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Landscape tie stair trip hazard (C) Daniel FriedmanSlippery Stair & Walk Surface Hazards
Coefficients of Friction or Slipperiness of Various Walking Surfaces

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. Page top photo: wet slippery landscape ties add to the trip hazard formed by the toe-catching design of these exterior stairs.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Algae, Ice, Fungus, Wet Surfaces & Other Stair Slip, Trip & Fall Hazards

Aglae slip hazard on wood steps (C) Daniel FriedmanDaniel Friedman

The wooden steps shown in our photograph are improperly constructed with too-narrow treads, loose components, and a flimsy "handrail" made of 1" diameter pipe (too narrow for safe grasp), placed too low, just 24" above the step tread surface.

Entering or leaving this Poughkeepsie New York home is unsafe at any speed, but more-so when the green algae-coated step is also wet, causing it to be incredibly slippery.

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. [29][30][31][32]

Article Series Contents

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.

Stair support unsafe (C) Daniel Friedman

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. This was my ladder and my slip [DF] that was luckily interrupted - else I'd have unable to write this article.

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.

Also see ALGAE, FUNGUS, LICHENS, MOSS for images of algae under the microscope.

Algae on Stair Treads vs Recommended Coefficient of Friction (COF) and Static Coefficient of Friction (SCOF) & Dynamic Coefficient of Friction (DCOF)

Unsafe exterior stair stringer and riser height (C) Daniel Friedman

Our photo of horribly-constructed wooden steps up to a deck 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 shallow - the tread depth is just a few inches, the step rise too high, there are uneven step riser heights, and there is no bottom 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". [28]

Definitions of Measurements of Friction - Slip Resistance

COF Coefficient of friction - tests giving a numeric value indicating the amount of friction or slip-resistance between two objects or surfaces

SCOF Static Coefficient of Friction - tests the friction of an object that moves from a standing or still position - its "break-away" point.

DCOF Dynamic Coefficient of Friction - (DCOF) tests the friction of an object that is already in motion - kinetic energy.

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) [43]

or

Slippery conditions on stairways must be corrected. - OSHA standard on temporary workplace stairways [42]

Use Glowing Stair Tread Tape to Reduce Fall Hazards

Glow-in-dark Non-slip tape marks edge of otherwise hard-to-see stair step edges (C) Daniel Friedman

Foster (2014) documented not only the added safety of using a tread-edge highlighter but also the optimal position for the highlight to reduce stair falls.

Above: we added and are testing the durability of this glow-in-the-dark non-slip stair tread tape placed along the front edge of these interior stairs made of brick.

Particularly because the upper walking surface is identically-colored brick, at night in low light it is difficult to see the edge of the individual steps.

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. [44] (Similar provisions are made for ramps in 1010.7.1 and 1010.7.2)

The florescent stair edge non-slip tape glows in the dark making the edges of the steps easier to spot.

Non-slip glow in the dark stair tread tape reduces fall hazards (C) Daniel Friedman at InspectApedia.com

Research & Standards for Phosphorescent Non-slip Tape for Steps

Comparing the Slipperiness of Algae with that of Ice & Other Slippery Walking Surfaces

Dry green algae on wood step (C) Daniel Friedman

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:

How Slippery are Various Stair & Walkway Surfaces?

Table Comparing Coefficients of Friction for Various Walking or Driving Surfaces & Materials Note 1

Materials / Surface
Alphabetic Order

Static Coefficient of
Friction (SCOF)
Friction before movement

Coefficient of Friction (COF)
Sliding friction

Algae 0.015 [32]  
Asphalt paving (frost covered) [39][40]   0.53 - 0.63
Asphalt paving (dry) [39][40]   0.72

Ice: Glare ice on asphalt paving
Smoothest ice, similar to wet ice, water not observed

(note for icy surfaces the nature of the material below is probably irrelevant in measuring friction)

  0.19

Ice on asphalt paving with sun exposure, not melted [39][40]

Ice, wet, on asphalt paving [39][40]

  0.24
Ice on Steel 0.03 0.03
Ice on Ice 0.05 - 0.5 0.02 - 0.09
Leather on Metal (e.g. a shoe sole on a dry steel stair tread) 0.4 - 0.56  
Leather on metal, wet or greased [41] 0.2  
Leather on Oak (shoe sole on an oak stair tread, presumably unfinished and unpolished tread surface) 0.27 - 0.38  
Marble Floor Tiles [indoors]

<= 0.8 SCOF bare foot
on dry Massaa tiles.

