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This article describes and includes illustrations of common causes of slippery walking surfaces on stairs and walkways. We provide citations of recommended anti-slip or anti-skid steps or other walking surfaces, we define COF - coefficient of friction and SCOF - static coefficient of friction, and we cite recommended COF or SCOF for stairs and walkways.
We provide a table that compares the slipperiness of different walking surfaces & surfaces such as dry versus wet concrete, steel or wood, and algae, snow, ice or water coated walking surfaces.
We also provide some stair and walkway maintenance suggestions to reduce slip, trip and fall hazards due to water, algae, snow, ice, etc.
Algae, Ice, Fungus, Wet Surfaces & Other Stair Slip, Trip & Fall Hazards
Algae on steps & other walking surfaces - trip hazard
Algae growth on steps or decks: green or sometimes black algae grows readily on wood, concrete, or stone surfaces in most climates, particularly where those surfaces are repeatedly wet and especially if the surface is also shaded.
Algae makes these walking surfaces dangerously slippery, particularly when wet - a slip, trip and fall hazard which is widely recognized. 
The green algae-covered deck shown at above left was inspected by the author who in fact nearly had a bad fall due to wet algae on the deck where the ladder was placed.
You can see the scrape marks of the ladder feet (where my pen is pointing in the photo, above right) and the good luck that the ladder slippage was stopped by the chimney base.
Algae, when wet, is one of the slipperiest substances known.
It is readily observed that algae growth on wood surfaces may seem harmless when the steps are dry, but when any stair surface, stone, brick, wood, or other, is covered with algae and becomes wet, the surface is extremely slippery, adding significantly to the risk of a serious fall and injury.
Algae on Stair Treads vs Recommended Coefficient of Friction (COF) and Static Coefficient of Friction (SCOF)
Our photo (left) illustrates that multiple hazards may combine at an individual stairway leading to a fall even by people who have used the stairs many times before.
The stair we show has treads that are too narrow, rise too high, uneven step riser heights, no tread nose, and no stair handrailing. But notice that at the time of our inspection the stair tread surfaces were also wet, adding to the fall hazard.
Stair Tread Anti-Slip or Coefficient of Friction Requirement for Safe Walking Surfaces
Various industry, ANSI, ASTM, OSHA, ADA standards recommend a static coefficient of friction (SCOF) of 0.5 or higher (ADA 0.6 or above) and define surfaces with SCOF of 0.4 or lower as "low traction", i.e. "slippery". 
Model building codes attempt to address the effects of algae, ice, snow, and water on stairs and walkways.
But because building codes & standards cannot anticipate every possible physical cause of slipperiness on walking surfaces or stairways, codes generally do not attempt to address every possible slippery substance that might be present such as algae, ice, snow, water, even spilled oil or food or wet paint on steps.
Since building codes cannot anticipate every possible slip trip and fall hazard, instead codes and standards typically state something like the text shown below.
1009.5.2 Outdoor conditions. Outdoor stairways and outdoor approaches to stairways shall be designed so that water will not accumulate on walking surfaces. In other than occupancies in Group R-3, and occupancies in Group U that are accessory to an occupancy in Group R-3, treads, platforms and landings that are part of exterior stairways in climates subject to snow or ice shall be protected to prevent the accumulation of same. - IBC (International Building Code) 
Slippery conditions on stairways must be corrected. - OSHA standard on temporary workplace stairways 
1003.4 Floor surface. Walking surfaces of the means of egress shall have a slip-resistant surface and be securely attached. - ICC and as adopted by various states throughout the U.S.  (Similar provisions are made for ramps in 1010.7.1 and 1010.7.2)
Comparing the Slipperiness of Algae with that of Ice & Other Slippery Walking Surfaces
Algal growth (green, brown, black) on a wood walking surface such as a stair, ramp, or deck may feel and look pretty harmless when it's dry (photo at left). But when there is even a modest level of dew, or water the surface can become more slippery than teflon.
Watch out: Wet algae may be in fact more slippery than ice:
The COF of kinetic (moving) rubber (say a car tire) on ice is standardized at 0.15.and depending on the material contacting the ice, the COF of ice may appear as low as 0.017 - less than Teflon. 
Gourdon et als. showed that algae can have a coefficient of friction (COF) as low as 0.015 - enormously slippery.
How Slippery are Various Stair & Walkway Surfaces?
Table Comparing Coefficients of Friction for Various Walking or Driving Surfaces & Materials Note 1
Materials / Surface
Static Coefficient of Friction (SCOF)
Friction before movement
Leather on Oak (shoe sole on an oak stair tread, presumably unfinished and unpolished tread surface)
0.27 - 0.38
Marble Floor Tiles [indoors]
up to 0.8 SCOF for bare foot on dry Massaa tiles. "Values of friction coefficient of bare
foot sliding against
tiles were 0.5, 0.43 and 0.4 at normal loads
of 200, 400 and 600 N."
