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

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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.

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

Daniel Friedman

Aglae slip hazard on wood steps (C) Daniel Friedman

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 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.

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)

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

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". [28]

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]


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

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)

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)


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

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

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] up to 0.8 SCOF for bare foot on dry Massaa tiles. "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."
A floor with a friction coefficient between 0.2 and 0.29 was ‘‘slip resistant’’
See: Ali, W. Y. "Friction Coefficient of Bare Foot Sliding Against Marble Flooring Tiles." - Files/ 320/Researches/56847_27169.pdf
and Ali, W. Y. "Friction Coefficient of Bare Foot Sliding Against Marble Flooring Tiles." - 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). 0.5  
Recommended Minimum SCOF for walking surfaces (ADA) [28] 0.6  
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]

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

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  
Wood, clean [41] 0.25 - 0.5  

Wood, wet [41]

Wood, waxed [41] 0.04  


1. Table adapted & expanded from "Friction", the Physics Hypertextbook, retrieved 8/29/12, original source:

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.

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 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

Collapsing exterior deck steps (C) D Friedman

More examples of exterior stair slip fall hazards are

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

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

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.

Dry green algae on wood step (C) Daniel Friedman Dry green algae on wood step (C) Daniel Friedman

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.

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 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.

Roof drip onto deck forms ice (C) Daniel Friedman Snow covered stone & gravel walk - hard to clear (C) Daniel Friedman

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.

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 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

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?


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

Citations - interesting & useful texts describing the measurement of friction


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