Note 8
Masonry on Brick (a brick set on concrete) 0.60 - 0.7  
Recommended Minimum SCOF for walking surfaces (OSHA etc). 0.5  
Recommended Minimum SCOF for walking surfaces (ADA) [28] 0.6 [Obsolete?]  
Rubber on Ice (tennis shoe on icy step or car tire on icy pavement) [37]   0.15
Rubber on wet concrete [41] 0.45 - 0.75  
Rubber (car tire or tennis shoe sole) on concrete (all surfaces dry, no loose sand, etc) 0.6 - 0.85 [41] 0.8 [38] - 1.02

Rubber Car tire on wet road [presumably asphalt] [41]

0.2  
Rubber (tennis shoe or car tire) on dry asphalt [41]   0.9

Rubber (car tire or tennis shoe sole) on grass (presumably dry)

Packed (car driven over) snow or fresh unpacked snow

  0.35
Skin (bare foot) on metal surface (presumably dry) 0.8 -1.0  

Snow on waxed hickory (similar to snow on a waxed or perhaps poly or painted stair tread) [28]

Snow or ice on asphalt [39][40]

0.04 - 0.4 0.04 - 0.4
Snow, wet [41] 0.14  
Teflon 0.04  
Recommended for
Tile, ceramic
 

0.60 4 [Obsolete]

>= 0.42 DCOF 6

Wood, clean [41] 0.25 - 0.5  

Wood, wet [41]

0.2  
Wood, waxed [41] 0.04  

Notes to the table above

1. Table - some portions are 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

4. Tile .60 static coefficient of friction (SCOF), determined by the ASTM C1028

5. Tile DCOF AcuTest, ANSI A137.1,

6. ANSI A137.1 (2012) tile walking surfaces for level interior spaces

7. Values of friction coefficient of bare foot sliding against Massaa tiles were 0.5, 0.43 and 0.4 at normal loads of 200, 400 and 600 N."
and
A floor with a friction coefficient between 0.2 and 0.29 was ‘‘slip resistant

8. 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

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.

Algae stains on concrete (C) Daniel Friedman ... brick steps (C) Daniel Friedman

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 (C) Daniel Friedman

Algae under the microscope

 has a distinctive appearance that easily distinguishes it from moss, lichens, and mold, as we show here.

This micro photograph 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.

Also see our black or blue-green algae illustrated and discussed near the end of

MOSS on ROOFS where we discuss the mix of moss, lichens, and algae often found on roofs./

How to Reduce the Hazard of Algae Growth on Stairs or Other Walking Surfaces

Collapsing exterior deck steps © D Friedman at InspectApedia.com

Slippery treacherous steps down to a gondola stopping dock in Cannaregio, Venice Italy (C) Daniel Friedman at InspectApedia.com ... Algae and seaweed on steps in Dorsoduro, Venice (C) Daniel Friedman at InspectApedia.com

More examples of exterior stair slip fall hazards are

at EXTERIOR STAIR FALLS

and at EXTERIOR STAIR CONSTRUCTION & CODES.

Ice, Snow or Even Plain Water on Steps Means Very Slippery Surfaces

Icy wooden stair treads (C) 2013 Daniel Friedman Paul Galow

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 above and found in Poughkeepsie, NY were pointed out to us by Paul Galow.

A very light snow dusting, atop ice on steps or other walking surfaces may be even more dangerous than when the ice was visible, as the hazard may not be noticed by the stair user.

Water means slippery stairs and walks, icy or not.

Icy stairs in Minnesota (C) Daniel Friedman

Above: ice-coated steps at Gooseberry Falls, Two Harbors Minnesota. Crampons are required to negotiate this stairway with any hope of avoiding a fall.

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.

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.

Dry green algae on wood step (C) Daniel Friedman

At below 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.

Dry green algae on wood step (C) Daniel Friedman

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.