A floor with a
between 0.2 and 0.29 was ‘‘slip resistant’’
See: Ali, W. Y. "Friction Coefficient of Bare Foot Sliding Against Marble Flooring Tiles." - https://www.kau.edu.sa/ Files/ 320/Researches/56847_27169.pdf
and Ali, W. Y. "Friction Coefficient of Bare Foot Sliding Against Marble Flooring Tiles." - https://www.kau.edu.sa/Files/ 320/Researches/ 55466_25788.pdf [PDF] files
Masonry on Brick (a brick set on concrete)
0.60 - 0.7
Recommended Minimum SCOF for walking surfaces (OSHA etc).
Recommended Minimum SCOF for walking surfaces (ADA) 
Rubber on Ice (tennis shoe on icy step or car tire on icy pavement) 
1. Table adapted & expanded from "Friction", the Physics Hypertextbook, retrieved 8/29/12, original source: http://physics.info/friction/
2. Friction measures the force between two surfaces that are in contact and measures the resistance to their slipping or tangential motion. There are two coefficients of friction, static friction (nothing is already moving) (SCOF or Us) and kinetic friction (moving or sliding) (COF or uK).
The measure of friction is independent of the surface area, speed (as long as speed is more than zero), and temperature. The amount of friction depends on the nature of the surfaces in contact with one another and the force between them (such as the weight of a person whose shoe sole is in contact with a stair tread surface. The "roughness" of a surface has a minor impact on friction, and friction can be higher between smooth surfaces.
3. The Ice on Ice example in the table above illustrates the reduced amount of friction when movement is present
Algae Appears on Other Building & Surrounding Surfaces: Concrete, Brick, Roofs, Siding
Green or black stains due to algae: green stains also appear on buildings including on roof shingles, tiles, slates, on building siding, and even on masonry walls, sidewalks, planters, and retaining walls: stone, concrete block, and concrete. If you see flat green stain on a building exterior and that is not producing any plant-like raised growth it is likely to be an algae.
In our photo at below left both the green on the concrete grate-surround and the black on the sidewalk may be species of algae. Why are they different? Perhaps different genera/species prefer different nutrients in the two pours of concrete, or perhaps because of moisture or other surface differences.
Watch out: on walks, decks, ramps and stairs, algae makes for a dangerously slippery surface, particularly when it is wet.
At above right our photo illustrates that bricks used in a stair or walkway may be quite uneven in their ability to host slippery algal growth. Two bricks in the foreground (above the 2012) have appear to have a modest algae growth while others did not show algae. But of course there are other trip hazards here - loose bricks.
Algae under the microscope has a distinctive appearance that easily distinguishes it from moss, lichens, and mold, as we show here.
This microphotograph of algae was made in our lab while examining a sample sent to us from our friend and mold lab expert Sue Flappan.
The original algae sample was collected from a concrete sidewalk using simple adhesive tape.
How to Reduce the Hazard of Algae Growth on Stairs or Other Walking Surfaces
Slippery Algae-coated stair tread hazard reduction.
The cedar wood steps shown at left were located at a home in the northeastern U.S. in an area of shade and dampness. The presence of lichens as well as algae illustrate that even rot-resistant cedar decking and steps are not immune to these slippery conditions.
Clean and remove algae growth. Algae on steps is often green, sometimes black in color.
Algal growth can be removed from outdoor steps using a power washer with or without deck cleaners such as the products described
at STONE SURFACE CLEANING METHODS
Do not direct roof runoff, downspouts, nor surface runoff onto stair surfaces. Such water promotes algae or fungus growth on the stair surface and in freezing climates, ice formation.
Use anti-slip additive in paint on outdoor stair treads, landings, and entry porches, such as a fine-ground sand powder which is mixed in with the paint. Anti-slip stick-on plastic tread covers are also available.
Masonry stair treads and entry platforms should be slightly pitched away from the riser (or away from the building for platforms) in order to drain water away from the riser side of the step - you don't need much pitch to drain, 1" in 45" of run is sufficient and won't violate building code. Good drainage on a masonry stair (or walk) also reduces damage from frost-related surface spalling and cracking.
