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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. Because a slip, trip or fall on stairs or walkways may involve more hazards than just the slipperiness of the walking surface, also see Exterior Stair Falls and STAIR TREAD DIMENSIONS and CODES for STAIRS & RAILINGS as well as other stairway construction, code, & inspection articles in the links listed at Related Topics .
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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 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.
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)
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.
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:
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.
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
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.
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.
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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Frequently Asked Questions (FAQs) about Slippery Walking Surfaces & Stairways
Questions & answers or comments about Algae, Ice, Fungus, Wet Surfaces & Other Stair Slip, Trip & Fall Hazards.
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Technical Reviewers & References
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