Exterior step and walk maintenance in snow and ice conditions (C) Daniel Friedman

Difficult-to-maintain snow and ice fall on decks, porches, steps, walks

We liked the photo above 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 of a wooden deck frame shown below (Volition NY), 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.


Roof drip onto deck forms ice (C) Daniel Friedman

Below you can just make out a stone and gravel walkway along side of the building.

Snow covered stone & gravel walk - hard to clear (C) Daniel Friedman

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.

At SNOW & ICE REMOVAL on WALKS & STAIRS we describe methods for keeping snow and ice off of ramps, stairs and walkways.

Snow covered front entry stair in Duluty Minnesota 2014 (C) Church Friedman \ ... Snow covered exterior stairway (C) Daniel Friedman

Spills of Water Softener Resin Can Lead to a Nasty Fall

Below we show close-up photographs of Purolite® water softener resin beads that we discuss

at WATER SOFTENER RESIN REPLACEMENT.

The resin particles vary in size but are about 0.4 to 1mm in diameter.

Purloite water softener resin beads under the microscope (C) Daniel Friedman ... Purloite water softener resin beads under the microscope (C) Daniel Friedman

Watch out: Spilling these water softener resin beads on a walking surface will make it about as slippery as wet algae, with a very low static coefficient of friction (SCOF). What that means for normal people is if you step in this stuff you will fall on your you-know-what.

I didn't want to have to visit the mothers of the other workers to explain why they fell off of the rooftop: we took great care to sweep up any spillage immediately and very thoroughly.

More about the hazards of working with these very tiny resin beads are

at WATER SOFTENER RESIN PROPERTIES & SOURCES

History & Methods of Measuring Friction

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?

Reply:

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 move 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

Friction & Slipperiness Research

Measurement of friction & walkway or stair falls

Tiled Walks & Stairs: Outdoor Slip & Fall Hazard

Moved to TILED WALKING SURFACE SLIP HAZARDS

 




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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-01-27 - by (mod) - there is help available and solutions to slippery surfaces.

Well-said, Richard;

We welcome comments, criticism, suggestions (not product advertisements) and are happy to cite and link to technical content contributors.

On 2021-01-26 by Richard Campbell

I own a Non-Slip business by adding non-slip treatments to various types of walking surfaces as well as stairs, metal loading docks, bathtubs and more for homeowners and businesses.

What people don't realize, is that there is help available and solutions to slippery surfaces.

On 2020-08-11 - by (mod) -

Bill

I don't understand the question, but in general if walkers might indeed walk on a slippery metal surface imo that's a hazard.

On 2020-08-11 by Bill

is there a hazard if the walking surface (exterior) has a non-slip surface and a metal surface 16" wide is introduced at the same level?

On 2018-04-11 - by (mod) - how to contact OSHA about slip & fall concerns

David

If this is a place of employment, OSHA rules apply; typically guardrails and handrails would be required.

OSHA Consultation Service Area Offices* For workplace safety and health questions or assistance from Cal/OSHA Consultation, call the toll-free assistance number 1(800) 963-9424 or email us at InfoCons@dir.ca.gov. You may also contact the Cal/OSHA Consultation Service Area Office nearest your workplace.

else your local building department sets the local approvals for stairs and rails.

On 2018-04-11 by David Howe

I slipped on icy steps coming out of MILK HOUSE after a farmer cleaner the steps in 19 dergree weather and never informed me or took precautions .

There is no handrails and the landing is washed out grave. what is OSHA code on the?

On 2017-06-08 - by (mod) - hazard of open grate stair treads

M

I think the concern with open grate stair treads is that they catch pointed heel shoes, causing a fall

On 2017-06-07 by Mom

I am considered exterior stairs to connect my first-floor front yard to my basement-level back yard, to run along the side of the house. I am concerned about slip hazards.

Is it acceptable to use metal treads with a grate-like surface, for drainage and traction? I ran across a reference to IBC that said stair treads had to be solid, but I am not sure that was relevant to my situation.

On 2017-02-21 - by (mod) - height requirement for railingds

Typically the requirement for railings is based on the total height above ground between the top of the stairs and the bottom. Often that's three feet. But your local building inspector is a final Authority.

On 2017-02-20 by Elizabeth

How many exterior stepslegally require a railing?


...

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