(Also see STONE SURFACE CLEANING METHODS and
for more algae images and data about algae on buildings
see ALGAE STAINS on ROOFS)
Masonry stairs should be protected from frost heaves by proper gravel, backfill, drainage, and other construction details
Regular stair safety inspections performed by someone familiar with stair, railing, and safety requirements should discover hazards due to poor maintenance, stair or step or handrail deterioration, loose or worn stair treads or railings, or even bad original design that has gone unattended. The steps shown in our photograph above are improperly constructed with too-narrow treads, loose components, and a flimsy "handrail" placed too low, just 24" above the step tread surface.
Ice, Snow or Even Plain Water on Steps Means Very Slippery Surfaces
Water means slippery stairs and walks, icy or not.
In our table of surface slip coefficients of friction (above) we indicated that water on a walking surface significantly reduces the COF or increases the surface slip hazard.
Water atop ice and "black ice" are still more slippery (lower SCOF or COF) than water alone or ice alone on most surfaces.
The ice-covered exterior steps shown just below were pointed out to us by Paul Galow.
In our photo (below-left) water is running over these stone stairs, combining water, possibly thin algal coatings on some stones, and debris to add to a serious slip hazard.
At above left the author tries out stone surfaced stairs in Girona, Spain. Early morning mist left a thin wet coating that made these steps actually more treacherous than the running stone steps on the hiking trail at above right.
When water is visibly running down the steps on a hiking trail the walker might expect trouble and may walk with more care than a casual stroller or worse, a runner up the Spanish steps shown at above right.
Difficult-to-maintain snow and ice fall on decks, porches, steps, walks
We liked the photo at left because it shows both green algal growth down the brick building wall and typical snow and ice conditions at a masonry walk and stair in the Northeastern U.S.
Notice that in recognition that the steps will experience snow melt and then ice re-freezing during the daily temperature cycle, the site maintenance crew have left a bag of ice-melting crystals by the entry door.
In our photo at below left, even before construction had been completed we wondered about future ice hazards on the deck shown in our photo at left. Ice and snow melting and dripping off of roof eaves onto a deck or porch where the water re-freezes can lead to a surprise trip hazard.
At above right you can just make out a stone and gravel walkway along side of the building.
Because shoveling snow off of combined surfaces (flat stones surrounded by gravel) can be difficult these walks may not be adequately cleared and may present a fall hazard more often than other surfaces exposed to winter and freezing weather.
Watch out: using a power snow-blower where loose gravel is present can throw a stone through a window or into an eye.
The owners cleared this walk by hand but later later decided to reduce the risk and hassle of this hard-to-clear stone walkway by installing a roof over the entire walkway.
Our photographs below illustrate the range of challenges for snow and ice removal on exterior stairs and walks. At below left is a deep snow-covered main entry stair to the front door of a home in Duluth Minnesota while at below right is an exterior stair with snow on its treads in Hyde Park, New York, both photographed during the winter of 2014.
Reader question: 1/29/14 Carlos Rivera said: Is there a simpler way of measuring the Coefficient of Friction, besides using a Surface Roughness Tester?
Carlos, I am not expert on friction measurement - a check with a text will almost certainly list a variety of ways people have measured friction, such as using a combination of known slopes, pulleys, and scales. A quick look at history shows that around 1500 Leonardo DaVinci experimented with friction measurements using just that approach.
Early work in friction measurement provides the formula relating the dead weight of the mlove of a block being dragged across a surface and the counterweight used with a rope and pulley to move it.
µ = Ff / N = Mass(dead weight) / Mass(block)
The long list of methods for measuring friction took off from there and include at least
block and pulley (described above)
tilted plane (friction angle at which the mlove begins to move)
Citations - interesting & useful texts describing the measurement of friction
Cartwright, David Edgar, and Paul Melchior. Tides: a scientific history. Vol. 7. Cambridge: Cambridge University Press, 1999.
Cottenden, A. M., W. K. Wong, D. J. Cottenden, and A. Farbrot. "Development and validation of a new method for measuring friction between skin and nonwoven materials." Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine 222, no. 5 (2008): 791-803.
Goodenow, Gary L., Gary L. Kolhoff, and Fraser D. Smithson. Tire-road Friction Measuring System: A Second Generation. No. 37. 1968.
Swartz, J. C. "Apparatus for Measuring Internal Friction and Modulus Changes of Metals at Low Frequencies." Review of Scientific Instruments 32, no. 3 (1961): 335-338.
Van Blovehuysen, Richard, and Fred Schaefer. Internal combustion engine handbook-basics, components, systems and perspectives. Vol. 345. 2004.
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Eric Galow, Galow Homes, Lagrangeville, NY. Mr. Galow can be reached by email: email@example.com or by telephone: 914-474-6613. Mr. Galow specializes in residential construction including both new homes and repairs, renovations, and additions.
 "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 NIS
 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 firstname.lastname@example.org 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 ] -
 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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