Studies have shown stair tread depth to be a risk factor of stairways. Understanding the safety of tread depth and the IBC and OSHA requirements will help stair designers ensure they choose the right stair design for optimal safety and ergonomics.
Tread Depth Definition
Before getting into the requirements, let's start by defining stair tread depth. OSHA states that "tread depth is measured horizontally between the vertical planes of the foremost projection of adjacent treads and at a right angle to the treads leading edge." It is a measure of the surface available for the user's foot to rest as they ascend or descend the stair.
Minimum Tread Depth
OSHA and IBC have different tread depth requirements for different stair types. OSHA introduced minimum tread depths in its 2017 update to walking-working surface standards. IBC has had them for many years. This chart below shows the minimum tread depths for each stair type.
You can see from the chart that the minimum tread depth varies greatly between different stair types. Generally, steeper angled stairs, like ship stairs or ships ladders, have less tread depth because as a stair gets steeper, the tread above becomes an obstacle to the tread below.
The result is that steep ship stairs and ship ladders leave the user with less than half a typical foot-length to place his or her foot.
So how did OSHA and IBC determine the minimum tread depth requirements? Beginning in the mid 1970's researchers began studying how stair design affects missteps, slips, and falls. One important study by John Templer in 1976 showed that reducing tread depth to less than about 11 inches resulted in more missteps. Other studies showed similar results, and the standard 11 inch tread depth was then adopted by the major building codes.
OSHA introduced it's own minimums for tread depth in the 2017 update to walking-working surfaces. OSHA determined that a 9.5 inch minimum tread depth would be sufficiently safe for workers in general industry.
Steeper, non-standard stair types have narrower minimum tread depths. Alternating tread stairs have a minimum of 8.5", and OSHA ship stairs have a minimum tread depth of 4". But does a 4" tread leave the user with enough space to safely and comfortably walk the stair?
Tread Depth Safety
A recent study measured the perceived difficulty in descent of stairways. This study, by Hisao Nagato, of the Ohara Memorial Institute for Science of Labor in Japan, looked at how difficult it was for users to descend stairways with varying tread depths. The result showed that stairs become increasingly difficult to descend as tread depth narrows. To measure the difficulty, the researchers used a metric called the Z-value, which increases as difficulty increases. Below is a chart showing the Z-values for different stair designs.
From this chart, you can see that ship stairs are 40x more difficult to descend than IBC stairs and nearly 3x as difficult as alternating tread stairs. So it is safe to say that a 4" tread depth does not provide enough tread depth to safely and comfortably walk the stair.