Code, International Code Council, Falls Church, Virginia.},
The Stairway Manufacturers Association website stairways.org provides free downloads of stairway handrailing profiles and dimensions
 Mold-Resistant Building Practices, advice from an expert on how to prevent mold after a building flood and how to prevent mold growth in buildings by selection of building materials and by anti-mold construction details.
 "The Dimensions of Stairs", J. M. Fitch et al., Scientific American, October 1974.
 Stair & Walkway Standards for Slipperiness or Coefficient of Friction (COF) or Static Coefficient of Friction (SCOF)
ASTM D-21, and ASTM D2047
UL-410 (similar to ASTM D-21)
NSFI 101-B (National Floor Safety Institute)
NSFI Walkway Auditing Guideline (WAG) Ref. 101-A& 101-B (may appear as ANSI B101.0) sets rules for measuring walkway slip resist
OSHA - (Dept of Labor CFR 1910.22 does not specify COF and pertains to workplaces) but recognizes the need for a "qualified person" to evaluate walkway slipperiness
ADA (relies on the ANSI and ASTM standards)
 A. Sacher, International Symposium on Slip Resistance: The Interface of Man, Footwear, and Walking Surfaces, Journal of Testing and Evaluation (JTE), ISSN: 1945-7553, January 1997 [more focused on slipperiness of polished surfaces
 Algae is widely recognized as a slippery surface - a Google web search for "how slippery is algae on steps" produced more than 15,000 results on 8/29/12)
 Slipperiness of algae on walking surfaces, warning, Royal Horticultural Society, retrieved 8/29/2012, original source: http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=418
 Slipperiness of algae: "Watch your step, wet rocks and algae are slippery" Oregon State University warning 1977 retrieved 8/29/2012, original source: http://www.worldcat.org/title/watch-your-step-wet-rocks-and-algae-are-slippery/oclc/663683915
 Coefficient of friction of algae on surfaces [like stair treads]: Delphine Gourdon, Qi Lin, Emin Oroudjev, Helen Hansma, Yuval Golan, Shoshana Arad, and Jacob Israelachvili, "Adhesion and Stable Low Friction Provided by a Subnanometer-Thick Monolayer of a Natural Polysaccharide", Langmuir, 2008 pp 1534-1540, American Chemical Society,
retrieved 8/29/2012, Abstract: Using a surface forces apparatus, we have investigated the adhesive and lubrication forces of mica surfaces separated by a molecularly thin, subnanometer film of a high-molecular-weight (2.3 MDa) anionic polysaccharide from the algae Porphyridium sp. adsorbed from aqueous solution. The adhesion and friction forces of the confined biopolymer were monitored as a function of time, shearing distance, and driving velocity under a large range of compressive loads (pressures). Although the thickness of the dilute polysaccharide was <1 nm, the friction was low (coefficient of friction = 0.015), and no wear was ever observed even at a pressure of 110 atm over 3 decades of velocity, so long as the shearing distances were less than twice the contact diameter. Atomic force microscopy in solution shows that the biopolymer is able to adsorb to the mica surface but remains mobile and easily dragged upon shearing. The adhesion (adsorption) of this polysaccharide even to negatively charged surfaces, its stable low friction, its robustness (high-load carrying capacity and good wear protection), and the weak (logarithmic) dependence of the friction force on the sliding velocity make this class of polyelectrolytes excellent candidates for use in water-based lubricant fluids and as potential additives to synovial fluid in joints and other biolubricating fluids. The physical reasons for the remarkable tribological properties of the ultrathin polysaccharide monolayer are discussed and appear to be quite different from those of other polyelectrolytes and proteins that act as thick “polymer brush” layers.
 Jason R. Stokes, Lubica Macakova, Agnieszka Chojnicka-Paszun, Cornelis G. de Kruif, and Harmen H. J. de Jongh, "Lubrication, Adsorption, and Rheology of Aqueous Polysaccharide Solutions, Langmuir 2011 27 (7), 3474-3484
 "Coefficients of Friction for Ice", The Physics Factbook™, Glenn Elert, Ed., retrieved 8/29/12, original source: http://hypertextbook.com/facts/2004/GennaAbleman.shtml
 "Coefficients of Friction for Ice", The University of the State of New York Reference Tables for Physical Setting/Physics. New York: The State Education Department, 2002. Op. Cit.
 Serway Physics for Scientists and Engineers 4th edition (p. 126.)
 "How Slippery Is It", retrieved 8/29/12, original source http://www.icebike.org/Articles/howslippery.htm
 John E. Hunter, "Friction Values", The Source, Society of Accident Reconstructionists, Winter 1998. Study of frictional values of car tires involved in collisions on snow or ice covered roadways.
 Frictional Coefficients of some Common Materials and Materials Combinations, The Engineering Toolbox, retrieved 8/29/2012, original source: http://www.engineeringtoolbox.com/friction-coefficients-d_778.html [copy on file as Friction and Coefficients of Friction.pdf ]
 Stairways and Ladders, A Guide to OSHA Rules, OSHA, U.S. Department of Labor, 3124-12R 2003 - Web Search 05/28/2010 original source: http://www.osha.gov/Publications/osha3124.pdf. OSHA regulations govern standards in the construction industry and in the workforce Quoting from OSHA whose focus is on workplace safety and so excludes discussion of falls and stair-falls in private homes:
OSHA estimates that there are 24,882 injuries and as many as 36 fatalities per year due to falls from stairways and ladders used in construction. Nearly half of these injuries are serious enough to require time off the job--11,570 lost workday injuries and 13,312 non-lost workday injuries occur annually due to falls from stairways and ladders used in construction. These data demonstrate that work on and around ladders and stairways is hazardous. More importantly, they show that compliance with OSHA's requirements for the safe use of ladders and stairways could have prevented many of these injuries. -osha.gov/doc/outreachtraining/htmlfiles/stairlad.html
 International Building Code, Stairway Provisions, Section 1009: Stairways and Handrails, retrieved 8/29/12, original source: http://www.amezz.com/ibc-stairs-code.htm [copy on file as IBC Stairs Code.pdf]
 Model Building Code, Chapter 10, Means of Egress, retrieved 8/29/12, original source: http://www2.iccsafe.org/states/newjersey/NJ_Building/PDFs/NJ_Bldg_Chapter10.pdf, [copy on file as NJ_Bldg_Chapter10.pdf] adopted, for example by New Jersey. International Code Council, 500 New Jersey Avenue, NW, 6th Floor, Washington, DC 20001, Tel: 800-786-4452
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